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

bclaff · Sep 5, 2018

JimKasson said:
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?

https://blog.kasson.com/?s=oof+psf

Is you site currently down?

JimKasson said:
JimKasson 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?

The pupils aren't perfectly flat although they are often flatter than in this example.

I'm unsure how or whether this contributes to focus curvature.

JimKasson said:
JimKasson said:

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

Click to expand...

I'm in total agreement with that sentence.

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

JimKasson · Sep 5, 2018

bclaff said:
JimKasson said:

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?

https://blog.kasson.com/?s=oof+psf

Click to expand...

Is you site currently down?

Hmm.. It's working for me.

Try this:

https://blog.kasson.com/a7riii/mitakon-zhongyi-speedmaster-50mm-f-0-95-oof-psfs/

Or this:

https://blog.kasson.com/

Or try it without SSL:

http://blog.kasson.com/

Thanks.

BTW, that test does not determine where the occlusion happens. Maybe the mount. Maybe somewhere else.

bclaff said:
bclaff said:

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?

Click to expand...

The pupils aren't perfectly flat although they are often flatter than in this example.

I'm unsure how or whether this contributes to focus curvature.

Thanks, Bill. Your diagrams are instructive and appreciated.

bclaff said:
bclaff said:

bclaff said:

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

Click to expand...

I'm in total agreement with that sentence.

Click to expand...

--
http://blog.kasson.com

bclaff · Sep 5, 2018

BJL said:
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.

I think you assume that the exit pupil location coincides with the rear principal plane but most lenses are not symmetrical.

For example:

b18dcd6abed949dcbf63ef8b6cb035fa.jpg.png

H' is 51mm from I because that's the focal length.
But P' is 25mm further away because this design isn't symmetrical.

Note:

387919215de344a489d6fb443892257c.jpg.png

The edge of the exit pupil isn't visible wide open.
(Top red ray not perpendicular to Image plane.)

Until we stop down from f/1.45 to about f/2.6:

e7121532d3124bbda2b23a2111801497.jpg.png

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

JimKasson · Sep 5, 2018

bclaff said:
BJL said:

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.

Click to expand...

I think you assume that the exit pupil location coincides with the rear principal plane but most lenses are not symmetrical.

For example:

H' is 51mm from I because that's the focal length.
But P' is 25mm further away because this design isn't symmetrical.

Note:

The edge of the exit pupil isn't visible wide open.
(Top red ray not perpendicular to Image plane.)

Until we stop down from f/1.45 to about f/2.6:

OK, bill, I think I need to go back to school. Can you recommend a See Spot Run primer on lens design that uses the notation above? Something simple for me to get started with.

--

the last word the last word - Photography meets digital computer technology. Photography wins -- most of the time.

Photography meets digital computer technology. Photography wins -- most of the time.

blog.kasson.com

bclaff · Sep 5, 2018

JimKasson said:
bclaff said:

BJL said:

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.

Click to expand...

I think you assume that the exit pupil location coincides with the rear principal plane but most lenses are not symmetrical.

For example:

H' is 51mm from I because that's the focal length.
But P' is 25mm further away because this design isn't symmetrical.

Note:

The edge of the exit pupil isn't visible wide open.
(Top red ray not perpendicular to Image plane.)

Until we stop down from f/1.45 to about f/2.6:

Click to expand...

OK, bill, I think I need to go back to school. Can you recommend a See Spot Run primer on lens design that uses the notation above? Something simple for me to get started with.

The H, P, F with/without apostrophe is pretty standard.
I also use I and O for the Image and Object planes.

For a really simple start you could look at my Optics Primer.

I'm sure others will have better references for a deeper understanding.

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

Antisthenes · Sep 5, 2018

JimKasson said:
bclaff said:

beatboxa said:

chambeshi said:

Good question in recurring topic that's been explored in threads since early in the year.

Some in depth posts with useful explanatory diagrams here, and citing older posts

A simple answer is the 16mm flange distance is one key to make the Z-Mount Future Proof. There also does appear to be a strategic advantage, because Nikon's is the widest, lens-mount closest to the sensor (just within Sony's).

Here's the instructive Figure by poster 9cx with [amended] earlier comparison after beatbox2:

Click to expand...

Here's another one:

https://www.dpreview.com/forums/post/61224903

A reminder that one must discuss both flange & throat diameter together when discussing optical constraints.

Flange distance affects minimum camera thickness.

Throat diameter affects minimum camera face size & lens diameter.

Click to expand...

In this diagram , produced from a Nikon patent for a 50mm f/0.9 lens, I have indicated the 55mm diameter mount at 16mm FFD:

Z-Mount is highlighted in yellow.

Note that this lens wouldn't even have any glass behind the mount.
Also note that because of housing no element will be as large as the throat.
The topmost green ray is nearly perpendicular to the Image plane.

Click to expand...

Nicely done, Bill. Looks to me like the second closest element to the sensor is close enough to the flange that there might not be room for sufficient metal to afford the kind of rigidity that a lens this heavy would need. It also looks like some of that element could be machined away with little optical effect, though.

Jim

I don't think practical issues like mechanical rigidity really matter for such a design, as the overall lens stack thickness of that 50/0.9 is about 260mm, meaning that it will probably only remain a theoretical, computer-based exercise.

The mock-up pictures of the planned 58/0.95 indicate, based on the Z mount dimensions as well as the front filter (82mm) diameter, a lens stack thickness of about 110mm.

See e.g. the picture on page 54 of this brochure:

http://cdn-4.nikon-cdn.com/e/Q5NM96...Qz5w==/Misc/Z_Engineer_Interview_Brochure.pdf

The 58/0.95 design looks thus far more realistic, from a marketability and usability point of view, than the 260mm+ monster in the patent.

In fact, the Nikkor 58/0.95 may well be shorter and thinner than the Canon R 28-70/2, which uses 95mm filters.

Antisthenes · Sep 5, 2018

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.)

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,

Note that, the shorter the FFD, the sharper the transition between the "well-illuminated" zone from which the exit pupil is fully visible, and the dark, "vignetted" areas away from the image center, where the pupil becomes partially obstructed.

A longer FFD (as in SLRs) projects a softer shadow, a fairly gentle penumbra, which is less visually objectionable than the comparatively sharper vignetting — i.e. a fairly abrupt shadow transition — one would get with a short FFD (as in mirrorless cameras).

This thus makes aspects like controlling the exit pupil distance and minimizing mechanical vignetting more important when designing lenses and camera mounts with short FFDs.

Alex Permit · Sep 5, 2018

JMD-70 said:
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?

I was discussing flange distance

bclaff · Sep 5, 2018

Antisthenes said:
Antisthenes said:

...

Click to expand...

I don't think practical issues like mechanical rigidity really matter for such a design, as the overall lens stack thickness of that 50/0.9 is about 260mm, meaning that it will probably only remain a theoretical, computer-based exercise.

The mock-up pictures of the planned 58/0.95 indicate, based on the Z mount dimensions as well as the front filter (82mm) diameter, a lens stack thickness of about 110mm.

See e.g. the picture on page 54 of this brochure:

http://cdn-4.nikon-cdn.com/e/Q5NM96...Qz5w==/Misc/Z_Engineer_Interview_Brochure.pdf

The 58/0.95 design looks thus far more realistic, from a marketability and usability point of view, than the 260mm+ monster in the patent.

In fact, the Nikkor 58/0.95 may well be shorter and thinner than the Canon R 28-70/2, which uses 95mm filters.

I agree. The lens design in the patent is not likely to be produced.

Do you (or does anyone) have any better patent references?
I'd be happy to get something more realistic into the Optical Bench.

BJL · Sep 5, 2018

Bill, I am assuming nothing about the exit pupil, I am just looking at rays after they leave the back of the lens and angle inwards to near the corners. If they leave the back of the lens further away than the deeper of the two mounts being compared, the similar triangles compare the throat diameters needed to avoid vignetting by the lens mount. Of course, a bit of vignetting is OK; there will be some cos^4 falloff anyway, and software can adjust for a small amount of such stuff

P.S. Of course for any exit pupil, once the f-stop is high enough, there are no rays “angling in”, and then the throat diameter does not need to be greater than image circle size; I am instead looking at the low f-stop regime where some rays do angle inward.

--
Smaller lenses, better in low light, more telephoto reach:
you can have any _two_ at one time.

bclaff · Sep 5, 2018

BJL said:
Bill, I am assuming nothing about the exit pupil, I am just looking at rays after they leave the back of the lens and angle inwards to near the corners. If they leave the back of the lens further away than the deeper of the two mounts being compared, the similar triangles compare the throat diameters needed to avoid vignetting by the lens mount. Of course, a bit of vignetting is OK; there will be some cos^4 falloff anyway, and software can adjust for a small amount of such stuff

P.S. Of course for any exit pupil, once the f-stop is high enough, there are no rays “angling in”, and then the throat diameter does not need to be greater than image circle size; I am instead looking at the low f-stop regime where some rays do angle inward.

You may not realize it but the angle at which the rays leave the last lens surface is influenced by the location of the exit pupil.

BJL · Sep 5, 2018

bclaff said:
BJL said:

Bill, I am assuming nothing about the exit pupil, I am just looking at rays after they leave the back of the lens and angle inwards to near the corners. If they leave the back of the lens further away than the deeper of the two mounts being compared, the similar triangles compare the throat diameters needed to avoid vignetting by the lens mount. Of course, a bit of vignetting is OK; there will be some cos^4 falloff anyway, and software can adjust for a small amount of such stuff

P.S. Of course for any exit pupil, once the f-stop is high enough, there are no rays “angling in”, and then the throat diameter does not need to be greater than image circle size; I am instead looking at the low f-stop regime where some rays do angle inward.

Click to expand...

You may not realize it but the angle at which the rays leave the last lens surface is influenced by the location of the exit pupil.

Of course I realize that, but once there are rays that "angle in" (true for low enough f-stop with any given exit pupil) and the rear element is beyond the mount, the similar triangles calculation works. Put it this way: I am comparing which combinations of [high] exit pupil and [low] f-stop can be used without vignetting by the lens mount.

bclaff · Sep 5, 2018

BJL said:
bclaff said:

BJL said:

Bill, I am assuming nothing about the exit pupil, I am just looking at rays after they leave the back of the lens and angle inwards to near the corners. If they leave the back of the lens further away than the deeper of the two mounts being compared, the similar triangles compare the throat diameters needed to avoid vignetting by the lens mount. Of course, a bit of vignetting is OK; there will be some cos^4 falloff anyway, and software can adjust for a small amount of such stuff

P.S. Of course for any exit pupil, once the f-stop is high enough, there are no rays “angling in”, and then the throat diameter does not need to be greater than image circle size; I am instead looking at the low f-stop regime where some rays do angle inward.

Click to expand...

You may not realize it but the angle at which the rays leave the last lens surface is influenced by the location of the exit pupil.

Click to expand...

Of course I realize that, but once there are rays that "angle in" (true for low enough f-stop with any given exit pupil) and the rear element is beyond the mount, the similar triangles calculation works. Put it this way: I am comparing which combinations of [high] exit pupil and [low] f-stop can be used without vignetting by the lens mount.

I don't think we're really disagreeing.
I just think it's a simplistic approximation.

Astrophotographer 10 · Sep 5, 2018

Good video, thanks.

Greg.

Astrophotographer 10 · Sep 5, 2018

JimKasson said:
jonikon said:

It seems counterintuiive to have a shorter flange focal distance (FFD) on a FF camera than that of a 1” sensor (17mm), or m43 (19.25mm). So why did Nikon chose 16mm for FFD. I have no doubt Nikon has a very good reason for their choice, but i am curious as to what it is. Has Nikon explained their choice of a16mm FFD?

Click to expand...

The official line, as stated in the Nikon optical engineer's video, is that that was as short as they could make it and still have room for the sensor stack and the shutter.

But that can't be the whole story, since two of the three announced lenses have optical elements that extend behind the flange. There must be some spec for how far mechanical and optical elements can project behind the flange, but I don't know what it is.

Jim

I think they just wanted the shortest flange distance knowing full well Canon was going with 18mm in advance so an adapter was possible to use a Canon lens on a Z body but not vice versa just like it is now with their DSLRs. Possibly even an adapter for Sony E to Nikon Z. Not sure if there is enough room for a smart adapter but perhaps as Z is considerably wider in aperture than Sony E.

Greg.

beatboxa · Sep 5, 2018

Astrophotographer 10 said:
JimKasson said:

jonikon said:

It seems counterintuiive to have a shorter flange focal distance (FFD) on a FF camera than that of a 1” sensor (17mm), or m43 (19.25mm). So why did Nikon chose 16mm for FFD. I have no doubt Nikon has a very good reason for their choice, but i am curious as to what it is. Has Nikon explained their choice of a16mm FFD?

Click to expand...

The official line, as stated in the Nikon optical engineer's video, is that that was as short as they could make it and still have room for the sensor stack and the shutter.

But that can't be the whole story, since two of the three announced lenses have optical elements that extend behind the flange. There must be some spec for how far mechanical and optical elements can project behind the flange, but I don't know what it is.

Jim

Click to expand...

I think they just wanted the shortest flange distance knowing full well Canon was going with 18mm in advance so an adapter was possible to use a Canon lens on a Z body but not vice versa just like it is now with their DSLRs. Possibly even an adapter for Sony E to Nikon Z. Not sure if there is enough room for a smart adapter but perhaps as Z is considerably wider in aperture than Sony E.

Greg.

Canon RF has a 20mm flange distance.

But one important difference is that all of Canon's RF lenses are / will require fully electronic communication. In other words, a smart adapter is required. This is unlike historically, where lenses had things like mechanical apertures.

This means that the 4mm thick RF to Z adapter would also have to have room for wiring & contacts, circuitry to convert signals between the two, and material to securely fasten to the camera & lens, including the lens' & camera's release lock mechanisms.

While it could be possible, I don't think we'll be seeing this. I'd bet we'll see adapters for every DSLR lens onto every mirrorless mount, but I can't foresee adapters between the various mirrorless cameras.

BJL · Sep 6, 2018

I agree that I am only considering one case, and the rigid demand of no vignetting at all; I just offered it as one possible advantage. Having seen a possible 50mm lens design with rear elements within 20mm or less of the focal plane, that design flexibility could be another factor, and even a more important one.

What is your best bet at the reason for 16mm rather than 18 or 20?

bclaff · Sep 6, 2018

BJL said:
...

What is your best bet at the reason for 16mm rather than 18 or 20?

I think Nikon chose the throat diameter and Flange to Focal Distance (FFD) to be as wide and short as they could because they wanted a mount specification that could last for many years (in the launch event they said "the next 100 years").
This is certainly consistent with how long the F-mount lasted; they are committed to a philosophy of introducing new mounts very infrequently (Nikon 1 not withstanding; that was a different necessity).

Just my opinion and of course it doesn't change the facts.

Dibyendu Majumdar · Sep 6, 2018

bclaff said:
BJL said:

...

What is your best bet at the reason for 16mm rather than 18 or 20?

Click to expand...

I think Nikon chose the throat diameter and Flange to Focal Distance (FFD) to be as wide and short as they could because they wanted a mount specification that could last for many years (in the launch event they said "the next 100 years").
This is certainly consistent with how long the F-mount lasted; they are committed to a philosophy of introducing new mounts very infrequently (Nikon 1 not withstanding; that was a different necessity).

I think that making the flange distance as short as physically possible also means that there is more space at the back of the sensor - this might be beneficial for a number of reasons as presumably it gives a few milli-meters of additional space.

Regards

Dibyendu

JimKasson · Sep 6, 2018

bclaff said:
BJL said:

...

What is your best bet at the reason for 16mm rather than 18 or 20?

Click to expand...

I think Nikon chose the throat diameter and Flange to Focal Distance (FFD) to be as wide and short as they could because they wanted a mount specification that could last for many years (in the launch event they said "the next 100 years").

If they'd have waited for inexpensive FF global shutters, it could have been even shorter, but I don't think they had that luxury.

bclaff said:
This is certainly consistent with how long the F-mount lasted; they are committed to a philosophy of introducing new mounts very infrequently (Nikon 1 not withstanding; that was a different necessity).

Jim

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

Forum Pro

Community Leader

Forum Pro

Community Leader

Forum Pro

Forum Enthusiast

Forum Enthusiast

Senior Member

Forum Pro

Veteran Member

Forum Pro

Veteran Member

Forum Pro

Forum Pro

Forum Pro

Veteran Member

Veteran Member

Forum Pro

Dibyendu Majumdar

Guest

Community Leader

Similar threads

Keyboard shortcuts