Why the very short flange focal distance of 16mm for Z mount?

JimKasson said:

Alex Permit said:

Leonard Migliore said:

Alex Permit said:

cosmicnode said:

Here is a photo of the rear of a Nikon lens. notice where the registration distance to the sensor is, it's that flat area with the rubber sealing ring this connects to the front of the camera mount. from which the registrationdistance is measured. The mechanism of the bayonette protrudes into the camera body by 5-6mm, in this case the lens elements also can protrude closer to the sensor. Every lens from any manufacturer which uses a bayonette mount of any description protrudes into the camera body by a similar 5-6mm reducing the distance to the sensor from the rear of the lens, it's not a standard but it seems to be difficult to reduce this size . if you look at the chromed part of the rear of the lens , the inside diameter of this is the maximum size any of an element at the rear of any lens. These are engineering constraints.

Click to expand...

Not every lens


Nikkor 7.5mm f/5.6 Fisheye

You could only use it with the mirror locked up.

Click to expand...

But will it work with the FTZ adapter?

Click to expand...

Gets back to Jim's point. We don't know how far they allow the lens to extend behind the mount. So we don't have sufficient information. If it does, it should work better then it ever did since you can use the EVF. And you don't have to worry about pulling a bone headed maneuver of turning off live view and smashing your mirror.

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All true except possibly the working better than ever. That lens was designed for a zero-thickness sensor stack. I expect those ray angles are going to interact with the sensor stack to create corner smear (or what would be corner smear in a cropped sensor, since that lens doesn't illuminate the Zx corners. There also might be some color casting off-axis due to CFA crosstalk, but a BSI sensor should help with that.

Jim

Click to expand...

I'm sure modern optics are better. Even when using film, it wasn't tack sharp, contrast was moderate and it suffered from flare and some vignetting. But it is cool. No retro focus so the lens is slim in front of the camera. And it looks like HAL.

Bottom line, it was optical and physical engineering decision by Nikon, many similar combinations also work. Sony uses 18mm for its E mount, so Nikon’s choice of 16mm is not much different. The larger flange diameter allows for shorter FFD distances, which is why the Nikon 1 FFD was longer, along with MFT.
Cheers

Alex Permit said:

I’m not sure if a couple of mm’s makes that big a difference.

Click to expand...

Which would you prefer,....a 28-70mm,...or 24-70mm lens?

JimKasson said:

cosmicnode said:

JimKasson said:

cosmicnode said:

JimKasson said:

jonikon said:

beatboxa said:

JimKasson said:

The sensor stack thickness plus the shutter thickness plus the distance that the lens can extend behind the flange plus some safety margin determines how short the FFD can be.

Jim

Click to expand...

I would think that this sums it all up.

Click to expand...

So does that mean that the other mirrorless cameras with longer FFDs are bad designs?Surely they could have made the FFD shorter on smaller sensor cameras as well.

Click to expand...

We don't know how far they allow the lens to extend behind the sensor. So we don't have sufficient information.

Jim

Click to expand...

On all lenses from any manufacturer the mechanics of the lens mount protrude 5-6mm into the camera body,

Click to expand...

I have not observed the standardization of which you write. In fact, I have observed differences among lenses and mounts.

Click to expand...

Here is a photo of the rear of a Nikon lens. notice where the registration distance to the sensor is, it's that flat area with the rubber sealing ring this connects to the front of the camera mount. from which the registrationdistance is measured. The mechanism of the bayonette protrudes into the camera body by 5-6mm, in this case the lens elements also can protrude closer to the sensor. Every lens from any manufacturer which uses a bayonette mount of any description protrudes into the camera body by a similar 5-6mm reducing the distance to the sensor from the rear of the lens,

Click to expand...

Did you miss the link below with a counterexample?

Click to expand...

It's not a counter example it's exactly what I have shown in the Nikon photo





Note th 26.7mm dimension to the front of the camera body mounting flange, see that the inside of the flange has the bayonette tabs inside at the lowest point, the tabs on the lens locate on these but further below them into the camera body . Notice also the rear element shown at a minimum back focus of 16.7mm, this is very similar to the nikon lens shown in the photo. It's impossible to mount a lens onto a bayonette mount without it protruding into it, Everyone on these forums only needs to look at a lens and how it attaches to their own camera body regardless of make to see the truth in this.

JimKasson said:

JimKasson said:

it's not a standard but it seems to be difficult to reduce this size . if you look at the chromed part of the rear of the lens , the inside diameter of this is the maximum size any of an element at the rear of any lens. These are engineering constraints.

JimKasson said:

Here's a spec that's got a 10mm delta:

http://www.fujifilm.com/products/digital_cameras/gfx/fujifilm_gfx_50s/features/index.html

JimKasson said:

this will place the lens barrel and sometimes the elements on the Z mount to within 10mm of the sensor, as in the video the engineer stated it allows only room for a thinner shutter mechanism

Click to expand...

Jim

--
http://blog.kasson.com

Click to expand...

--
Mike.
"I say we take off and nuke the entire site from orbit, it's the only way to be sure."

Click to expand...

--
http://blog.kasson.com

Click to expand...

--
Mike.
"I say we take off and nuke the entire site from orbit, it's the only way to be sure."

Alex Permit said:

cosmicnode said:

Here is a photo of the rear of a Nikon lens. notice where the registration distance to the sensor is, it's that flat area with the rubber sealing ring this connects to the front of the camera mount. from which the registrationdistance is measured. The mechanism of the bayonette protrudes into the camera body by 5-6mm, in this case the lens elements also can protrude closer to the sensor. Every lens from any manufacturer which uses a bayonette mount of any description protrudes into the camera body by a similar 5-6mm reducing the distance to the sensor from the rear of the lens, it's not a standard but it seems to be difficult to reduce this size . if you look at the chromed part of the rear of the lens , the inside diameter of this is the maximum size any of an element at the rear of any lens. These are engineering constraints.



--
Mike.
"I say we take off and nuke the entire site from orbit, it's the only way to be sure."

Click to expand...

Not every lens


Nikkor 7.5mm f/5.6 Fisheye

You could only use it with the mirror locked up.

Click to expand...

That is the same, the rear element is smaller than the inside of th mount and both mount and element protrude into the camera body, the fact it protrudes further is irrelavant to what I said

--
Mike.
"I say we take off and nuke the entire site from orbit, it's the only way to be sure."

cosmicnode said:

Alex Permit said:

cosmicnode said:

Here is a photo of the rear of a Nikon lens. notice where the registration distance to the sensor is, it's that flat area with the rubber sealing ring this connects to the front of the camera mount. from which the registrationdistance is measured. The mechanism of the bayonette protrudes into the camera body by 5-6mm, in this case the lens elements also can protrude closer to the sensor. Every lens from any manufacturer which uses a bayonette mount of any description protrudes into the camera body by a similar 5-6mm reducing the distance to the sensor from the rear of the lens, it's not a standard but it seems to be difficult to reduce this size . if you look at the chromed part of the rear of the lens , the inside diameter of this is the maximum size any of an element at the rear of any lens. These are engineering constraints.

Click to expand...

Not every lens


Nikkor 7.5mm f/5.6 Fisheye

You could only use it with the mirror locked up.

Click to expand...

That is the same, the rear element is smaller than the inside of th mount and both mount and element protrude into the camera body, the fact it protrudes further is irrelavant to what I said

Click to expand...

I misunderstood. When you said:

"The mechanism of the bayonette protrudes into the camera body by 5-6mm, in this case the lens elements also can protrude closer to the sensor. Every lens from any manufacturer which uses a bayonette mount of any description protrudes into the camera body by a similar 5-6mm "

I thought you meant every lens from every manufacturer protrudes into the camera body by 5-6mm.

cosmicnode said:

JimKasson said:

cosmicnode said:

JimKasson said:

cosmicnode said:

JimKasson said:

jonikon said:

beatboxa said:

JimKasson said:

The sensor stack thickness plus the shutter thickness plus the distance that the lens can extend behind the flange plus some safety margin determines how short the FFD can be.

Jim

Click to expand...

I would think that this sums it all up.

Click to expand...

So does that mean that the other mirrorless cameras with longer FFDs are bad designs?Surely they could have made the FFD shorter on smaller sensor cameras as well.

Click to expand...

We don't know how far they allow the lens to extend behind the sensor. So we don't have sufficient information.

Jim

Click to expand...

On all lenses from any manufacturer the mechanics of the lens mount protrude 5-6mm into the camera body,

Click to expand...

I have not observed the standardization of which you write. In fact, I have observed differences among lenses and mounts.

Click to expand...

Here is a photo of the rear of a Nikon lens. notice where the registration distance to the sensor is, it's that flat area with the rubber sealing ring this connects to the front of the camera mount. from which the registrationdistance is measured. The mechanism of the bayonette protrudes into the camera body by 5-6mm, in this case the lens elements also can protrude closer to the sensor. Every lens from any manufacturer which uses a bayonette mount of any description protrudes into the camera body by a similar 5-6mm reducing the distance to the sensor from the rear of the lens,

Click to expand...

Did you miss the link below with a counterexample?

Click to expand...

It's not a counter example it's exactly what I have shown in the Nikon photo



Note th 26.7mm dimension to the front of the camera body mounting flange, see that the inside of the flange has the bayonette tabs inside at the lowest point, the tabs on the lens locate on these but further below them into the camera body . Notice also the rear element shown at a minimum back focus of 16.7mm, this is very similar to the nikon lens shown in the photo. It's impossible to mount a lens onto a bayonette mount without it protruding into it, Everyone on these forums only needs to look at a lens and how it attaches to their own camera body regardless of make to see the truth in this.

Click to expand...

So now I'm really confused as to what you are saying. You're saying the lens of all bayonet mounts protrudes into the camera the same amount. Are you saying that they all protrude past the flange 5-6 mm, or that they all protrude past the flange 10 mm?

cosmicnode said:

cosmicnode said:

cosmicnode said:

it's not a standard but it seems to be difficult to reduce this size . if you look at the chromed part of the rear of the lens , the inside diameter of this is the maximum size any of an element at the rear of any lens. These are engineering constraints.

cosmicnode said:

Here's a spec that's got a 10mm delta:

http://www.fujifilm.com/products/digital_cameras/gfx/fujifilm_gfx_50s/features/index.html

cosmicnode said:

this will place the lens barrel and sometimes the elements on the Z mount to within 10mm of the sensor, as in the video the engineer stated it allows only room for a thinner shutter mechanism

Click to expand...

Jim

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--

Alex Permit said:

cosmicnode said:

Alex Permit said:

cosmicnode said:

Here is a photo of the rear of a Nikon lens. notice where the registration distance to the sensor is, it's that flat area with the rubber sealing ring this connects to the front of the camera mount. from which the registrationdistance is measured. The mechanism of the bayonette protrudes into the camera body by 5-6mm, in this case the lens elements also can protrude closer to the sensor. Every lens from any manufacturer which uses a bayonette mount of any description protrudes into the camera body by a similar 5-6mm reducing the distance to the sensor from the rear of the lens, it's not a standard but it seems to be difficult to reduce this size . if you look at the chromed part of the rear of the lens , the inside diameter of this is the maximum size any of an element at the rear of any lens. These are engineering constraints.

Click to expand...

Not every lens


Nikkor 7.5mm f/5.6 Fisheye

You could only use it with the mirror locked up.

Click to expand...

That is the same, the rear element is smaller than the inside of th mount and both mount and element protrude into the camera body, the fact it protrudes further is irrelavant to what I said

Click to expand...

I misunderstood. When you said:

"The mechanism of the bayonette protrudes into the camera body by 5-6mm, in this case the lens elements also can protrude closer to the sensor. Every lens from any manufacturer which uses a bayonette mount of any description protrudes into the camera body by a similar 5-6mm "

I thought you meant every lens from every manufacturer protrudes into the camera body by 5-6mm.

Click to expand...

That's what I thought too, and I thought the reference was the flange.

--

JimKasson said:

cosmicnode said:

JimKasson said:

cosmicnode said:

JimKasson said:

cosmicnode said:

JimKasson said:

jonikon said:

beatboxa said:

JimKasson said:

The sensor stack thickness plus the shutter thickness plus the distance that the lens can extend behind the flange plus some safety margin determines how short the FFD can be.

Jim

Click to expand...

I would think that this sums it all up.

Click to expand...

So does that mean that the other mirrorless cameras with longer FFDs are bad designs?Surely they could have made the FFD shorter on smaller sensor cameras as well.

Click to expand...

We don't know how far they allow the lens to extend behind the sensor. So we don't have sufficient information.

Jim

Click to expand...

On all lenses from any manufacturer the mechanics of the lens mount protrude 5-6mm into the camera body,

Click to expand...

I have not observed the standardization of which you write. In fact, I have observed differences among lenses and mounts.

Click to expand...

Here is a photo of the rear of a Nikon lens. notice where the registration distance to the sensor is, it's that flat area with the rubber sealing ring this connects to the front of the camera mount. from which the registrationdistance is measured. The mechanism of the bayonette protrudes into the camera body by 5-6mm, in this case the lens elements also can protrude closer to the sensor. Every lens from any manufacturer which uses a bayonette mount of any description protrudes into the camera body by a similar 5-6mm reducing the distance to the sensor from the rear of the lens,

Click to expand...

Did you miss the link below with a counterexample?

Click to expand...

It's not a counter example it's exactly what I have shown in the Nikon photo



Note th 26.7mm dimension to the front of the camera body mounting flange, see that the inside of the flange has the bayonette tabs inside at the lowest point, the tabs on the lens locate on these but further below them into the camera body . Notice also the rear element shown at a minimum back focus of 16.7mm, this is very similar to the nikon lens shown in the photo. It's impossible to mount a lens onto a bayonette mount without it protruding into it, Everyone on these forums only needs to look at a lens and how it attaches to their own camera body regardless of make to see the truth in this.

Click to expand...

So now I'm really confused as to what you are saying. You're saying the lens of all bayonet mounts protrudes into the camera the same amount. Are you saying that they all protrude past the flange 5-6 mm, or that they all protrude past the flange 10 mm?

JimKasson said:

JimKasson said:

JimKasson said:

it's not a standard but it seems to be difficult to reduce this size . if you look at the chromed part of the rear of the lens , the inside diameter of this is the maximum size any of an element at the rear of any lens. These are engineering constraints.

JimKasson said:

Here's a spec that's got a 10mm delta:

http://www.fujifilm.com/products/digital_cameras/gfx/fujifilm_gfx_50s/features/index.html

JimKasson said:

this will place the lens barrel and sometimes the elements on the Z mount to within 10mm of the sensor, as in the video the engineer stated it allows only room for a thinner shutter mechanism

Click to expand...

Jim

Click to expand...

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Maybe he is saying so or so. but my lenses do not produde into the space behind the flange, because I do not like to Scratch my lenses if I put them on a desk or car seat without back lens cover … Maybe they protrude in certain Focusing condition that I do not know.

BUT: it is known that, for Canon, of you mount an af-s lens onto an ef-mount, then the Mirror of the ef-cam may crash into the ef-s lens.

br gusti

mcabato said:

mcabato said:

...

Click to expand...

Maybe he is saying so or so. but my lenses do not produde into the space behind the flange, because I do not like to Scratch my lenses if I put them on a desk or car seat without back lens cover … Maybe they protrude in certain Focusing condition that I do not know.

Click to expand...

Perhaps this is a language issue but your lenses do protrude behind the flange.

For example:


Olympus Zuiko Digital 14-54mm 1:2.8-3.5

Red line approximates the flange.
On bayonet mounts there is always something behind the flange but not always optics.

--
Bill ( Your trusted source for independent sensor data at PhotonsToPhotos )

JimKasson said:

cosmicnode said:

JimKasson said:

cosmicnode said:

JimKasson said:

cosmicnode said:

JimKasson said:

jonikon said:

beatboxa said:

JimKasson said:

The sensor stack thickness plus the shutter thickness plus the distance that the lens can extend behind the flange plus some safety margin determines how short the FFD can be.

Jim

Click to expand...

I would think that this sums it all up.

Click to expand...

So does that mean that the other mirrorless cameras with longer FFDs are bad designs?Surely they could have made the FFD shorter on smaller sensor cameras as well.

Click to expand...

We don't know how far they allow the lens to extend behind the sensor. So we don't have sufficient information.

Jim

Click to expand...

On all lenses from any manufacturer the mechanics of the lens mount protrude 5-6mm into the camera body,

Click to expand...

I have not observed the standardization of which you write. In fact, I have observed differences among lenses and mounts.

Click to expand...

Here is a photo of the rear of a Nikon lens. notice where the registration distance to the sensor is, it's that flat area with the rubber sealing ring this connects to the front of the camera mount. from which the registrationdistance is measured. The mechanism of the bayonette protrudes into the camera body by 5-6mm, in this case the lens elements also can protrude closer to the sensor. Every lens from any manufacturer which uses a bayonette mount of any description protrudes into the camera body by a similar 5-6mm reducing the distance to the sensor from the rear of the lens,

Click to expand...

Did you miss the link below with a counterexample?

Click to expand...

It's not a counter example it's exactly what I have shown in the Nikon photo



Note th 26.7mm dimension to the front of the camera body mounting flange, see that the inside of the flange has the bayonette tabs inside at the lowest point, the tabs on the lens locate on these but further below them into the camera body . Notice also the rear element shown at a minimum back focus of 16.7mm, this is very similar to the nikon lens shown in the photo. It's impossible to mount a lens onto a bayonette mount without it protruding into it, Everyone on these forums only needs to look at a lens and how it attaches to their own camera body regardless of make to see the truth in this.

Click to expand...

So now I'm really confused as to what you are saying. You're saying the lens of all bayonet mounts protrudes into the camera the same amount. Are you saying that they all protrude past the flange 5-6 mm, or that they all protrude past the flange 10 mm?

Click to expand...

I have said that the mechanism of the lens mount, ie the bayonette tabs protrude into the camera body by 5-6mm. This means that the end of the lens body, the metal or plastic parts are now 10mm off the sensor, the example you highlighted from Fuji shows for their lens design the rear element can protrude into the mount beyond the sensor registration distance by 10mm. All lens bayonette mounts protrude into the camera body by around 5-6mm. you simply have to look at how your own camera and lens fit together to see this.

The lens registration distance is a specific distance away from the sensor plane, it's a mechanical registration distance to the front face of the lens mount, it's not optical, the lens body has to position optics the correct distance in front or behind this distance using the mount face on the lens body as the datum for the registration distance. Telephoto lenses can have the last element 100mm away from the lens mount and wide angles can be inside the lens mount.

The fact that the solid componants of the lens body protrude into the body beyond the lens registration distance means that the max diameter available for a glass element in the rear of a lens body is reduced by the material required for the lens mount ring and lens barrel tube, this is a reduction of around 6mm. So the rear of a Z lens (not element) will have a available clear light path of around reduced from 55mm to 49mm diameter 10mm off the surface of the sensor approximately, in reality the sensor is never vingnetted in the corners even with up to 2.5mm diagonal movement of the sensor either direction of the centre point of the sensor for IBIS. light can hit the sensor at any angle unobstructed with the rear element placed any distance from the sensor.

JimKasson said:

JimKasson said:

JimKasson said:

JimKasson said:

it's not a standard but it seems to be difficult to reduce this size . if you look at the chromed part of the rear of the lens , the inside diameter of this is the maximum size any of an element at the rear of any lens. These are engineering constraints.

JimKasson said:

Here's a spec that's got a 10mm delta:

http://www.fujifilm.com/products/digital_cameras/gfx/fujifilm_gfx_50s/features/index.html

JimKasson said:

this will place the lens barrel and sometimes the elements on the Z mount to within 10mm of the sensor, as in the video the engineer stated it allows only room for a thinner shutter mechanism

Click to expand...

Jim

Click to expand...

Click to expand...

Click to expand...

Click to expand...

Click to expand...

--
Mike.
"I say we take off and nuke the entire site from orbit, it's the only way to be sure."

Short version: to accommodate the near tele-centric lens designs needed by sensors, the mount diameter needs to exceed the sensor diagonal, by an amount proportional to mount depth, so lower depth also allows a less wide throat.

Details: If at 16mm, some incoming light rays are near the edge of that 55mm diameter opening, they are angling in to hit the 43.3mm diameter of the sensor at a slope of (55-43.3)/16, so even if the rear lens elements are far further than 16mm, when the incoming rays are further the focal plane, they are even further from the central axis, so a throat further out would have to be wider to let them through.

For example, at 20mm depth, the throat diameter needed would be

43.3 + (55-43.3) x 20/16 = 58mm.

As a more extreme case, for the 46.5mm depth of F-mount, a lens with rear elements at least that 46.5mm back and pushing the limits of Z mount for light angles would require a throat diameter of

43.3 + (55-43.3) x 46.5/16 = 77mm !

This similar triangles reasoning is flawed because it assumes that rays are coming from the flange when what really matters is the lens surface closest to the sensor.

For example, even if the Flange to Focal Distance (FFD) were 20mm a 55mm diameter lens element positioned 16mm from the sensor would be sufficient; the throat would not have to be larger.

For example, here a pretty aggressive 50mm f/0.9 from a Nikon patent:



I added black tick marks at the Z-mount (16mm) and red tick marks further out at 32mm.
So a 32mm FFD that allowed for a 16mm Back Focal (BF) distance would be fine.

--
Bill ( Your trusted source for independent sensor data at PhotonsToPhotos )

Nice explanations by Canon:

https://downloads.canon.com/nw/camera/misc-pages/eos-r/pdf/canon_eos_r_white_paper.pdf

Consider the following:



The red block is the flange in the camera body. The blue is metal in the barrel of the lens, some of which mates with the left side of the flange, and some of which supports the lens by sticking through the flange. Lets say that barrel needs to be 2 or 3 mm thick. The various shades of light blue indicate the diagonals of three different sized sensors (I found it easier to explain this by changing the sensor size than by changing the throat, but you could copy and scale the drawing to represent the situation that way).

The FFD is the distance from the left side of the red block to the left side of the light blue blocks. The sensor stack is left out of this drawing.

Lets call the distance between the bottom of the blue block labeled Lens Barrel and that block on the other side of the lens (not shown here) the Optical Throat (made up term). The optical throat will be 4 to 6 mm smaller than what we have been calling the throat. Eliding how the elements are attached, light can pass freely through the optical throat.

The three sensor side shown are

1. (bottom) smaller than the optical throat.
2. (middle) equal to the optical throat.
3. (top) greater than the optical throat.

Let's take the middle case first, because it's the simplest. The sensor's view of the exit pupil will not be obscured if the diameter of the exit pupil is equal to or smaller than the optical throat. If the exit pupil is sufficiently near the sensor, the lens can be fast and the sensor will be fully illuminated. If the exit pupil is far from the sensor, the sensor will be fully illuminated only by slower lenses.

For sensors smaller than the optical throat, the size and position of the exit pupil that can fully illuminate the sensor depends on the angle that is limited by the top of the sensor, the optical throat, and the thickness of the dark blue part of the lens that mates with the flange.

For sensors larger than the optical throat, the size and position of the exit pupil that can fully illuminate the sensor depends on the angle that is limited by the top of the sensor, the optical throat, and the distance that the dark blue part of the lens extends into the camera beyond the flange.

Accommodating IBIS can be accomplished by increasing the size of the sensor.

Does that make sense?

Jim

--

As far as exit pupil visibility is concerned the diameter of the last lens surface is what matters. Naturally that is constrained by the throat diameter.

But there is no need for the exit pupil to be fully visible and for many lens designs it is not when the aperture is wide open.
(I'll bet you can prove with to yourself with lenses you have handy.)

Here's an example from Optics Primer - More on the Pupils:


50mm f/1.4 Nikon lens from US patent 8,934,182 B2 Example 2

Clearly a wider throat is desirable but it is not necessary.

The entrance pupil is another matter; the front element must always be able to accommodate it.

Regards,

--
Bill ( Your trusted source for independent sensor data at PhotonsToPhotos )

bclaff said:

As far as exit pupil visibility is concerned the diameter of the last lens surface is what matters. Naturally that is constrained by the throat diameter.

But there is no need for the exit pupil to be fully visible and for many lens designs it is not when the aperture is wide open.
(I'll bet you can prove with to yourself with lenses you have handy.)

Click to expand...

Like this?

bclaff said:

Here's an example from Optics Primer - More on the Pupils:


50mm f/1.4 Nikon lens from US patent 8,934,182 B2 Example 2

Click to expand...

That's interesting. Would you care to comment on the implications of the exit pupil not being planar? Is that the source (or a source) of focus curvature?

bclaff said:

Clearly a wider throat is desirable but it is not necessary.

Click to expand...

I'm in total agreement with that sentence.

--

JimKasson said:

Consider the following:


...
The three sensor side shown are

1. (bottom) smaller than the optical throat.
2. (middle) equal to the optical throat.
3. (top) greater than the optical throat.

Let's take the middle case first, because it's the simplest. The sensor's view of the exit pupil will not be obscured if the diameter of the exit pupil is equal to or smaller than the optical throat. If the exit pupil is sufficiently near the sensor, the lens can be fast and the sensor will be fully illuminated. If the exit pupil is far from the sensor, the sensor will be fully illuminated only by slower lenses.
...
Does that make sense?

Jim

Click to expand...

A possible translation being that the wider throat allows for the design and building more telecentric lenses? For edge to edge consistency, such telecentric designs like those used the in the Olympus 4/3 HG & SHG lenses showed the benefit of such an approach, especially wide open with the f/2 lenses.

Probably a stretch for my take on the design but it would seem that the wider throat would allow for the possibility of a line of more telecentric lenses...

You are right — for the case where the lens can be positioned with rear elements that close to the sensor. However or all but short focal lengths, the rear element and exit pupil will be further than 20mm away, and then the "similar triangles" calculation applies.

For your example plus a variant, if the rays at the outer edge of the light cone going to a point near the corner of the sensor have already left the rear element of the lens before range 20mm from the sensor, and then pass the 20mm mark at diameter

(a) 55mm (b) 58mm

they then angle in and so by the time they get to 16mm, they are at diameter

(a) 52.6mm (b) 55mm.

On the other hand, some lens designs with rear elements (and exit pupil) inside the throat diameter can even work with a threat diameter less than the sensor diagonal; that was true of some older 35mm film cameras and lenses, but not with the higher exit pupils needed by current mainstream sensors.

Take a 21mm lens with a flange distance of 40mm. Then you cannot focus simply to infinity if the front focal distance is equal to the back focal distance of the lens. In order to solve the problem, you need additional lens elements to compensate for that to elongate the back focal distance of a 21mm lens.

You can see that explicitly by comparing the Zeiss ZF 21mm lens design and the ZM 21mm lens for the Leica M system, a mirrorless range finder with a much shorter flange distance than the F-mount. The ZM lens is much more compact and significantly lighter even it illuminates a FX sensor.

16mm FFD even gives you additional advantages. For tele lenses you do gain much.