Finding f-stops for remounted lens

[Chauncey Walden]

Pere, what kind of lux readings do you get through the lens off a normally lit wall? 2 digits, 3 digits, 4 digits? How much change in lux is 1 stop?

[C. D. Keth]

Pere, what kind of lux readings do you get through the lens off a normally lit wall? 2 digits, 3 digits, 4 digits? How much change in lux is 1 stop?

Difference between stops is a doubling or halving of light allowed through. If f4 lets 850 lux to your groundglass, then f2.8 allows 1700 lux and f5.6 lets 425 lux through and so on.

[Kiwi7475]

I have a related but different question. How do I know if a lens requires spacers/shims on either end when mounting it on a new shutter (or when swapping lenses on one shutter). I mean if it’s really off I assume the GG will show but I won’t be able to check if the quality is degraded due to uncorrected aberrations. Is there a way to find out/optimize the inner spacing without specialized equipment?
(to be clear I’m talking about needing to possibly adjust the distance between the front and back elements beyond what the shutter naturally creates).

[Pere Casals]

Pere, what kind of lux readings do you get through the lens off a normally lit wall? 2 digits, 3 digits, 4 digits? How much change in lux is 1 stop?

Hello Chauncey,

Time ago I asked that question to Sammuel Bigler, from his kind and extensive answer, simplified for the image center of the image circle:

(lux in focal plane) = R . (incident lux) . 1/(1+4N^2)

R is the reflectance of the wall (we consider lambertian reflectiveness) and N is the f-number.

We consider that lens transmission is near 100% (multicoated), an uncoated petzval woud transmit the half.


Off-center the lens always may have some fall-off...


So considering a white wall reflecting 80%, if we illuminate the wall with 300 lux, at f/5.6 we get: 300*0.8 / (1 + 4 * 5.6^2) = 240 /126 = 2 LUX aprox.


So for the smallest aperture we may need a very bright wall and/or a 0.01 precision luxmeter ($20 ...). We always may use sunlight, which throws 30,000 to 100.000 lux on walls.

I asked that to Sammuel to be aware about how spot meterings in the scene relates to a film calibration curve, to learn, I wanted to predict the exact density in the negative I was to obtain from the spot metering, just an exercise to evaluate the process precission, of course this is less a need for regular shooting.



Regards

[Pere Casals]

I have a related but different question. How do I know if a lens requires spacers/shims on either end when mounting it on a new shutter (or when swapping lenses on one shutter). I mean if it’s really off I assume the GG will show but I won’t be able to check if the quality is degraded due to uncorrected aberrations. Is there a way to find out/optimize the inner spacing without specialized equipment?
(to be clear I’m talking about needing to possibly adjust the distance between the front and back elements beyond what the shutter naturally creates).

There are lenses that include shims and others don't. For example Fujinons don't include shims, it would be rare.


Shims are a bit a mess, but some designs do include shimming in the product design to allow to refine performance, remarkably many Sironars do. Those designs are specifically intended to allow shimming do an specific optimization job.


Some Ebay sellers (and other) are not aware about the importance of shims, and they may use the shutter of a broken glass for a lens with a destroyed shutter, and perhaps the shims remained in the old shutter and were lost... beyond aperture scale missmatch...


You may suspect shimming is wrong if your corners are sub-optimal, you may unscreew the font cell until you have a good sharpness balance across all image circle. To make that job you may attach a DSLR in the back of the view camera (tethering), and you unscreew (front cell) and re-focus to see the effect. Depart with the lens with no shim and unscreew until optimal in the corners/mid (mid is midpoint between center and corner), possibly center will be always good, but check it. Then... from the number of tours it takes to screew the front cell and the thread pitch you know the shim you need.


It is possible that some manufacturer service is still offering a shimming service, some $ hundreds, but a refined job.

Inter-cell spacing may also be used to optimize for a certain subject distance range, many reproduction lenses (optimized for close subjects) can be optimized for distant subjects by adding a shim.

[Kiwi7475]

There are lenses that include shims and others don't. For example Fujinons don't include shims, it would be rare.


Shims are a bit a mess, but some designs do include shimming in the product design to allow to refine performance, remarkably many Sironars do. Those designs are specifically intended to allow shimming do an specific optimization job.


Some Ebay sellers (and other) are not aware about the importance of shims, and they may use the shutter of a broken glass for a lens with a destroyed shutter, and perhaps the shims remained in the old shutter and were lost... beyond aperture scale missmatch...


You may suspect shimming is wrong if your corners are sub-optimal, you may unscreew the font cell until you have a good sharpness balance across all image circle. To make that job you may attach a DSLR in the back of the view camera (tethering), and you unscreew (front cell) and re-focus to see the effect. Depart with the lens with no shim and unscreew until optimal in the corners/mid (mid is midpoint between center and corner), possibly center will be always good, but check it. Then... from the number of tours it takes to screew the front cell and the thread pitch you know the shim you need.


It is possible that some manufacturer service is still offering a shimming service, some $ hundreds, but a refined job.

Inter-cell spacing may also be used to optimize for a certain subject distance range, many reproduction lenses (optimized for close subjects) can be optimized for distant subjects by adding a shim.

Thank you Pere! Very useful. Do you know if Nikkor’s usually use shims or if it’s rare? That’s the one I’m currently pondering on. I’ll follow the process you describe anyway.

[Pere Casals]

Thank you Pere! Very useful. Do you know if Nikkor’s usually use shims or if it’s rare? That’s the one I’m currently pondering on. I’ll follow the process you describe anyway.

Some Nikon models may have shims, I use a Nikon W210 than has one, and a W360 has 3 shims. Also Nikon SW 90mm f/8 reportedly often has shims...

[Iga]

https://apenasimagens.com/en/measuring-lens-aperture/

Hope this helps.
Best,
Igor

[Pere Casals]

https://apenasimagens.com/en/measuring-lens-aperture/

Hope this helps.
Best,
Igor

This is a sound way to do it !

[Iga]

This guy was teacher, so he can explain well :-)

Best,
Igor.