A few words on Equivalence and comparing systems

Started Apr 2, 2013 | Discussions
Macx
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Re: A few words on Equivalence and comparing systems
In reply to Anders W, Apr 3, 2013

Anders W wrote:

Macx wrote:

texinwien wrote:

olliess wrote:

texinwien wrote:

Just another Canon shooter wrote:

texinwien wrote:

This is your mistake. You think a smaller equivalent f-number equals better performance. That is simply not the case, unless shallow DOF is your measure of performance.

More light is not?

Nope.

"Didn't want that light anyway."

It's the sensor not the lens, and JACS is talking about measures of lens performance w/r/t equivalence.

But I'd wager you knew this and were just trying to be witty. Unfortunately, you just ended up sounding obtuse.

tex

Keep the sensor out of it. The sensor size is interesting because the FOV of a particular lens is dependant on it, but for the same FOV and the same physical aperture the systems are equivalent and they get the same amount of light. If we had lenses with the same FOV and the same physical aperture for micro four-thirds as we have for 135, there would be no innate shot noise advantage of the larger sensor.

In other words, it's the lenses, not the sensor, and if we had a 12-35/2 our exposure envelope or gamut would be equal to having a 24-70/4 for our 135. The loss of a stop of total light when going from a 24-70/4 on a 135 to a 12-35/2.8 on M43 is dependant on the latter being f/2.8 and not f/2.

Macx,

It seems that your point (FoV instead of FL, absolute size of the aperture rather than relative size) is a pedagogical one. Your idea, if I understand you right, is that it simplifies things to use FoV and absolute aperture size because you can state equivalence in terms of equalities.

Yes, and because these are optical qualities that define what we want in a lens. It's field of view and it's depth of field and/or diffraction properties. We've learned to make the mental arithmetic needed to translate focal length and focal ratio into these values, but why the need for such translation when we could use these "proper" values as the basis for discussing how a lens behaves?

I see that point but I am not sure I agree that this is the more pedagogical solution all things considered. For example, FL and max relative aperture (max f-stop) are known properties of a lens and independent of the sensor to which the lens is fitted. FoV and max absolute aperture are properties that have to be derived (calculated) from those already known and the first of them (FoV) varies depending on the size of the sensor the lens is mated with.

The known properties are the focal length and the maximum absolute (virtual) aperture. The maximum f-stop is derived from these, not the other way around. It is true that to calculate the FoV we need to know what sensor the lens is used for. Even so, at least we would know it, instead of just having an intuitive understanding about the field of view of a 19 mm lens on a 135, for example.

I don't propose that we keep focal length or the f-stops secret. But these values aren't what is interesting about how our lens behaves on our camera, and especially when discussing lens properties across systems it seems to me to be much more pertinent to concentrate on these values instead of having to work with "equivalent focal lengths" and other thought-experiments.

And from a performance standpoint, isn't the performance of a lens at a certain absolute aperture more interesting that what it is at a relative aperture? I think so, at least.

Both ways of thinking about the matter are of course factually correct. It's just a matter of conceptualization. But in the end, I am not sure that a shift of attention away from FL and relative aperture to FoV and absolute aperture makes things any better from a pedagogical point of view.

Maybe not. The lens manufacturers isn't making it easy for us, that's for sure.

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Bob Tullis
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Re: Perhaps the real question should be.....
In reply to jalywol, Apr 3, 2013

jalywol wrote:

Great Bustard wrote:

In my opinion, best to acknowledge the differences between the systems based on the both the facts and the types of photos one takes.  That is, while one system may clearly be superior to another for a particular purpose, if that particular purpose is not an aspect of your photography, then it has no bearing for you.

That was the entire point of my post......

The rounder we go, the faster we get.   [g]

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Anders W
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Re: A few words on Equivalence and comparing systems
In reply to Macx, Apr 3, 2013

Macx wrote:

Anders W wrote:

Macx wrote:

texinwien wrote:

olliess wrote:

texinwien wrote:

Just another Canon shooter wrote:

texinwien wrote:

This is your mistake. You think a smaller equivalent f-number equals better performance. That is simply not the case, unless shallow DOF is your measure of performance.

More light is not?

Nope.

"Didn't want that light anyway."

It's the sensor not the lens, and JACS is talking about measures of lens performance w/r/t equivalence.

But I'd wager you knew this and were just trying to be witty. Unfortunately, you just ended up sounding obtuse.

tex

Keep the sensor out of it. The sensor size is interesting because the FOV of a particular lens is dependant on it, but for the same FOV and the same physical aperture the systems are equivalent and they get the same amount of light. If we had lenses with the same FOV and the same physical aperture for micro four-thirds as we have for 135, there would be no innate shot noise advantage of the larger sensor.

In other words, it's the lenses, not the sensor, and if we had a 12-35/2 our exposure envelope or gamut would be equal to having a 24-70/4 for our 135. The loss of a stop of total light when going from a 24-70/4 on a 135 to a 12-35/2.8 on M43 is dependant on the latter being f/2.8 and not f/2.

Macx,

It seems that your point (FoV instead of FL, absolute size of the aperture rather than relative size) is a pedagogical one. Your idea, if I understand you right, is that it simplifies things to use FoV and absolute aperture size because you can state equivalence in terms of equalities.

Yes, and because these are optical qualities that define what we want in a lens. It's field of view and it's depth of field and/or diffraction properties. We've learned to make the mental arithmetic needed to translate focal length and focal ratio into these values, but why the need for such translation when we could use these "proper" values as the basis for discussing how a lens behaves?

Personally, I have no trouble going between FL and FoV. And I actually find it easier to define DoF and diffraction properties on the basis of f-stops than absolute aperture. Could you perhaps tell me how to convert the rule of thumb that MFT lenses shouldn't be stopped down further than f/8 if you want to avoid significant diffraction effects into your conceptual scheme?

I see that point but I am not sure I agree that this is the more pedagogical solution all things considered. For example, FL and max relative aperture (max f-stop) are known properties of a lens and independent of the sensor to which the lens is fitted. FoV and max absolute aperture are properties that have to be derived (calculated) from those already known and the first of them (FoV) varies depending on the size of the sensor the lens is mated with.

The known properties are the focal length and the maximum absolute (virtual) aperture. The maximum f-stop is derived from these, not the other way around.

When it comes to aperture, it works perfectly well either way. Start with the relative number and define the absolute number if you want it (I rarely do).

It is true that to calculate the FoV we need to know what sensor the lens is used for. Even so, at least we would know it, instead of just having an intuitive understanding about the field of view of a 19 mm lens on a 135, for example.

I know that anyway.

I don't propose that we keep focal length or the f-stops secret. But these values aren't what is interesting about how our lens behaves on our camera, and especially when discussing lens properties across systems it seems to me to be much more pertinent to concentrate on these values instead of having to work with "equivalent focal lengths" and other thought-experiments.

You say that f-stops aren't what we are interested in. So tell me, how would you convert the "sunny 16" rule into your conceptual machinery?

Another thing here that I am curious about. Equivalence isn't only about equality but also about how far apart things are. Say we are comparing a 12/2 on MFT with a 24/2.8 on FF. Clearly, these two lenses are not equivalent. The 12/2 is equivalent to a 24/4, not a 24/2.8. But how would you, using your conceptual machinery, express the one f-stop difference between the 12/2 and the 24/2.8? And of what practical help would that difference, as expressed in your language, be?

And from a performance standpoint, isn't the performance of a lens at a certain absolute aperture more interesting that what it is at a relative aperture? I think so, at least.

So if you are comparing a 25 mm and 50 mm lens on MFT, which comparison would you find more interesting. A comparison at the same f-stop or a comparison at the same absolute aperture?

Both ways of thinking about the matter are of course factually correct. It's just a matter of conceptualization. But in the end, I am not sure that a shift of attention away from FL and relative aperture to FoV and absolute aperture makes things any better from a pedagogical point of view.

Maybe not. The lens manufacturers isn't making it easy for us, that's for sure.

It seems I am making it difficult for you as well. It's a hard life.

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richarddd
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Re: Perhaps the real question should be.....
In reply to Bob Tullis, Apr 3, 2013

Bob Tullis wrote:

jalywol wrote:

Great Bustard wrote:

In my opinion, best to acknowledge the differences between the systems based on the both the facts and the types of photos one takes.  That is, while one system may clearly be superior to another for a particular purpose, if that particular purpose is not an aspect of your photography, then it has no bearing for you.

That was the entire point of my post......

The rounder we go, the faster we get.   [g]

Wheeeeee!!

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Dr_Jon
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Re: The role of ISO
In reply to Great Bustard, Apr 3, 2013

Another way to explain what I was saying, as you seem not to have understood my point at all. I have a 5DmkII and a GH3 and I will choose which to take along based on several criteria, but when I take the 5DmkII I don't feel any need to use it in a GH3 compatible higher-ISO mode so it would be equivalent to what I'd get with the GH3. I'll happily use an ISO that would be incompatible with anything the GH3 can do (and indeed often do).

Oh and I'm quite happy using A or M modes for almost all my photography thank you all the same, plus quite happy to put my technical knowledge up against yours any day (which I wouldn't usually mention, but as you took my comment as meaning something quite different to what I intended I thought I'd throw it in).

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Great Bustard
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Re: The role of ISO
In reply to Dr_Jon, Apr 3, 2013

Dr_Jon wrote:

I'm sorry, my whole point was when I'm using camera A I really don't give a toss how it might relate to camera B which I'm not using and so I don't care about equivalence in that sense, I just want to get the best picture I can with that camera.

I concur.

In other words I was suggesting it really isn't relevant all that much in the real World but just something people seem to like to debate, at length.

Equivalence has relevance when comparing systems, not when using any particular system.

Oh and I'm quite happy using A or M modes for almost all my photography thank you all the same, plus quite happy to put my technical knowledge up against yours any day (which I wouldn't usually mention, but as you took my comment as meaning something quite different to what I intended I thought I'd throw it in).

I'm not sure where this is coming from.  Can you link and quote the post of mine that provoked the above comments?

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Great Bustard
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Depends.
In reply to Anders W, Apr 3, 2013

Anders W wrote:

So if you are comparing a 25 mm and 50 mm lens on MFT, which comparison would you find more interesting. A comparison at the same f-stop or a comparison at the same absolute aperture?

It depends on the point of the comparison.  I would compare 25mm f/1.4 on mFT to 50mm f/1.4 on FF if I was interested in the shallowest DOF I could get at that AOV and/or the least amount of noise I could get.

On the other hand, if I were looking for overall sharpness, I'd compare the 25 / 1.4 on mFT at its sharpest aperture to the 50 / 1.4 at its sharpest aperture.

However, if I were looking for overall sharpness in a light limited scene then I'd compare 25 / 1.4 on mFT to 50mm f/2.8 on FF.

In other words, Equivalent settings are simply the starting point for a comparison -- we generally don't choose one system over another based on where they are most similar, but based on where they are most different.

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Dr_Jon
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Re: The role of ISO
In reply to Great Bustard, Apr 3, 2013

Please note I wasn't happy with my second post and rephrased the first part somewhat to more clearly explain my initial post.

I wasn't thrilled with "That said, many take some very nice photos in Auto mode", but perhaps I read too much into it. Although it is true I've seen some very nice shots taken that way. Also I've been a slight convert to sometimes using Auto-ISO on my RX100 in A or M modes recently, after more years than I can remember of always choosing the ISO explicitly, partly as getting quick shots of "something happening right now" in bright sunlight where the screen is hard to see can go badly wrong otherwise. (Actually the GH3 getting Auto-ISO in M would be very useful for movies, well if the ISO steps were small.)

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69chevy
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Re: No you did not, or ...
In reply to forpetessake, Apr 3, 2013

forpetessake wrote:

69chevy wrote:

Great Bustard wrote:

69chevy wrote:

Great Bustard wrote:

69chevy wrote:

So...

My camera phone has a crop factor of 7.64, the lens is 3.85mm and the aperture is f2.4.

The FF equivalent is 30mm f2.4?

No -- the FF equivalent is 29mm f/18 (3.85mm x 7.64 = 29mm, f/2.4 x 7.64 = f/18).

No disrespect, but if you are strictly talking DOF, this makes a little sense. If you are talking about diffraction effects or exposure effects it is not true.

...read the OP.  It's all discussed there (and yes, diffraction and exposure are discussed).

I did.

You can compare diffraction between two cameras with the same entrance pupil and different sensor sizes, but not with different flange distances and sensor sizes. This is where your equivalency formula falls short.

You either didn't read or did not understand. If it's the latter, then there is no shame in asking questions, but making arrogant statements is a completely different matter.

Diffraction isn't just related to aperture. It is also a function of sensor size. It is not arrogance, it is true.

When diffraction occurs the light is bent into a disc shape. As sensor size decreases, the pixel size does as well.

This means that the closer together the pixels are, the more diffraction affects the image.

This is why the equivalence is not accurate. Here is a little drawing I borrowed to illustrate. Credit goes to http://www.digitalversus.com for dumbing it down.

Full Frame 12 MP                       APSC 18 MP

1/2.3"  14 MP

I won't even get into the effects of how changing the flange distance changes the convex shape required by the lens. (Which also affects diffraction).

I will gladly ask for your expertise when you demonstrate it.

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Great Bustard
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Re: The role of ISO
In reply to Dr_Jon, Apr 3, 2013

Dr_Jon wrote:

Please note I wasn't happy with my second post and rephrased the first part somewhat to more clearly explain my initial post.

I wasn't thrilled with "That said, many take some very nice photos in Auto mode", but perhaps I read too much into it.

I believe you did "read too much into it".  What I meant is that all these technical points are often unimportant in terms of the overall impact of the final photo.  In other words, IQ usually plays a distant second to the recorded scene.

For example, let's say I took a photo at f/2.8 1/100 ISO 400 and everyone thought it was great.  If I took the same photo at f/5.6 1/50 ISO 800, how many would notice, much less care?  That is, there's usually a rather large range of parameters that doesn't affect the appeal of the final photo.

Although it is true I've seem some very nice shots taken that way. Also I've been a slight convert to sometimes using Auto-ISO on my RX100 in A or M modes recently, after more years than I can remember of always choosing the ISO explicitly, partly as getting quick shots of "something happening right now" in bright sunlight where the screen is hard to see can go badly wrong otherwise. (Actually the GH3 getting Auto-ISO in M would be very useful for movies, well if the ISO steps were small.)

M mode + Auto ISO is awesome, and something I very much looked forward to when I upgraded from the 5Dc to the 6D.  However, Canon's failure to allow EC with that particular combination limits its utility, unfortunately.

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Macx
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Re: A few words on Equivalence and comparing systems
In reply to Anders W, Apr 3, 2013

Anders W wrote:

Macx wrote:

Anders W wrote:

Macx wrote:

texinwien wrote:

olliess wrote:

"Didn't want that light anyway."

It's the sensor not the lens, and JACS is talking about measures of lens performance w/r/t equivalence.

But I'd wager you knew this and were just trying to be witty. Unfortunately, you just ended up sounding obtuse.

tex

Keep the sensor out of it. The sensor size is interesting because the FOV of a particular lens is dependant on it, but for the same FOV and the same physical aperture the systems are equivalent and they get the same amount of light. If we had lenses with the same FOV and the same physical aperture for micro four-thirds as we have for 135, there would be no innate shot noise advantage of the larger sensor.

In other words, it's the lenses, not the sensor, and if we had a 12-35/2 our exposure envelope or gamut would be equal to having a 24-70/4 for our 135. The loss of a stop of total light when going from a 24-70/4 on a 135 to a 12-35/2.8 on M43 is dependant on the latter being f/2.8 and not f/2.

Macx,

It seems that your point (FoV instead of FL, absolute size of the aperture rather than relative size) is a pedagogical one. Your idea, if I understand you right, is that it simplifies things to use FoV and absolute aperture size because you can state equivalence in terms of equalities.

Yes, and because these are optical qualities that define what we want in a lens. It's field of view and it's depth of field and/or diffraction properties. We've learned to make the mental arithmetic needed to translate focal length and focal ratio into these values, but why the need for such translation when we could use these "proper" values as the basis for discussing how a lens behaves?

Personally, I have no trouble going between FL and FoV. And I actually find it easier to define DoF and diffraction properties on the basis of f-stops than absolute aperture. Could you perhaps tell me how to convert the rule of thumb that MFT lenses shouldn't be stopped down further than f/8 if you want to avoid significant diffraction effects into your conceptual scheme?

If I really had my way, I would use a logarithmic scale for the stops. So I would say that Av shouldn't be more than 6. That might be taking it too far though. As I wrote, I'm not asking for us to forget or make focal length and relative aperture secret. It's not either-or. Relative aperture is also useful for calculating depth of field using the same framing, for example.

I see that point but I am not sure I agree that this is the more pedagogical solution all things considered. For example, FL and max relative aperture (max f-stop) are known properties of a lens and independent of the sensor to which the lens is fitted. FoV and max absolute aperture are properties that have to be derived (calculated) from those already known and the first of them (FoV) varies depending on the size of the sensor the lens is mated with.

The known properties are the focal length and the maximum absolute (virtual) aperture. The maximum f-stop is derived from these, not the other way around.

When it comes to aperture, it works perfectly well either way. Start with the relative number and define the absolute number if you want it (I rarely do).

It does indeed. No special advantage either way.

It is true that to calculate the FoV we need to know what sensor the lens is used for. Even so, at least we would know it, instead of just having an intuitive understanding about the field of view of a 19 mm lens on a 135, for example.

I know that anyway.

Well, it was an easy one. What is the Field of View of a 3 mm focal length phone camera? NO PEEKING AT WHAT THE CROP FACTOR IS! I suppose we're blessed that we can use such simple arithmetic to convert between 135 and 4:3 values. It would be harder to do on the fly if it was an irrational number.

I don't propose that we keep focal length or the f-stops secret. But these values aren't what is interesting about how our lens behaves on our camera, and especially when discussing lens properties across systems it seems to me to be much more pertinent to concentrate on these values instead of having to work with "equivalent focal lengths" and other thought-experiments.

You say that f-stops aren't what we are interested in. So tell me, how would you convert the "sunny 16" rule into your conceptual machinery?

I wouldn't really. Is there a need to when talking about the optical qualities of a lens on different systems?

Another thing here that I am curious about. Equivalence isn't only about equality but also about how far apart things are. Say we are comparing a 12/2 on MFT with a 24/2.8 on FF. Clearly, these two lenses are not equivalent. The 12/2 is equivalent to a 24/4, not a 24/2.8. But how would you, using your conceptual machinery, express the one f-stop difference between the 12/2 and the 24/2.8? And of what practical help would that difference, as expressed in your language, be?

If the absolute aperture is half the area I would say that it has one stop less light. But I'd probably just use the focal ratio here for ease of calculation. Again, I would have changed it to a logarithmic scale if I had my way, (If it wasn't for those meddling kids!), but since I have memorised the f-stop progression (and the 1/3 points and the 1/2 points) I can hobble along using focal ratios.

And from a performance standpoint, isn't the performance of a lens at a certain absolute aperture more interesting that what it is at a relative aperture? I think so, at least.

So if you are comparing a 25 mm and 50 mm lens on MFT, which comparison would you find more interesting. A comparison at the same f-stop or a comparison at the same absolute aperture?

A 25 mm on 4:3 and a 50 mm on 135, absolute aperture here, definitely. I wouldn't really have much use comparing specific f-stops on a 25 and 50 mm to each other, I would be much more interested in how they performed individually across the range. I don't think I would often be in doubt whether to use a 25 or a 50, and use their performance at specific stops to decide. I suppose I could crop the 25 to get the same FOV as the 50?

Both ways of thinking about the matter are of course factually correct. It's just a matter of conceptualization. But in the end, I am not sure that a shift of attention away from FL and relative aperture to FoV and absolute aperture makes things any better from a pedagogical point of view.

Maybe not. The lens manufacturers isn't making it easy for us, that's for sure.

It seems I am making it difficult for you as well. It's a hard life.

Yes, but I hope I eventually learn something from all the beat downs, so don't show mercy. As long as you're polite about it, I can take it, I think.

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Great Bustard
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Re: No you did not, or ...
In reply to 69chevy, Apr 3, 2013

69chevy wrote:

forpetessake wrote:

69chevy wrote:

Great Bustard wrote:

69chevy wrote:

Great Bustard wrote:

69chevy wrote:

So...

My camera phone has a crop factor of 7.64, the lens is 3.85mm and the aperture is f2.4.

The FF equivalent is 30mm f2.4?

No -- the FF equivalent is 29mm f/18 (3.85mm x 7.64 = 29mm, f/2.4 x 7.64 = f/18).

No disrespect, but if you are strictly talking DOF, this makes a little sense. If you are talking about diffraction effects or exposure effects it is not true.

...read the OP.  It's all discussed there (and yes, diffraction and exposure are discussed).

I did.

You can compare diffraction between two cameras with the same entrance pupil and different sensor sizes, but not with different flange distances and sensor sizes. This is where your equivalency formula falls short.

You either didn't read or did not understand. If it's the latter, then there is no shame in asking questions, but making arrogant statements is a completely different matter.

Diffraction isn't just related to aperture. It is also a function of sensor size. It is not arrogance, it is true.

When diffraction occurs the light is bent into a disc shape. As sensor size decreases, the pixel size does as well.

This means that the closer together the pixels are, the more diffraction affects the image.

Sorry, but that's all incorrect.  Again:

http://www.josephjamesphotography.com/equivalence/#diffraction

In short, diffraction softening is a result of the proportion of the photo that the Airy Disk spans, and that proportion is the same for all systems at the same DOF, regardless of sensor size or pixel count.

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Dr_Jon
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Re: The role of ISO
In reply to Great Bustard, Apr 3, 2013

Okay, sorry about the misunderstanding, but perhaps you can see why I might have read it another way?

I think you can take photographs with the majority of decent or semi-decent cameras that you can shove up on a wall at a pretty fair size and they will look great. My sister has one of the Panasonic TZ models (it was the top one a generation ago) and has a 30" wide pic (maybe 12" high) she took up on her wall that look stunning and most people assume it's a professional print she bought. It's the composition in a large part, but actually it's also sharp and low noise (gotta love a bright sunny day). Where the better kit helps is as the conditions become trickier.

Oh, and when deciding which camera to take weight and lens availability (I have some very nice Canon glass) is probably more of an effect than equivalence. So is noise for low-light work, but let's leave that discussion for another day...

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69chevy
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Re: No you did not, or ...
In reply to Great Bustard, Apr 3, 2013

Great Bustard wrote:

69chevy wrote:

forpetessake wrote:

69chevy wrote:

Great Bustard wrote:

69chevy wrote:

Great Bustard wrote:

69chevy wrote:

So...

My camera phone has a crop factor of 7.64, the lens is 3.85mm and the aperture is f2.4.

The FF equivalent is 30mm f2.4?

No -- the FF equivalent is 29mm f/18 (3.85mm x 7.64 = 29mm, f/2.4 x 7.64 = f/18).

No disrespect, but if you are strictly talking DOF, this makes a little sense. If you are talking about diffraction effects or exposure effects it is not true.

...read the OP.  It's all discussed there (and yes, diffraction and exposure are discussed).

I did.

You can compare diffraction between two cameras with the same entrance pupil and different sensor sizes, but not with different flange distances and sensor sizes. This is where your equivalency formula falls short.

You either didn't read or did not understand. If it's the latter, then there is no shame in asking questions, but making arrogant statements is a completely different matter.

Diffraction isn't just related to aperture. It is also a function of sensor size. It is not arrogance, it is true.

When diffraction occurs the light is bent into a disc shape. As sensor size decreases, the pixel size does as well.

This means that the closer together the pixels are, the more diffraction affects the image.

Sorry, but that's all incorrect.  Again:

http://www.josephjamesphotography.com/equivalence/#diffraction

In short, diffraction softening is a result of the proportion of the photo that the Airy Disk spans, and that proportion is the same for all systems at the same DOF, regardless of sensor size or pixel count.

So you agree the disc size grows?

Then how do you not understand that as that disc covers more pixels (smaller ones) it degrades the image further than it would if the pixels were larger?

I can't open your link at work, or I would be happy to look at it.

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klopus
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Is that so?
In reply to Great Bustard, Apr 3, 2013

Great Bustard wrote:

Indeed.  If we took a photo at 100mm f/2.8 on FF, and cropped it to the same framing as a photo taken at 100mm f/2.8 on mFT, the photos would be equivalent (although the mFT photo would be more detailed since the sensor has a higher pixel density and the lens is sharper).\

Is it true that higher pixel density always makes for more detail? By that logic any tiny sensor point-n-shoot at same ISO will always be more detailed than mFT not speaking of FF. In reality for smaller and noisy pixels any theoretical extra density detail (plus good chunk of base detail) will be destroyed by noise that those small but low "quality" pixels produce.

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Detail Man
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Re: Photosite Density in relation to Diffraction Effects
In reply to Great Bustard, Apr 3, 2013

Great Bustard wrote:

... the effects of diffraction softening are also included, and the pixel size has nothing to do with it:  the proportion of the photo that the Airy Disk takes up when the FF photo is cropped by a factor of two also doubles (as does the DOF when the photo is displayed at the original size).

... diffraction softening is a result of the proportion of the photo that the Airy Disk spans, and that proportion is the same for all systems at the same DOF, regardless of sensor size or pixel count.

For a given sensor size and lens, more pixels always result in more detail -- that's a fact.  As we stop down and the DOF deepens, we reach a point where we begin to lose detail due to diffraction softening.  As a consequence, photos made with more pixels will begin to lose their detail advantage earlier and quicker than images made with fewer pixels, but they will always retain more detail ...

... the effects of diffraction softening affect all systems equally at the same AOV and DOF, not the same f-ratio.  This is because the diameter of the Airy Disk takes up the same proportion of the sensor (and displayed photo) at the same DOF on all systems.  However, that does not mean that all systems record the same detail at the same AOV and DOF.  Even though diffraction softening affects all systems equally at the same AOV and DOF, the system that began with more detail will retain more detail (although, as the DOF deepens, all systems asymptotically approach zero detail).

While the diameter of the Airy Disk is the same for the same color and f-ratio, regardless of the sensor size, the effect of the diffraction softening is not the same across formats.  The reason is that the proportion of the sensor that is covered by the Airy Disk is not the same since the sensors are not the same size ...

... the system that began with more resolution will always retain more resolution, but that resolution will asymptotically vanish as the DOF deepens.  In absolute terms, the earliest we will notice the effects of diffraction softening is when the diameter of the Airy Disk exceeds that of a pixel (two pixels for a Bayer CFA), but, depending on how large the photo is displayed, we may not notice until the diameter of the Airy Disk is much larger.

... for two sensors of a given size, the sensor with a greater pixel density does not suffer more from diffraction softening due to the smaller pixels.  We will simply notice the effects of diffraction softening earlier (at wider apertures) since we had more resolution to begin with as a result of the smaller pixels (presuming, of course, that we display the photo large enough that we can resolve individual pixels) ...

It seems important to note that composite system resolution (as assessed via the composite system MTF response) is not something independent of lens-system diffraction effects in relation to individual photosite size - in a manner that is independent of proportion of image-frame.

When the MTF response of the photosite (combined with any AA filter assembly that may exist on the image-sensor) decreases to a point where the product of its multiplication by the MTF response resulting from diffraction through a circular aperture opening reduces the composite MTF of the system in total, photosite size does in that case limit resolution of the composite system.

Higher photosite density can result in more detail (that the image-sensor can potentially resolve) - but that has little or even no effect in situations where the MTF response due to diffraction through a circular aperture has already reduced the composite MTF response to an extent where loss of resolution due to diffraction dominates relative to the MTF response of the photosite (combined with any AA filter assembly that may exist on the image-sensor).

The above effect is independent of what relative proportion of the recorded image-frame the diameter of the Airy disk or the photosite (combined with any AA filter assembly that may exist on the image-sensor) represents - it relates to the relative proportions of their individual physical dimensions.

"The sensor with a greater pixel density does not suffer more from diffraction" - but the composite system MTF response also does not (necessarily) benefit from greater pixel density.

Resolution that vanishes as a result of lens-system diffraction is not meaningful. Little to be gained from increasing photosite density beyond amounts that correspond to what are dominating effects of diffraction through a circular aperture of the lens-system used at its widest opening.

.

Am interested in your thoughts (related to composite MTF as a function of photosite density), GB.

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69chevy
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Re: No you did not, or ...
In reply to Great Bustard, Apr 3, 2013

Great Bustard wrote:

Sorry, but that's all incorrect.  Again:

http://www.josephjamesphotography.com/equivalence/#diffraction

In short, diffraction softening is a result of the proportion of the photo that the Airy Disk spans, and that proportion is the same for all systems at the same DOF, regardless of sensor size or pixel count.

Also, the airy disc size only stays proportional when the relative size of the fstop is also in proportion.

In your scenario, you give the smaller sensors the same size entrance pupil, which means the same size airy disc, which means more apparent diffraction for a smaller sensor with higher pixel density.

The airy disc blurs more pixels in your equivalent scenaio, so diffraction is very different (not equivalent).

I'm not wrong on this, I promise.

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69chevy
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Re: No you did not, or ...
In reply to Great Bustard, Apr 3, 2013

Great Bustard wrote:

Sorry, but that's all incorrect.  Again:

http://www.josephjamesphotography.com/equivalence/#diffraction

In short, diffraction softening is a result of the proportion of the photo that the Airy Disk spans, and that proportion is the same for all systems at the same DOF, regardless of sensor size or pixel count.

Also, the airy disc size only stays proportional when the relative size of the fstop is also in proportion.

In your scenario, you give the smaller sensors the same size entrance pupil, which means the same size airy disc, which means more apparent diffraction for a smaller sensor with higher pixel density.

The airy disc blurs more pixels in your equivalent scenaio, so diffraction is very different (not equivalent).

I'm not wrong on this, I promise, so you dont have to say sorry..

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olliess
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Re: A few words on Equivalence and comparing systems
In reply to Macx, Apr 3, 2013

Macx wrote:

If I really had my way, I would use a logarithmic scale for the stops.

That's what the clicky stops are for (or used to be for, in case your chosen system no longer has them). If you take f/1.4 as the baseline aperture (for instance), then the f-stops happen at 1, 2, 3, ..., * (log 1.4).  

So I would say that Av shouldn't be more than 6.

The numerical f/stops, which you can read off the camera, are useful for some other things, e.g., manual flash exposures.

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Great Bustard
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Re: No you did not, or ...
In reply to 69chevy, Apr 3, 2013

69chevy wrote:

Great Bustard wrote:

69chevy wrote:

forpetessake wrote:

69chevy wrote:

Great Bustard wrote:

69chevy wrote:

Great Bustard wrote:

69chevy wrote:

So...

My camera phone has a crop factor of 7.64, the lens is 3.85mm and the aperture is f2.4.

The FF equivalent is 30mm f2.4?

No -- the FF equivalent is 29mm f/18 (3.85mm x 7.64 = 29mm, f/2.4 x 7.64 = f/18).

No disrespect, but if you are strictly talking DOF, this makes a little sense. If you are talking about diffraction effects or exposure effects it is not true.

...read the OP.  It's all discussed there (and yes, diffraction and exposure are discussed).

I did.

You can compare diffraction between two cameras with the same entrance pupil and different sensor sizes, but not with different flange distances and sensor sizes. This is where your equivalency formula falls short.

You either didn't read or did not understand. If it's the latter, then there is no shame in asking questions, but making arrogant statements is a completely different matter.

Diffraction isn't just related to aperture. It is also a function of sensor size. It is not arrogance, it is true.

When diffraction occurs the light is bent into a disc shape. As sensor size decreases, the pixel size does as well.

This means that the closer together the pixels are, the more diffraction affects the image.

Sorry, but that's all incorrect.  Again:

http://www.josephjamesphotography.com/equivalence/#diffraction

In short, diffraction softening is a result of the proportion of the photo that the Airy Disk spans, and that proportion is the same for all systems at the same DOF, regardless of sensor size or pixel count.

So you agree the disc size grows?

No.  For a given DOF, the size of the Airy Disk spans the same proportion of the sensor on all systems.  How many pixels that disk covers is irrelevant.

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