The plastic in your camera

bobn2 · Mar 7, 2013

Member said:
howardroark wrote:

Member said:

bobn2 wrote:

Member said:

Wyville wrote:

Member said:

howardroark wrote:

The housing, plate, and mirror box all serve important structural functions, but boiling it down to the mirror box being the most important one is ignoring the various load paths a camera body is subjected to.

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I agree. My sister is a structural engineer and from her I have learned a lot about how structures gain their strength. It's never just one part, but rather the collective of all parts. That's why I asked my question below. Engineering these days is about making structures light, if all the strength has to come from one part it will have to be disproportionately heavy.

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Yes, but design is also design for production. A unit has to be designed to have sufficient strength to be assembled, and also to be maintained. Making the skin structural is a bad idea, because it means that the unit is very fragile without it. That means that production necessitates all kinds of jigs to hold the bits together until the skin is put on. Likewise maintenance, because as soon as the skin is taken off, the unit cannot be easily worked on.

--

Bob

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More than one compnenent lending structural integrity, man. Something being in once piece without the skin during production doesn't proclude stress in-service from being different and benefiting from that exoskeleton/housing. That steel plate in the middle can have every single thing attached to it, things attached to the mirror box, all holding together in one piece without the housing, but a sub-assembly sitting on a bench is much different from an item being used in the field.

I really am not sure why you and the others are devoting so much energy to this. I would think that most people are concerned with what holds the lens on when they talk about the robustness of a DSLR. There is not a lot of evidence of other structural failures. What holds the lens on is the lens mount which is attached only to the mirror box. Even if the contention that the magnesium skin makes everything else much stronger (I don't think it does) that is largely irrelevant to the security of the lens mount.

You might say I'm obsessive about the lens mount, but in my experience that is what most photographers are concerned about.

Wyville · Mar 7, 2013

bobn2 said:
bobn2 wrote:

Yes, but design is also design for production. A unit has to be designed to have sufficient strength to be assembled, and also to be maintained. Making the skin structural is a bad idea, because it means that the unit is very fragile without it. That means that production necessitates all kinds of jigs to hold the bits together until the skin is put on. Likewise maintenance, because as soon as the skin is taken off, the unit cannot be easily worked on.

No, I wasn't talking about it falling appart under its own weight or the independent components being very fragile. It's not a delicate balance. The individual parts can(must) be solid and strong to really increase the overall strength to a level that allows abuse like the 7D is capable of handling.

I strongly doubt you would replace the aluminum shell of a 7D with cheap, thin plastic. You must know as well as I do that, that would seriously compromise the overall strength of the camera.

bobn2 · Mar 7, 2013

Member said:
howardroark wrote:

Member said:

I don't. You do. I never ever denied that the skin is screwed to the mirror box. I missed a screw, that is all. Since I never denied it, I can't concede it. The key point is that it is screwed to the mirror box, not the lens mount. That means that all the loads from the lens must go through the mirror box. I'm beginning to think maybe you just don't understand this point.

Click to expand...

What I don't get is that you don't think those three screws attached to the mirror box could allow a rigid magnesium structure to preven the mirror box from twisting or transferring that load to the handle....

because that job is being done mainly by the big steel bracket in the base. Those screws are clearly not there for structural reasons, they are tiny and there are only three of them. If the intention was really to transfer load to the skin, the lens mount would have been designed with an outer flange, and the skin secured (screwed) to that. It would have not made assembly particularly more difficult, and would have transferred loads directly from the metal lens mount to metal skin. So, either Canon's designers are poor, or they never intended for the skin to be structural. I think the latter.

Member said:
as well as the point at which it attaches to the central steel plate.

putting the steel plate there actually increases loads on the mirror box, since it secures one end (if you think that plate is separately secured to the skin, I don't think it is, the circuit board is in the way)

Member said:
There doens't have to be only one load path. Those attachment points from the mirror box to a piece of metal have absolutely no choice but to take load.

The only 'take load' if there is a stress at that point to take, and if the thing they are attached to provides a reaction force. As it is, I think the thin mag skin is pretty flexible and will just twist if the mirror box twists, rather than transfer load down a ling, flat piece of thin metal. It's just the wrong shape to be load bearing, so once again, either Canon engineers didn't know what they are doing or else that was not their intention.

TTMartin · Mar 7, 2013

bobn2 said:
bobn2 wrote:

bobn2 said:

TTMartin wrote:

bobn2 said:

bobn2 wrote:

bobn2 said:

TTMartin wrote:

The mirror box, is held in place at the back by the steel cross member, and in the front by the magnesium body and the steel bracket extending to the tripod mount and then tied into the magnesium exoskeleton body.
It is not connected to the magnesium shell at all. There are no mounting points. There is simply a hole at the front of the shell through which the lens mount, which is screwed to the mirror box, protrudes. That is the simple truth. You can pretend otherwise as much as you like, but it doesn't change the fact. It is inaccurate to describe the magnesium skin as an 'exoskeleton'. The 'skeleton' is on the inside.
WRONG!!! More BabelN2 Fantasy!!! No matter how smart it sounds, when it is made up, it is just fantasy!!!

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It is true. The lens mount is not attached at all to the skin. It is attached to the mirror box.

Where did I say anything about the lens mount?

I said 'The mirror box, is held in place at the back by the steel cross member, and in the front by the magnesium body and the steel bracket extending to the tripod mount and then tied into the magnesium exoskeleton body.'

You fixate on the lens mount, because the crux of your argument is that because the lens mount attaches to the mirror box, therefor the mirror box is a key structural element.

Unfortunately, you some how seem to extrapolate that to the mirror box is the only structural element, which is where your argument falls apart. The mirror box is supported front and rear by metal elements (steel in the rear, magnesium in the front).

Again I'll use the analogy of 1940's mechanic or automotive engineer looking at a modern day uni-body constructed car, and trying to project a conventional frame into the car, when there is none. Yes, there are sub-frames, yes they bear loads, but, they also transfer those loads into the entire body.

It's like comparing a Ford Ranger pick-up truck and a Honda Rigdeline pick-up truck. Both have similar load and towing capacities, but, the Ford Ranger using a traditional frame and the Honda Ridgeline uses uni-body construction. You can't project a convention frame into the Honda Ridgeline, it's just not there.

You are doing the same thing with the Canon 7D. Nikon uses an internal chassis that carries the load. Canon distributes the load differently. You can't project the Nikon design into the Canon 7D, any more than you can project the Ford Ranger design, into the Honda Ridgeline.

bobn2 · Mar 7, 2013

Member said:
Wyville wrote:

Member said:

bobn2 wrote:

Yes, but design is also design for production. A unit has to be designed to have sufficient strength to be assembled, and also to be maintained. Making the skin structural is a bad idea, because it means that the unit is very fragile without it. That means that production necessitates all kinds of jigs to hold the bits together until the skin is put on. Likewise maintenance, because as soon as the skin is taken off, the unit cannot be easily worked on.

Click to expand...

No, I wasn't talking about it falling appart under its own weight or the independent components being very fragile. It's not a delicate balance. The individual parts can(must) be solid and strong to really increase the overall strength to a level that allows abuse like the 7D is capable of handling.

I strongly doubt you would replace the aluminum shell of a 7D with cheap, thin plastic. You must know as well as I do that, that would seriously compromise the overall strength of the camera.

I think that it would be quite possible to produce a plastic skin just as strong as the magnesium one. It would be thicker but probably lighter. The main advantage of the magnesium skin is marketing value, which should never be underestimated. By using magnesium rather than plastic, Canon can charge much more per camera, and people are more satisfied by the product, which is all a win for Canon. The first professional EOS, the EOS-1 had a plastic shell (aluminium chassis). The preceding Canon F-1 was metal shelled (brass not magnesium). At that time Canon clearly thought plastic shell was in engineering terms a good solution. They went magnesium with the EOS-1V, on which was based the original 1D. It was a marketing masterstroke.

howardroark · Mar 7, 2013

bobn2 said:
bobn2 wrote:

bobn2 said:

howardroark wrote:

bobn2 said:

I don't. You do. I never ever denied that the skin is screwed to the mirror box. I missed a screw, that is all. Since I never denied it, I can't concede it. The key point is that it is screwed to the mirror box, not the lens mount. That means that all the loads from the lens must go through the mirror box. I'm beginning to think maybe you just don't understand this point.

Click to expand...

What I don't get is that you don't think those three screws attached to the mirror box could allow a rigid magnesium structure to preven the mirror box from twisting or transferring that load to the handle....

Click to expand...

because that job is being done mainly by the big steel bracket in the base. Those screws are clearly not there for structural reasons, they are tiny and there are only three of them. If the intention was really to transfer load to the skin, the lens mount would have been designed with an outer flange, and the skin secured (screwed) to that. It would have not made assembly particularly more difficult, and would have transferred loads directly from the metal lens mount to metal skin. So, either Canon's designers are poor, or they never intended for the skin to be structural. I think the latter.

bobn2 said:

as well as the point at which it attaches to the central steel plate.

Click to expand...

putting the steel plate there actually increases loads on the mirror box, since it secures one end (if you think that plate is separately secured to the skin, I don't think it is, the circuit board is in the way)

bobn2 said:

There doens't have to be only one load path. Those attachment points from the mirror box to a piece of metal have absolutely no choice but to take load.

Click to expand...

The only 'take load' if there is a stress at that point to take, and if the thing they are attached to provides a reaction force. As it is, I think the thin mag skin is pretty flexible and will just twist if the mirror box twists, rather than transfer load down a ling, flat piece of thin metal. It's just the wrong shape to be load bearing, so once again, either Canon engineers didn't know what they are doing or else that was not their intention.

An aluminum can bear a huge load....from the top. The metal is extremely thin, much thinner than this mag camera body. The direction of the force and the shape of the housing determine in what direction the strength lies. The can would be easily crushed from the sides, or from the top if the geometry was changed (dented, allowing the can to buckle) and then loaded from the top.

I assure you a thin piece of magnesium when attached to another piece in order to form a shell has exceptional rigidity, resistance to crushing forces and flexing, and certainly resistance to torsional stresses. I really don't understand what you think the right shape for load bearing should be. And different loads require completely different shapes....in this case it is basically a big box.

bobn2 · Mar 7, 2013

TTMartin said:
TTMartin wrote:

TTMartin said:

bobn2 wrote:

TTMartin said:

TTMartin wrote:

TTMartin said:

bobn2 wrote:

TTMartin said:

TTMartin wrote:

The mirror box, is held in place at the back by the steel cross member, and in the front by the magnesium body and the steel bracket extending to the tripod mount and then tied into the magnesium exoskeleton body.
It is not connected to the magnesium shell at all. There are no mounting points. There is simply a hole at the front of the shell through which the lens mount, which is screwed to the mirror box, protrudes. That is the simple truth. You can pretend otherwise as much as you like, but it doesn't change the fact. It is inaccurate to describe the magnesium skin as an 'exoskeleton'. The 'skeleton' is on the inside.
WRONG!!! More BabelN2 Fantasy!!! No matter how smart it sounds, when it is made up, it is just fantasy!!!

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It is true. The lens mount is not attached at all to the skin. It is attached to the mirror box.

Click to expand...

Where did I say anything about the lens mount?

Easy lessons in having a coherent discussion:

1. Follow the conversation.

2. Note which points were raised, by whom.

3. Understand that there will be a logical connection between successive contributions to a discussion. So..

Bob: There is simply a hole at the front of the shell through which the lens mount, which is screwed to the mirror box, protrudes.

TT: WRONG!!! More BabelN2 Fantasy!!! No matter how smart it sounds, when it is made up, it is just fantasy!!!

Bob: It is true. The lens mount is not attached at all to the skin.

See how the flow works. Put a bit of effort into this, one day you might succeed in having a coherent conversation.

TTMartin said:
I said 'The mirror box, is held in place at the back by the steel cross member, and in the front by the magnesium body and the steel bracket extending to the tripod mount and then tied into the magnesium exoskeleton body.'

The mirror box is not 'held in place ay the back by the steel cross member. That steel plate is located by being screwed into the mirror box.

TTMartin said:
You fixate on the lens mount, because the crux of your argument is that because the lens mount attaches to the mirror box, therefor the mirror box is a key structural element.

No, my argument is that what is important in a camera is the location of the sensor relative to the lens. They go skew-whiff, your pictures go all blurry. Everything else, it doesn't really matter if it bends a bit.

TTMartin said:
Unfortunately, you some how seem to extrapolate that to the mirror box is the only structural element, which is where your argument falls apart. The mirror box is supported front and rear by metal elements (steel in the rear, magnesium in the front).

I have never said that the mirror box is the only structural element. I just said that it is the most important one, because it does the most important job in a camera, holding the lens in relation to the sensor.

TTMartin said:
Again I'll use the analogy of 1940's mechanic or automotive engineer looking at a modern day uni-body constructed car, and trying to project a conventional frame into the car, when there is none. Yes, there are sub-frames, yes they bear loads, but, they also transfer those loads into the entire body.

It's like comparing a Ford Ranger pick-up truck and a Honda Rigdeline pick-up truck. Both have similar load and towing capacities, but, the Ford Ranger using a traditional frame and the Honda Ridgeline uses uni-body construction. You can't project a convention frame into the Honda Ridgeline, it's just not there.

You are doing the same thing with the Canon 7D. Nikon uses an internal chassis that carries the load. Canon distributes the load differently. You can't project the Nikon design into the Canon 7D, any more than you can project the Ford Ranger design, into the Honda Ridgeline.

You are wrong about that. The skin of these cameras is not structural. If it were, it would be designed differently. It would also attach to the lens mount, as it does in cameras like the E-M5 (in which it is structural). But the only fix point in your argument is that you are right and the skin is structural. You have no evidence, but that of course is of no importance to you. Because you know you are right. But that is only in your own world.

howardroark · Mar 7, 2013

bobn2 said:
bobn2 wrote:

bobn2 said:

Wyville wrote:

bobn2 said:

bobn2 wrote:

Yes, but design is also design for production. A unit has to be designed to have sufficient strength to be assembled, and also to be maintained. Making the skin structural is a bad idea, because it means that the unit is very fragile without it. That means that production necessitates all kinds of jigs to hold the bits together until the skin is put on. Likewise maintenance, because as soon as the skin is taken off, the unit cannot be easily worked on.

Click to expand...

No, I wasn't talking about it falling appart under its own weight or the independent components being very fragile. It's not a delicate balance. The individual parts can(must) be solid and strong to really increase the overall strength to a level that allows abuse like the 7D is capable of handling.

I strongly doubt you would replace the aluminum shell of a 7D with cheap, thin plastic. You must know as well as I do that, that would seriously compromise the overall strength of the camera.

Click to expand...

I think that it would be quite possible to produce a plastic skin just as strong as the magnesium one. It would be thicker but probably lighter. The main advantage of the magnesium skin is marketing value, which should never be underestimated.

Sorry, that's just wrong. Magnesium is truly exception in its strength-to-weight ratio. Nobody uses magnesium if they don't have to. The investment in tooling is huge. It eats into their profit margin. It is not done as a gimmick. They do make much cheaper plastic housings and they're called Rebels. They're smaller making the moment arm of applied forces smaller increasing stiffness. It is still a perfectly good material to use, but they can be crushed more easily, fractured more easily, and are not designed to last as long (shutter mechinism rating is proof enough of that, but plastic degrades over time losing its mechanical properties).

bobn2 said:
By using magnesium rather than plastic, Canon can charge much more per camera, and people are more satisfied by the product, which is all a win for Canon.

The camera can also withstand much greater impact without fracturing. It lends itself to supporting larger camera bodies and larger lenses for reasons stated above.

bobn2 said:
The first professional EOS, the EOS-1 had a plastic shell (aluminium chassis). The preceding Canon F-1 was metal shelled (brass not magnesium). At that time Canon clearly thought plastic shell was in engineering terms a good solution. They went magnesium with the EOS-1V, on which was based the original 1D. It was a marketing masterstroke.

bobn2 · Mar 7, 2013

Member said:
howardroark wrote:

Member said:

bobn2 wrote:

Member said:

howardroark wrote:

Member said:

I don't. You do. I never ever denied that the skin is screwed to the mirror box. I missed a screw, that is all. Since I never denied it, I can't concede it. The key point is that it is screwed to the mirror box, not the lens mount. That means that all the loads from the lens must go through the mirror box. I'm beginning to think maybe you just don't understand this point.

Click to expand...

What I don't get is that you don't think those three screws attached to the mirror box could allow a rigid magnesium structure to preven the mirror box from twisting or transferring that load to the handle....

Click to expand...

because that job is being done mainly by the big steel bracket in the base. Those screws are clearly not there for structural reasons, they are tiny and there are only three of them. If the intention was really to transfer load to the skin, the lens mount would have been designed with an outer flange, and the skin secured (screwed) to that. It would have not made assembly particularly more difficult, and would have transferred loads directly from the metal lens mount to metal skin. So, either Canon's designers are poor, or they never intended for the skin to be structural. I think the latter.

Member said:

as well as the point at which it attaches to the central steel plate.

Click to expand...

putting the steel plate there actually increases loads on the mirror box, since it secures one end (if you think that plate is separately secured to the skin, I don't think it is, the circuit board is in the way)

Member said:

There doens't have to be only one load path. Those attachment points from the mirror box to a piece of metal have absolutely no choice but to take load.

Click to expand...

The only 'take load' if there is a stress at that point to take, and if the thing they are attached to provides a reaction force. As it is, I think the thin mag skin is pretty flexible and will just twist if the mirror box twists, rather than transfer load down a ling, flat piece of thin metal. It's just the wrong shape to be load bearing, so once again, either Canon engineers didn't know what they are doing or else that was not their intention.

Click to expand...

An aluminum can bear a huge load....from the top.

An aluminium can is under internal pressure. An unpressurised can cannot take a huge load. The inside of a 7D is not pressurised.

howardroark · Mar 7, 2013

bobn2 said:
bobn2 wrote:

bobn2 said:

howardroark wrote:

bobn2 said:

bobn2 wrote:

bobn2 said:

howardroark wrote:

bobn2 said:

I don't. You do. I never ever denied that the skin is screwed to the mirror box. I missed a screw, that is all. Since I never denied it, I can't concede it. The key point is that it is screwed to the mirror box, not the lens mount. That means that all the loads from the lens must go through the mirror box. I'm beginning to think maybe you just don't understand this point.

Click to expand...

What I don't get is that you don't think those three screws attached to the mirror box could allow a rigid magnesium structure to preven the mirror box from twisting or transferring that load to the handle....

Click to expand...

because that job is being done mainly by the big steel bracket in the base. Those screws are clearly not there for structural reasons, they are tiny and there are only three of them. If the intention was really to transfer load to the skin, the lens mount would have been designed with an outer flange, and the skin secured (screwed) to that. It would have not made assembly particularly more difficult, and would have transferred loads directly from the metal lens mount to metal skin. So, either Canon's designers are poor, or they never intended for the skin to be structural. I think the latter.

bobn2 said:

as well as the point at which it attaches to the central steel plate.

Click to expand...

putting the steel plate there actually increases loads on the mirror box, since it secures one end (if you think that plate is separately secured to the skin, I don't think it is, the circuit board is in the way)

bobn2 said:

There doens't have to be only one load path. Those attachment points from the mirror box to a piece of metal have absolutely no choice but to take load.

Click to expand...

The only 'take load' if there is a stress at that point to take, and if the thing they are attached to provides a reaction force. As it is, I think the thin mag skin is pretty flexible and will just twist if the mirror box twists, rather than transfer load down a ling, flat piece of thin metal. It's just the wrong shape to be load bearing, so once again, either Canon engineers didn't know what they are doing or else that was not their intention.

Click to expand...

An aluminum can bear a huge load....from the top.

Click to expand...

An aluminium can is under internal pressure. An unpressurised can cannot take a huge load. The inside of a 7D is not pressurised.

Notice how I was only talking about a can. Crush an empty aluminum can on your head (in line with the long axis)....then cut through it to see that it was basically just aluminum foil, then tell me how shocked you are that the cylindrcal shape combined with the mechanical properties of aluminum aren't a shockingly effective combination. Compared to its weight, an empty aluminum can loaded in line with its long axis can take an enormous load. It's like the shell of an egg: the geometry and material create a surpisingly strong and light structure. The can analogy is the same concept as the honeycomb aluminum structure of aircraft skin. It is made up of extremely thin foil that flexes like an according transverse to the hollow pockets, but in line with that geometry you could stand on it and it would never buckle. You're arguing just to argue....or you just can't connect one dot to the next and then realize that connecting all the dots together is the only way to make a picture.

bobn2 said:
--
Bob

howardroark · Mar 7, 2013

bobn2 said:
bobn2 wrote:

bobn2 said:

howardroark wrote:
An exoskeleton and a skeleton, then. The housing for many devices is the only load bearing part of the assembly.

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That is true. For instance, the typical mirrorless camera assembly is based around a flat metal tube that forms the skin and chassis. That has been the preferred design since the first Leica. The inner components are loaded from the top and bottom and then screwed in place, finally top and bottom plates added. Assembly starts with the skin. An SLR is different. The chassis is the mirror box and the camera is built around that, and it is what holds it together. The skin is put on last. That has been the case since the Contax S.

bobn2 said:

In the case of such a complex device as a camera, it requires internal and external rigid, load bearing structures. It has internal parts that must be held in place, so unlike animals it has the option for both internal and external support structures.

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The primary structure is what assembly starts with (or disassembly ends with) in the case of an SLR, the mirror box.

Brutal reductionist oversimplification. All done.

trekkeruss · Mar 7, 2013

bobn2 said:
bobn2 wrote:

bobn2 said:

howardroark wrote:

bobn2 said:

bobn2 wrote:

bobn2 said:

Wyville wrote:

bobn2 said:

howardroark wrote:

The housing, plate, and mirror box all serve important structural functions, but boiling it down to the mirror box being the most important one is ignoring the various load paths a camera body is subjected to.

Click to expand...

I agree. My sister is a structural engineer and from her I have learned a lot about how structures gain their strength. It's never just one part, but rather the collective of all parts. That's why I asked my question below. Engineering these days is about making structures light, if all the strength has to come from one part it will have to be disproportionately heavy.

Click to expand...

Yes, but design is also design for production. A unit has to be designed to have sufficient strength to be assembled, and also to be maintained. Making the skin structural is a bad idea, because it means that the unit is very fragile without it. That means that production necessitates all kinds of jigs to hold the bits together until the skin is put on. Likewise maintenance, because as soon as the skin is taken off, the unit cannot be easily worked on.

--

Bob

Click to expand...

More than one compnenent lending structural integrity, man. Something being in once piece without the skin during production doesn't proclude stress in-service from being different and benefiting from that exoskeleton/housing. That steel plate in the middle can have every single thing attached to it, things attached to the mirror box, all holding together in one piece without the housing, but a sub-assembly sitting on a bench is much different from an item being used in the field.

Click to expand...

I really am not sure why you and the others are devoting so much energy to this. I would think that most people are concerned with what holds the lens on when they talk about the robustness of a DSLR. There is not a lot of evidence of other structural failures. What holds the lens on is the lens mount which is attached only to the mirror box. Even if the contention that the magnesium skin makes everything else much stronger (I don't think it does) that is largely irrelevant to the security of the lens mount.

You might say I'm obsessive about the lens mount, but in my experience that is what most photographers are concerned about.

bobn2 · Mar 7, 2013

Member said:
howardroark wrote:

Member said:

bobn2 wrote:

Member said:

Wyville wrote:

Member said:

bobn2 wrote:

Yes, but design is also design for production. A unit has to be designed to have sufficient strength to be assembled, and also to be maintained. Making the skin structural is a bad idea, because it means that the unit is very fragile without it. That means that production necessitates all kinds of jigs to hold the bits together until the skin is put on. Likewise maintenance, because as soon as the skin is taken off, the unit cannot be easily worked on.

Click to expand...

No, I wasn't talking about it falling appart under its own weight or the independent components being very fragile. It's not a delicate balance. The individual parts can(must) be solid and strong to really increase the overall strength to a level that allows abuse like the 7D is capable of handling.

I strongly doubt you would replace the aluminum shell of a 7D with cheap, thin plastic. You must know as well as I do that, that would seriously compromise the overall strength of the camera.

Click to expand...

I think that it would be quite possible to produce a plastic skin just as strong as the magnesium one. It would be thicker but probably lighter. The main advantage of the magnesium skin is marketing value, which should never be underestimated.

Click to expand...

Sorry, that's just wrong. Magnesium is truly exception in its strength-to-weight ratio.

Not according to this article : The strength –to-weight ratio of the precipitation-hardened magnesium alloys is comparable with that of the strong alloys of aluminum or with the alloy steels.

Member said:
Nobody uses magnesium if they don't have to.

So, those nice magnesium alloy wheels I see on souped up hatchbacks are there because the acre really needs it, and not just for show?

Member said:
The investment in tooling is huge.

No more than injection moulding for polymers. In fact with the advent of thixomoulding (which is what allowed the mag skin in the first place), it is the same tooling, pretty much.

Member said:
It eats into their profit margin.

Not if it allows you to charge more for the camera. Canon can charge $1000 more for a 7D than for a Rebel.

Member said:
It is not done as a gimmick.

Oh no?

Member said:
They do make much cheaper plastic housings and they're called Rebels.

And they sell for much less - less product margin.

Member said:
They're smaller making the moment arm of applied forces smaller increasing stiffness. It is still a perfectly good material to use, but they can be crushed more easily, fractured more easily, and are not designed to last as long (shutter mechinism rating is proof enough of that, but plastic degrades over time losing its mechanical properties).

What do you think is the cost difference between a 100,000 operation shutter and a 200,000 operation shutter, and why?

Member said:
Member said:

By using magnesium rather than plastic, Canon can charge much more per camera, and people are more satisfied by the product, which is all a win for Canon.

Click to expand...

The camera can also withstand much greater impact without fracturing. It lends itself to supporting larger camera bodies and larger lenses for reasons stated above.

That is what you assert, but so far we have seen no evidence that it is true. To truly make a claim that it can 'withstand much greater impact' you would need some real test data, or maybe service record data. I haven't seen any yet. So what you are actually saying is 'I presume that it will withstand much greater impact'. In fact that is what this discussion is all about, so what you are saying is 'it will withstand much greater impact because it will withstand much greater impact'

bobn2 · Mar 7, 2013

Member said:
howardroark wrote:

Member said:

bobn2 wrote:

Member said:

howardroark wrote:

Member said:

bobn2 wrote:

Member said:

howardroark wrote:

Member said:

I don't. You do. I never ever denied that the skin is screwed to the mirror box. I missed a screw, that is all. Since I never denied it, I can't concede it. The key point is that it is screwed to the mirror box, not the lens mount. That means that all the loads from the lens must go through the mirror box. I'm beginning to think maybe you just don't understand this point.

Click to expand...

What I don't get is that you don't think those three screws attached to the mirror box could allow a rigid magnesium structure to preven the mirror box from twisting or transferring that load to the handle....

Click to expand...

because that job is being done mainly by the big steel bracket in the base. Those screws are clearly not there for structural reasons, they are tiny and there are only three of them. If the intention was really to transfer load to the skin, the lens mount would have been designed with an outer flange, and the skin secured (screwed) to that. It would have not made assembly particularly more difficult, and would have transferred loads directly from the metal lens mount to metal skin. So, either Canon's designers are poor, or they never intended for the skin to be structural. I think the latter.

Member said:

as well as the point at which it attaches to the central steel plate.

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putting the steel plate there actually increases loads on the mirror box, since it secures one end (if you think that plate is separately secured to the skin, I don't think it is, the circuit board is in the way)

Member said:

There doens't have to be only one load path. Those attachment points from the mirror box to a piece of metal have absolutely no choice but to take load.

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The only 'take load' if there is a stress at that point to take, and if the thing they are attached to provides a reaction force. As it is, I think the thin mag skin is pretty flexible and will just twist if the mirror box twists, rather than transfer load down a ling, flat piece of thin metal. It's just the wrong shape to be load bearing, so once again, either Canon engineers didn't know what they are doing or else that was not their intention.

Click to expand...

An aluminum can bear a huge load....from the top.

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An aluminium can is under internal pressure. An unpressurised can cannot take a huge load. The inside of a 7D is not pressurised.

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Notice how I was only talking about a can.

I noticed that yoiu wer talking about a can.

Member said:
Crush an empty aluminum can on your head (in line with the long axis)....

Which you could do without too much pain.

Member said:
then cut through it

What, your head? You sure?

Member said:
to see that it was basically just aluminum foil,

It's much thicker than foil. I'll get the micrometer out and measure, but I wouldn't be surprised if it wasn't 10 time the thickness.

Member said:
then tell me how shocked you are that the cylindrcal shape combined with the mechanical properties of aluminum aren't a shockingly effective combination.

That sounds a bit tautological. I am not at all shocked since I understand the loads it takes, so since I am not shocked it is not a shockingly effective combination.

Member said:
Compared to its weight, an empty aluminum can loaded in line with its long axis can take an enormous load.

But here is the point - it has to be loaded exactly along its axis, else it buckles. The skin of a camera is not a complete tube, is not an even structure and is not stressed exactly along its axis. There are many crease points and it would buckle if stressed.

Member said:
You're arguing just to argue....or you just can't connect one dot to the next and then realize that connecting all the dots together is the only way to make a picture.

Once again, you describe what you do. You are arguing just to argue. All I do is refute the misinformed of just plain unformed points that you are making. Each time I do, you come up with a few more, which is pretty good evidence that you are in it just for the argument.

bobn2 · Mar 7, 2013

Member said:
howardroark wrote:

Member said:

bobn2 wrote:

Member said:

howardroark wrote:
An exoskeleton and a skeleton, then. The housing for many devices is the only load bearing part of the assembly.

Click to expand...

That is true. For instance, the typical mirrorless camera assembly is based around a flat metal tube that forms the skin and chassis. That has been the preferred design since the first Leica. The inner components are loaded from the top and bottom and then screwed in place, finally top and bottom plates added. Assembly starts with the skin. An SLR is different. The chassis is the mirror box and the camera is built around that, and it is what holds it together. The skin is put on last. That has been the case since the Contax S.

Member said:

In the case of such a complex device as a camera, it requires internal and external rigid, load bearing structures. It has internal parts that must be held in place, so unlike animals it has the option for both internal and external support structures.

Click to expand...

The primary structure is what assembly starts with (or disassembly ends with) in the case of an SLR, the mirror box.

Click to expand...

Brutal reductionist oversimplification. All done.

The truth, though. Actually, custom and practice is very powerful in engineering design. Given a new job, likely thing is most engineers will do it how they are used to, unless given an instruction to the contrary.

bobn2 · Mar 7, 2013

trekkeruss said:
trekkeruss wrote:

trekkeruss said:

bobn2 wrote:

trekkeruss said:

howardroark wrote:

trekkeruss said:

bobn2 wrote:

trekkeruss said:

Wyville wrote:

trekkeruss said:

howardroark wrote:

The housing, plate, and mirror box all serve important structural functions, but boiling it down to the mirror box being the most important one is ignoring the various load paths a camera body is subjected to.

Click to expand...

I agree. My sister is a structural engineer and from her I have learned a lot about how structures gain their strength. It's never just one part, but rather the collective of all parts. That's why I asked my question below. Engineering these days is about making structures light, if all the strength has to come from one part it will have to be disproportionately heavy.

Click to expand...

Yes, but design is also design for production. A unit has to be designed to have sufficient strength to be assembled, and also to be maintained. Making the skin structural is a bad idea, because it means that the unit is very fragile without it. That means that production necessitates all kinds of jigs to hold the bits together until the skin is put on. Likewise maintenance, because as soon as the skin is taken off, the unit cannot be easily worked on.

--

Bob

Click to expand...

More than one compnenent lending structural integrity, man. Something being in once piece without the skin during production doesn't proclude stress in-service from being different and benefiting from that exoskeleton/housing. That steel plate in the middle can have every single thing attached to it, things attached to the mirror box, all holding together in one piece without the housing, but a sub-assembly sitting on a bench is much different from an item being used in the field.

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I really am not sure why you and the others are devoting so much energy to this. I would think that most people are concerned with what holds the lens on when they talk about the robustness of a DSLR. There is not a lot of evidence of other structural failures. What holds the lens on is the lens mount which is attached only to the mirror box. Even if the contention that the magnesium skin makes everything else much stronger (I don't think it does) that is largely irrelevant to the security of the lens mount.

You might say I'm obsessive about the lens mount, but in my experience that is what most photographers are concerned about.

Click to expand...

TTMartin · Mar 7, 2013

bobn2 said:
bobn2 wrote:

So, those nice magnesium alloy wheels I see on souped up hatchbacks are there because the acre really needs it, and not just for show?

Another case of looking at something and not understanding what you are seeing!!!

Those wheels are aluminum, NOT, magnesium.

From Wikipedia: True magnesium wheels [for cars] are no longer produced, being found only on classic cars.

Josh152 · Mar 7, 2013

bobn2 said:
bobn2 wrote:

bobn2 said:

howardroark wrote:

bobn2 said:

bobn2 wrote:

bobn2 said:

Wyville wrote:

bobn2 said:

bobn2 wrote:

Yes, but design is also design for production. A unit has to be designed to have sufficient strength to be assembled, and also to be maintained. Making the skin structural is a bad idea, because it means that the unit is very fragile without it. That means that production necessitates all kinds of jigs to hold the bits together until the skin is put on. Likewise maintenance, because as soon as the skin is taken off, the unit cannot be easily worked on.

Click to expand...

No, I wasn't talking about it falling appart under its own weight or the independent components being very fragile. It's not a delicate balance. The individual parts can(must) be solid and strong to really increase the overall strength to a level that allows abuse like the 7D is capable of handling.

I strongly doubt you would replace the aluminum shell of a 7D with cheap, thin plastic. You must know as well as I do that, that would seriously compromise the overall strength of the camera.

Click to expand...

I think that it would be quite possible to produce a plastic skin just as strong as the magnesium one. It would be thicker but probably lighter. The main advantage of the magnesium skin is marketing value, which should never be underestimated.

Click to expand...

Sorry, that's just wrong. Magnesium is truly exception in its strength-to-weight ratio.

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Not according to this article : The strength –to-weight ratio of the precipitation-hardened magnesium alloys is comparable with that of the strong alloys of aluminum or with the alloy steels.

bobn2 said:

Nobody uses magnesium if they don't have to.

Click to expand...

So, those nice magnesium alloy wheels I see on souped up hatchbacks are there because the acre really needs it, and not just for show?

bobn2 said:

The investment in tooling is huge.

Click to expand...

No more than injection moulding for polymers. In fact with the advent of thixomoulding (which is what allowed the mag skin in the first place), it is the same tooling, pretty much.

bobn2 said:

It eats into their profit margin.

Click to expand...

Not if it allows you to charge more for the camera. Canon can charge $1000 more for a 7D than for a Rebel.

bobn2 said:

It is not done as a gimmick.

Click to expand...

Oh no?

bobn2 said:

They do make much cheaper plastic housings and they're called Rebels.

Click to expand...

And they sell for much less - less product margin.

bobn2 said:

They're smaller making the moment arm of applied forces smaller increasing stiffness. It is still a perfectly good material to use, but they can be crushed more easily, fractured more easily, and are not designed to last as long (shutter mechinism rating is proof enough of that, but plastic degrades over time losing its mechanical properties).

Click to expand...

What do you think is the cost difference between a 100,000 operation shutter and a 200,000 operation shutter, and why?

bobn2 said:

bobn2 said:

By using magnesium rather than plastic, Canon can charge much more per camera, and people are more satisfied by the product, which is all a win for Canon.

Click to expand...

The camera can also withstand much greater impact without fracturing. It lends itself to supporting larger camera bodies and larger lenses for reasons stated above.

Click to expand...

That is what you assert, but so far we have seen no evidence that it is true. To truly make a claim that it can 'withstand much greater impact' you would need some real test data, or maybe service record data. I haven't seen any yet. So what you are actually saying is 'I presume that it will withstand much greater impact'. In fact that is what this discussion is all about, so what you are saying is 'it will withstand much greater impact because it will withstand much greater impact'

bobn2 · Mar 7, 2013

TTMartin said:
TTMartin wrote:

TTMartin said:

bobn2 wrote:

So, those nice magnesium alloy wheels I see on souped up hatchbacks are there because the acre really needs it, and not just for show?

Click to expand...

Another case of looking at something and not understanding what you are seeing!!!

Those wheels are aluminum, NOT, magnesium.

Well, I guess these people have wasted their time writing a whole article on the pros and cons of magnesium and aluminium wheels, then.

TTMartin said:
From Wikipedia: True magnesium wheels [for cars] are no longer produced, being found only on classic cars.

I'd guess then that these people are selling plastic replicas, then.

You keep popping them up, I'll keep hitting them.

bobn2 · Mar 7, 2013

Josh152 said:
Josh152 wrote:

Josh152 said:

bobn2 wrote:

Josh152 said:

howardroark wrote:

Josh152 said:

bobn2 wrote:

Josh152 said:

Wyville wrote:

Josh152 said:

bobn2 wrote:

Yes, but design is also design for production. A unit has to be designed to have sufficient strength to be assembled, and also to be maintained. Making the skin structural is a bad idea, because it means that the unit is very fragile without it. That means that production necessitates all kinds of jigs to hold the bits together until the skin is put on. Likewise maintenance, because as soon as the skin is taken off, the unit cannot be easily worked on.

Click to expand...

No, I wasn't talking about it falling appart under its own weight or the independent components being very fragile. It's not a delicate balance. The individual parts can(must) be solid and strong to really increase the overall strength to a level that allows abuse like the 7D is capable of handling.

I strongly doubt you would replace the aluminum shell of a 7D with cheap, thin plastic. You must know as well as I do that, that would seriously compromise the overall strength of the camera.

Click to expand...

I think that it would be quite possible to produce a plastic skin just as strong as the magnesium one. It would be thicker but probably lighter. The main advantage of the magnesium skin is marketing value, which should never be underestimated.

Click to expand...

Sorry, that's just wrong. Magnesium is truly exception in its strength-to-weight ratio.

Click to expand...

Not according to this article : The strength –to-weight ratio of the precipitation-hardened magnesium alloys is comparable with that of the strong alloys of aluminum or with the alloy steels.

Josh152 said:

Nobody uses magnesium if they don't have to.

Click to expand...

So, those nice magnesium alloy wheels I see on souped up hatchbacks are there because the acre really needs it, and not just for show?

Josh152 said:

The investment in tooling is huge.

Click to expand...

No more than injection moulding for polymers. In fact with the advent of thixomoulding (which is what allowed the mag skin in the first place), it is the same tooling, pretty much.

Josh152 said:

It eats into their profit margin.

Click to expand...

Not if it allows you to charge more for the camera. Canon can charge $1000 more for a 7D than for a Rebel.

Josh152 said:

It is not done as a gimmick.

Click to expand...

Oh no?

Josh152 said:

They do make much cheaper plastic housings and they're called Rebels.

Click to expand...

And they sell for much less - less product margin.

Josh152 said:

They're smaller making the moment arm of applied forces smaller increasing stiffness. It is still a perfectly good material to use, but they can be crushed more easily, fractured more easily, and are not designed to last as long (shutter mechinism rating is proof enough of that, but plastic degrades over time losing its mechanical properties).

Click to expand...

What do you think is the cost difference between a 100,000 operation shutter and a 200,000 operation shutter, and why?

Josh152 said:

Josh152 said:

By using magnesium rather than plastic, Canon can charge much more per camera, and people are more satisfied by the product, which is all a win for Canon.

Click to expand...

The camera can also withstand much greater impact without fracturing. It lends itself to supporting larger camera bodies and larger lenses for reasons stated above.

Click to expand...

That is what you assert, but so far we have seen no evidence that it is true. To truly make a claim that it can 'withstand much greater impact' you would need some real test data, or maybe service record data. I haven't seen any yet. So what you are actually saying is 'I presume that it will withstand much greater impact'. In fact that is what this discussion is all about, so what you are saying is 'it will withstand much greater impact because it will withstand much greater impact'

Click to expand...

The plastic in your camera

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