Finding f-stops for remounted lens

[Jim Noel]

Any suggestions for best way to find f-stops for lens mounted in a different shutter?
I just moved my Protar IIa from an almost worthless Compound to a modern shutter. I know I can send it off and pay a fortune to get it done,but there has to be a reasonable method for DIY. If nothing else, I will aim it at a constant bright area, mount it in the old shutter, check the brightness on the GG, then move it to the new shutter, and close the aperture until it reads the same. A log and tedious process.
Thanks for your ideas.

[Dugan]

Focal length measurement in mm. divided by apparent aperture measurement in mm. = f stop.
Aperture measured from front of lens, through elements.

[Chauncey Walden]

Precompute the required apertures by dividing the focal length of the lens by the desired f/stop number - 5.6 - 8 - 11 -16 - 22 - 32 and so forth. Looking through the front of the lens (the glass elements must be in place) measure the diameter of the seen aperture. As you close down the aperture and each precomputed diameter is reached mark your shutter's position. Simple.

[C. D. Keth]

These guys hit it on the head. If your aperture isn’t very round, like an octagon, I just consider the average of the ‘flat to flat’ and the ‘point to point’ measurements and figure that’s very close.


Sent from my iPhone using Tapatalk

[Bob Salomon]

I just use my light meter. First determine the light absorption of the gg/fresnel. Enter that as a filter factor. Point the camera at an evenly illuminated surface, use the microscope attachment and open the aperture all the way. Note the aperture on the meter and then close the aperture while noting each 1/3rd change on the meter. Now your lens is calibrated in 1/3rd stops. Just make your scale marked accordingly. No math needed. Just make sure that you adjust your meter to take any bellows factor into consideration. Or point the camera at clear sky for no bellows factor.

[Jim Noel]

I know well how to compute f-stops by measuring the apparent aperture and applying the formula. I was looking for a method which is a little faster more practical. This Is a triple convertible lens.
Bob seems to have the most practical method. I had considered a very similar method, and will use his suggestion.
THanks to everyone for your assistance.

[B.S.Kumar]

A similar approach, using a DSLR:

https://www.largeformatphotography.info/forum/showthread.php?139119-Some-Questions-and-Observations-About-Making-an-Aperture-Scale

Kumar

[Jim Noel]

I have nothing to do with DSLR's, or the images they produce.

[Pere Casals]

Any suggestions for best way to find f-stops for lens mounted in a different shutter?


Jim, let me tell the way I used several times to calibrate apertures, as I rescued many of my lenses from trash I ended swapping several shutters, also I upgrades several lenses by purchasing the cells alone...


1) Prepare a "wall" (indoors is possible) that has an stable and uniform illumination

2) Take a patron lens that has well calibrated apertures, mounted in the view camera focus at infinite.

3) Point (with the patron lens) to the wall you prepared, use a luxometer to measure the LUX reading in the GG plane (GG removed may be better perhaps, id the patron lens has very different focal) in all apertures , you may sustitute the GG by a clear glass for total precision in the sensor position, you may use a $20 luxometer for that:

202088

4) Rest is easy, isn't it ? Mount the lens you want you calibrate, focus infinite, point to the approx. same point on the wall, ajust aperture to match the LUX reading in the patron lens for each aperture, and make the marks in the aperture scale.


It is a transmission based aperture, not a geometric based aperture, but the newly calibrated lens will expose like the patron lens, speed accuracy apart.


I have this one https://www.amazon.com/-/es/2000-Cuentas-200000-Lux-Resoluci%C3%B3n-Lumenmeter-luminometer/dp/B01J2KM3LW/ref=sr_1_45?__mk_es_US=%C3%85M%C3%85%C5%BD%C3%95%C3%91&keywords=luxometer&qid=1585504212&sr=8-45

But the one having the cord for the sensor is more convenient, I got that one because it is takes 0.01 readings, which I use to calibrate film in LIRF conditions.


Another choice is mounting a (say) Nikon F-Mount extension ring on a monorail camera lensboard, place that lensboard in the rear standard and mount an SLR on it, (so you don't need to touch a DSLR :)) use the camera meter readings to match the reading of the apertures of the two lenses, the patron one and the one you adjust.


You may also use a probe meter if you have one:

202089


...but the numeric reading in the luxometer will allow a more accurate matching.

_____


Not necessary to tell you that the view camera has to focused at infinite which each lens to avoid the compensation factor... I point it because somebody else may use that procedure in the future...

____

After doing that and calibrating shutter speeds exposures were totally perfect.

[Greg]

Beware of some older lenses that the iris opened up wider than the listed aperture for the lens. I have a 12 3/5" Protar Series VII which is listed as a f/7.7 optic. The iris actually needs to be closed down about one stop to give you that f/7.7 (starting) aperture. On my 5.9" No. 5 Periscope lens, its listed "maximum" aperture is f/14. On this lens the iris has to be closed more than one f stop from its maximum physical opening to give you its listed f/14 "maximum" aperture. Read that for the Periscope lens, the aperture could be opened up wider that its listed "maximum" aperture for "ease in focusing".

[Kiwi7475]

Jim, let me tell the way I used several times to calibrate apertures, as I rescued many of my lenses from trash I ended swapping several shutters, also I upgrades several lenses by purchasing the cells alone...


1) Prepare a "wall" (indoors is possible) that has an stable and uniform illumination

2) Take a patron lens that has well calibrated apertures, mounted in the view camera focus at infinite.

3) Point (with the patron lens) to the wall you prepared, use a luxometer to measure the LUX reading in the GG plane (GG removed may be better perhaps, id the patron lens has very different focal) in all apertures , you may sustitute the GG by a clear glass for total precision in the sensor position, you may use a $20 luxometer for that:

202088

4) Rest is easy, isn't it ? Mount the lens you want you calibrate, focus infinite, point to the approx. same point on the wall, ajust aperture to match the LUX reading in the patron lens for each aperture, and make the marks in the aperture scale.


It is a transmission based aperture, not a geometric based aperture, but the newly calibrated lens will expose like the patron lens, speed accuracy apart.


I have this one https://www.amazon.com/-/es/2000-Cuentas-200000-Lux-Resoluci%C3%B3n-Lumenmeter-luminometer/dp/B01J2KM3LW/ref=sr_1_45?__mk_es_US=%C3%85M%C3%85%C5%BD%C3%95%C3%91&keywords=luxometer&qid=1585504212&sr=8-45

But the one having the cord for the sensor is more convenient, I got that one because it is takes 0.01 readings, which I use to calibrate film in LIRF conditions.


Another choice is mounting a (say) Nikon F-Mount extension ring on a monorail camera lensboard, place that lensboard in the rear standard and mount an SLR on it, (so you don't need to touch a DSLR :)) use the camera meter readings to match the reading of the apertures of the two lenses, the patron one and the one you adjust.


You may also use a probe meter if you have one:

202089


...but the numeric reading in the luxometer will allow a more accurate matching.

_____


Not necessary to tell you that the view camera has to focused at infinite which each lens to avoid the compensation factor... I point it because somebody else may use that procedure in the future...

____

After doing that and calibrating shutter speeds exposures were totally perfect.

Interesting approach, thanks for sharing. What’s the advantage of using a luxometer vs a meter (since we already have one) for this application?

[Peter De Smidt]

Bob's method works great. I use a related method. I have an incident meter that has a magnet. I simply put the whole meter inside my camera. It reads to my phone in 1/10th stops, which is close enough for me.

[Pere Casals]

Interesting approach, thanks for sharing. What’s the advantage of using a luxometer vs a meter (since we already have one) for this application?

If you use a meter (depending on the particular model) then you may spect a 1/3 stop precision when matching the apertures of the patron lens with the serviced lens, instead with the Luxometer you can nail a 1/1000 precision if you want, you have more precision than required, because the meter will give you 3 significative digits.

Some meters like this Sekonik also tells LUX and works like a luxometer and sensor can be rotated to face the lens while taking the reading, but other meters don't tell such an accurate reading as fractions of stops are told.

202093


The trasmission approach is the most precise, easy and effective for DIY. The geometric approach is simply worse if our priority is making accurate exposures. I tried both ways and compared, with a DIY geometric aproach you may spect 1/2 stops shifts in the middle apertures, and a very high error in the small apertures, even 1 stop, while with the transmission way you nail 100% the matching. Of course an optical lab has the equipment to adjust the aperture gemetrically with total perfection, but in the DIY realm you have to go to transmission way if you want good precision, in special for the smaller apertures, errors from the DIY geometric way are not acceptable.

The Luxometer is also interesting to see how precise is our aperture scale, as lux have to follow the right progression, also we can check what effective fall-off we have at different apertures in the corners.


Also I often use the Luxometer to measure fall-off on the enlarger's easel, the enlarger I use, a 138, can move the bulb x-y-z which has an effect on the fall-off, and of course I use it for film/paper calibrations with absolute H scale, so those $20 were quite well invested.

[Kiwi7475]

If you use a metter (depending on the particular model) then you may spect a 1/3 stop precision when matching the apertures of the patron lens with the serviced lens, instead with the Luxometer you can nail a 1/1000 precision if you want, you have more precision than required, because the meter will give you 3 significative digits.

Some meters like this Sekonik also tells LUX and works like a luxometer and sensor can be rotated to face the lens while taking the reading, but other meters don't tell sach an accurate reading as fractions of stops is told.

202093


The trasmission approach is the most precise, easy and effective. The geometric approach is simply worse if our priority is making accurate exposures. I tried both ways and compared, with a DIY geometric aproach you may spect 1/2 stops shifts in the middle apertures, and a very high error in the small apertures, even 1 stop, while with the transmission way you nail 100% the matching. Of course an optical lab has the equipment to adjust the aperture gemetrically with total perfection, but in the DIY realm you have to go to transmission way if you want good precision, in special for the smaller apertures, errors from the DIY geometric way are not acceptable. Some are not aware...

The Luxometer is also interesting to see how precise is our aperture scale, as lux have to follow the rights progression, also we can check what effective fall-off we have at different apertures.


Also I often use the Luxometer to measure fall-off on the enlarger's easel, the enlarger I use, a 138, can move the bulb x-y-z which has an effect on the fall-off, and of course I use it for film/paper calibrations with absolute H scale, so those $20 were quite well invested.

Thank you! Very useful and clear.

[Pere Casals]

Thank you! Very useful and clear.

One thing else, with the transmission way you have to take the reading in the center of the image circle, to avoid fall-off influence in the reading.

Regards

[Bob Salomon]

One thing else, with the transmission way you have to take the reading in the center of the image circle, to avoid fall-off influence in the reading.

Regards

Actually, if you have a modern Linhof - one made after the 3, then you can use their Focus/Metering Bellows. This attaches to the gg frame and, on older ones, had a 2x magnifier built in. The later one had two 2x magnifiers screwed together.
The bellows eyepiece accepted a Gossen with the microscope attachment. This allowed metering an area the diameter of a quarter on any point of the gg or, by extending the bellows out, the entire gg. To use the later version for metering you unscrewed the upper magnifier.
Of course, for this to work at any point of the gg, you have to have the fresnel on the gg installed.

[Pere Casals]

Focus/Metering Bellows.

The great thing of metering in that way is that we don't even need an aperture scale to have perfect exposures...

[Greg]

One thing else, with the transmission way you have to take the reading in the center of the image circle, to avoid fall-off influence in the reading.

Regards

Personally am able to remove the GG and fresnel from the equation by using a Horseman Exposure Meter 45. It slips in as a film holder does and measures an area 4 5/8" x 3 1/2". Actually preferred it to a Sinar Six when I was doing a lot of Photomicrography and Photomacrography and shooting 4x5 Chromes. It was interesting to mount it on the back of my 4x5 and mount my lenses on the front all focused at infinity facing a constantly lit white wall. All f/16s are not the same! Pretty much the same for modern lenses, but on some of my pre 1900 lenses, you would easily see a variation of plus or minus 1/2 f/stop. A brass Petzval I had used warehouse stops (home made by its previous owner), they were beautifully machined from brass but were all off by a whole f/stop, of which I always had a hunch of.

[Jim Noel]

I used a combination of Bob and Greg's ideas to complete my task. I then compared the diameter of the openings with the results of computations and the few legible markings on original shutter.
Thanks to all who contributed and got me thinking of the quickest and most accurate method of solving my problem.

[Pere Casals]

All f/16s are not the same! Pretty much the same for modern lenses, but on some of my pre 1900 lenses, you would easily see a variation of plus or minus 1/2 f/stop. A brass Petzval I had used warehouse stops (home made by its previous owner), they were beautifully machined from brass but were all off by a whole f/stop, of which I always had a hunch of.

Of course, coatings changed that game, mainly after WWII. Single coated have a little loss, and a multicoated lenses require a lot of gropus to notice a loss



A brass Petzval I had used warehouse stops (home made by its previous owner), they were beautifully machined from brass but were all off by a whole f/stop, of which I always had a hunch of.

Yes... 3 groups are 6 uncoated surfaces, so we may expect 0.9^6 = 0.53 , to near exactly one stop less, what you say. An uncoated petzval transmits 50% of theoric light power !


... but a MC plasmat may not loss a 2% of the light, I guess.

[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.

[Iga]

This is a sound way to do it !

Pere, tienes tu buzo de entrada lleno. No puedo enviarte el PM.
Un saludo,
Igor.