Some Questions and Observations About Making an Aperture Scale

[Paul Kinzer]

I recently bought the cells for a 90mm f/5.6 Super Angulon lens, and I've put them into a Seiko 0 shutter. (The Seiko 0 and the Copal 0 have the same specs, so the cells fit just fine). The Seiko shutter once held a different lens, and the aperture scale goes from f/6.3-64. I have to close the aperture past that f/6.3 setting before the blades show on the SA. I want to make my own aperture scale for this lens, and have some questions that I hope folks here can advise me on.

I've done a bunch of reading on this, and want to make a few points before getting to my questions. First, I will not send the lens in to have a scale professionally made. I got very good deals on both the glass and the shutter, and after checking on the cost of pro-made scales, I found that I would add nearly 50% to the cost of the lens by buying one. I'm cheap, so that's right out. Second, I know that shutters are not perfectly accurate, and that imaging is at least somewhat forgiving, so a perfectly accurate scale is not needed (one more reason to forego a pro-made scale). Finally, the only light meter I own is in my DSLR, so the descriptions of making scales based on the actual light coming through is not easy for me to understand, and may be impossible to do.

In my reading, several folks expressed the view, based on some of my points above, that simply using a ruler to measure the opening as seen through the front of the lens is good enough to use as a tool for marking a DIY aperture scale. Dividing the focal length by the f/stop gives what the diameter of the opening should be. This is what I've been messing about with so far, and my questions concern what I've found.

First, when the lens is wide open, the aperture should be 16.07 mm across (90/5.6 = 16.07). But when I use a ruler to measure the wide open lens, it looks to be 15mm across. Does this mean that the glass in the lens bends the light in such a way that the diameter I'm seeing is off of the 'true' aperture? (I imagine this is so; either that, or Schneider just gave their f/6 lens [90/15 = 6] a generous rounding off to f/5.6.) If this is true, should I, when making my calculations, subtract a proportional amount from each calculation? What I mean is, 15 is 0.93 x 16.07, so should I multiply each calculation by that amount before taking me measurements? 90/8 = 11.25; 11.25 x 0.93 = 10.46. So I would look for about 10.5 mm as the diameter of my f/8 setting?

Next, how do I measure the 'diameter' of the aperture when the opening is a pentagon? That 15mm measurement is the wide open circular view through the glass in the cells, but when I stop down even to about f/8, the sides of the five-bladed aperture get pretty flat. I'm not a math guy, so I looked online to try to find a formula for figuring this out. This confirmed that, yes, I'm not a math guy. Still, I found a calculator that showed me the area of pentagons drawn on virtual graph paper. I found that, if I measure from a point of the pentagon straight across to the middle of the opposite side, that 'diameter' is very close. A circle with the same diameter has 1.07 times the area. Is this close enough?

And here's an odd thing: if the first thing I described -- the 16.07 supposed aperture measuring a visual 15mm -- means I should shrink my measurements a bit (x 0.93), then the second thing -- a measurement across a pentagon is somewhat smaller than the diameter of a circle that has the same area -- means I should make my measurements a bit bigger (x 1.07). Again, I'm not a math guy, but it seems to me that, for this particular glass in this shutter, I should multiply those two numbers together to tell me how to measure the apparent aperture. And it turns out that 0.93 x 1.07 = 0.9951! So I should just use the ruler and go by the distance across the pentagon and mark based on that.

I hope someone can check my thinking for me.

I also hope that what I've written, if it makes sense and is close enough, might help others who want to make their own aperture scales.

[LabRat]

The EZ way for this is that you know that the lens is f5.6, so you have a baseline... I have posted this before, so you can look for this there, or here is the quicky method... Focus lens to INF, aim at a very bright evenly lit wall, make a label strip under the f- pointer, press a spotmeter on the center of the GG, and with a dark cloth over you, take a reading of that spot on the GG... Set meter for the reading for f5.6... Keep the exact spot, but stop down until meter reads -1 stop, then make a dot on the white strip, and repeat until stopped down completely... Add the #'s and you will have a decent scale to work with... Then some film tests, but this will get you an OK seat in the ballpark...

Good Luck!!!

Steve K

[B.S.Kumar]

Paul, I think I suggested the same approach, but with a DSLR filling the frame, and in spot meter mode...

Kumar

[Paul Kinzer]

Hmmm...

Thanks, Steve and Kumar. Unfortunately, my tired and not overly bright brain is not getting it.

Do I set the lens at its infinity focus, but then aim it at said wall at a closer distance? And just how should I use the DSLR as a light meter in this situation? When I've used it as a meter for photography with un-metered film cameras, I use a lens on it with a similar focal length to the film camera, and frame the two cameras similarly on the view to be imaged. I've been spoiled by the auto-everything world, and am trying to learn.

My ignorance is large and a bit embarrassed, but I can live with it if I can get this done. I'd rather ask questions that show my denseness than flounder, and my public questions might help others.

I'd be happy to try this method if I have the proper gear -- and it sounds like I do. If this has all been explained before, I'd love a link, or terms to search for.

[B.S.Kumar]

No doubt someone more technically oriented will correct me...

1. Light a gray or white background in a darkened studio as if you were copying something.
2. Point your view camera (on a tripod) with the 90mm lens at this surface. A defocused image is best.
3. Tape a piece of paper on the aperture scale.
4. Get hold of a fine point marker pen.
5. Close the aperture until the point where the aperture leaves would intrude into a perfect circle. Mark that spot as f/5.6.
6. On a second tripod behind the view camera, mount a DSLR with any lens.
7. Fill the frame with the ground glass of the view camera. Again, it is best to have a defocused image.
8. Put the DSLR into spot metering mode.
9. Take a reading from the center of the ground glass and note it down. Let's assume it is f/2 at 1 seconds.
10. Have someone close down the aperture on the 90mm Super Angulon until you get a reading of f/2 at 2 seconds. Mark it as f/8.
11. Continue until the aperture on the SA is at the smallest opening.
12. Double check your DSLR readings against the markings.
13. When all the apertures are marked, you can decide whether to do this for intermediate stops.
14. Test, using roll film.
15. Make a more permanent scale by using a laser printer.

Kumar

[Paul Kinzer]

I've done more searching, and found different schools of thought. Some folks describe something similar to what you two are describing. Others say that measuring the 'apparent aperture' can be done, and that it is more accurate. I am not concerned with perfection. Some say that doing what I suggest in my OP worked for them and was good enough (but is it?).

Here's a quote from a thread on the topic that seems easy enough:

"You can measure the apparent aperture by putting the lens assm. on a lightbox and put a sheet of frosted glass/ acetate/ paper across the front lens. You can do the math to find the needed sizes and draw them out on the paper. Stop down until they match and mark your tape.

(Actually this should be done with a point-light source at the focal length for inf. but the lightbox is easier.)"

I found the quote in this (http://www.largeformatphotography.info/forum/showthread.php?16346-calculating-aperture-scale-an-easy-way/page2&highlight=aperture+scale) thread.

Is there a reason not to do this? I have a light table about five feet away from me....

[Paul Kinzer]

I wrote my post while you wrote yours, Kumar.

Thanks for being really clear in your description (I need clarity!)

If your way is the best way to go about this, then I can do that, but is there a reason not to do what I quoted in my last post?

In addition to being not very bright (when it comes to this topic, at least) I'm also lazy, and the light table is right there.

[B.S.Kumar]

Math is tough for me, but if it's easier for you, go ahead. You can also use both methods for cross verification.

Kumar

[Paul Kinzer]

Well, it's 3:00 am here, so stringing words together is tough for me right now! And someone in my home seems to have borrowed my light table, too....

The math I think he's referring to in that quote (if that's what you're referring to) is just the focal length divided by the f/number: 90/5.6 = 16.07, and so on.

[B.S.Kumar]

In the past, I've tried the "apparent aperture" method, and it was a pain trying to measure it, considering that I have only two hands. Using a vernier caliper without having it actually touch something is also hard for me. The method I outlined above works for me.

Kumar

[Paul Kinzer]

In another thread (http://www.largeformatphotography.info/forum/showthread.php?75124-How-do-you-create-an-aperture-scale-for-a-lens&highlight=aperture+scale), Jim Galli wrote:

'You folks always strain at gnats in these discussions, and you are of course correct. But in the practical world of getting some pictures made, the errors we live with in our meters, + or - 15-18%, and in our shutters, + or - 30% plus the human error in our zone calcs + or - unknown%, make the pupil error of no consequence at all really. Just get a ruler and start making some photos with it. You'll be very very close doing it the way I outlined.'

He was talking about just measuring the aperture as seen.

Ther next commenter wrote:

'... and sometimes what we hope are precise or accurate methods may not be as precise or accurate as they might appear and we may as well use an approximate method. A caveat with the method of measuring at the film plane is that angle of incidence may affect the light meter reading more than it affects exposure of the film. A spot meter pointed at the ground glass and aligned with the lens axis may be over-influenced by rays close to the lens axis, thus taking too little account of rays coming from the periphery of the exit pupil when the lens is wide open.'

I guess this is my concern as well.

I realize I'm just repeating an old thread by this point, but one more commenter wrote:

'...mount the lens, focus the camera at infinity, and lock everything down. Take the camera into the darkroom. Take a piece of cardboard, poke a pinhole throught the center of it, then tape it to the center of the GG. Hold a piece of photo paper up against the front of the lens and a flashlight against the cardboard and turn the flashlight on for a bit. Develop the paper and measure the resulting circle. That's your entrance pupil opening. Divide it into the focal length and you've got your f/stop.'

Why couldn't I just tape a piece of paper to the front of the lens and use calipers to measure the spot of light shining out of the front of the lens as i stop the aperture down?

I have no good idea of what I'm talking about. I'm just curious and trying to learn. I appreciate the help, and am going to sleep on it!

[B.S.Kumar]

'... and sometimes what we hope are precise or accurate methods may not be as precise or accurate as they might appear and we may as well use an approximate method. A caveat with the method of measuring at the film plane is that angle of incidence may affect the light meter reading more than it affects exposure of the film. A spot meter pointed at the ground glass and aligned with the lens axis may be over-influenced by rays close to the lens axis, thus taking too little account of rays coming from the periphery of the exit pupil when the lens is wide open.'

I guess this is my concern as well.

If that is a concern, you can use the DSLR meter in averaging mode.

Kumar

[LabRat]

No doubt someone more technically oriented will correct me...

1. Light a gray or white background in a darkened studio as if you were copying something.
2. Point your view camera (on a tripod) with the 90mm lens at this surface. A defocused image is best.
3. Tape a piece of paper on the aperture scale.
4. Get hold of a fine point marker pen.
5. Close the aperture until the point where the aperture leaves would intrude into a perfect circle. Mark that spot as f/5.6.
6. On a second tripod behind the view camera, mount a DSLR with any lens.
7. Fill the frame with the ground glass of the view camera. Again, it is best to have a defocused image.
8. Put the DSLR into spot metering mode.
9. Take a reading from the center of the ground glass and note it down. Let's assume it is f/2 at 1 seconds.
10. Have someone close down the aperture on the 90mm Super Angulon until you get a reading of f/2 at 2 seconds. Mark it as f/8.
11. Continue until the aperture on the SA is at the smallest opening.
12. Double check your DSLR readings against the markings.
13. When all the apertures are marked, you can decide whether to do this for intermediate stops.
14. Test, using roll film.
15. Make a more permanent scale by using a laser printer.

Kumar

#2 is to make sure there are no bellows factors involved, so camera is focused to INF...

For #1, I just go outside and measure my white garage door, so as you stop down, it is bright enough for a meter reading where you can still read the smallest stops...

On axis is fine... The GG acts as a diffuser, so a spotmeter or DSLR is fine... Just make sure the measuring is on the same spot and not tilted... Confirm with a film test, but you will see it is VERY close in practice if you do it right... All that you are trying to read is the point where as you stop down, it is a stop under wide open, then two, and so on...

Follow Kumar's great directions above... Try it!!!

Steve K

[Jim Jones]

In another thread (http://www.largeformatphotography.info/forum/showthread.php?75124-How-do-you-create-an-aperture-scale-for-a-lens&highlight=aperture+scale) . . . '... and sometimes what we hope are precise or accurate methods may not be as precise or accurate as they might appear and we may as well use an approximate method. A caveat with the method of measuring at the film plane is that angle of incidence may affect the light meter reading more than it affects exposure of the film. A spot meter pointed at the ground glass and aligned with the lens axis may be over-influenced by rays close to the lens axis, thus taking too little account of rays coming from the periphery of the exit pupil when the lens is wide open.'

I guess this is my concern as well.

I realize I'm just repeating an old thread by this point, but one more commenter wrote:

'...mount the lens, focus the camera at infinity, and lock everything down. Take the camera into the darkroom. Take a piece of cardboard, poke a pinhole throught the center of it, then tape it to the center of the GG. Hold a piece of photo paper up against the front of the lens and a flashlight against the cardboard and turn the flashlight on for a bit. Develop the paper and measure the resulting circle. That's your entrance pupil opening. Divide it into the focal length and you've got your f/stop.'

Why couldn't I just tape a piece of paper to the front of the lens and use calipers to measure the spot of light shining out of the front of the lens as i stop the aperture down?

I have no good idea of what I'm talking about. I'm just curious and trying to learn. I appreciate the help, and am going to sleep on it!

Good thinking. Note that the pinhole should be mounted on the ground surface of the ground glass. Mounting it on the outside of the ground glass will defocus the system.

Measuring the approximate diameter of the aperture as seen through the front of the lens is the simplest and most direct beginning to calculating the f/number. A simple transparent metric ruler may be more practical than a calipers. Simplicity is good. Complicating measurements by introducing variables such as lens flare and complex DSLRs may introduce possible errors that will be difficult to recognize. As you have determined, the error due to a non-circular aperture is well within the error of the several variables involved in lenses and shutters. Don't waste sleep over it. When I have to bracket difficult exposures on most film, it is usually in one-stop increments. This is greater than the cumulative total of most variables in apertures, shutters, and light meters.

[Paul Kinzer]

Okay, then! My reading, and all the helpful comments here, give me plenty to work with. I'm going to try both methods and see how they compare. Obviously, others have used both ways -- light meter and ruler -- to good effect, so I should be able to get close enough.

Thanks, everyone!

[lungovw]

I measure the entrance pupil by taking digital pictures of the lens having the GG back lit. For that I use a long focus (100 mm for APC sensor is OK). Later I analise the digital image. It is easy to measure in pixels (using Adobe InDesign, for instance) the entrance pupil and something else external, more accessible (like filter thread) that I measure in the image as well and in real with a caliper directly from the lens. Having these three figures I can find by proportion the entrance pupil in mm. I tested it with known lenses and the results are quite OK. I am uploading a picture that shows how it goes and I have it described in details in my website: https://apenasimagens.com/en/measuring-lens-aperture/. I've been using this mostly for old brass lenses having Waterhouse stops or just to assess what it is its full aperture. When the question is about making a full scale with the regular f stops figures, I think one can first find the how much the lens front element increments the actual diaphragm diameter and use that figure to calculate what is the series of diameter that would yield f5.6, f,. f11, etc. and 165122

[Jim Jones]

Lungovw -- Clever! Including a ruler in the photo should simplify the procedure. Also, the longer the focal length on the camera used to make the photograph, the less parallax error you will have.

[B.S.Kumar]

Lungovw, thank you for the explanation.

Kumar

[Jac@stafford.net]

Lungovw -- Clever! Including a ruler in the photo should simplify the procedure. Also, the longer the focal length on the camera used to make the photograph, the less parallax error you will have.

Yes, Lungovw has the trick! FWIW, if you have the Advanced version of Photoshop it includes, among other things, a measuring option for plane and irregular shapes.

[Paul Kinzer]

Another great idea; thanks!

[jim10219]

One thing to consider is that a lens labeled 90mm, is probably not exactly 90mm. So if your math turns up different numbers, that might be fine, so long as they're close. Also, the original aperture scales probably weren't all that accurate. I've calculated some of my older lenses and found small discrepancies like a lens listed with a max aperture of 5.6 might actually be 5.3 or 6. Even smaller format lenses with detented apertures or large format lenses with Waterhouse stops can be off. They still operate fine in the field though.

Another way to try it is to skip the large format camera all together and just temporarily mount it to your DSLR. Maybe use a cardboard tube and some light tight tape. Cut the tube to length, so it focuses on something to act as your baseline. Then snap some photos and compare them to what a regular DSLR lens would do at the same shutter speed. Be sure to use your DSLR's shutter and leave the LF lens open, of course. You can use the histogram to compare them.

Just do whatever is easiest. In any case, don't go nuts trying to make it 100% accurate. Just make it accurate enough that you like the results and feel confident using the lens.

[Paul Kinzer]

Thanks, jim10219!

I agree that getting too hung up on being spot on is probably not warranted, since so many other factors are not spot on anyway.

I've gotten sidetracked by other life issues, but hope to get to this on the long weekend here. I'll post what I find as soon as I get it done.

[Jac@stafford.net]

Since all my LF shutters are inaccurate (and yours probably are, too), I find an accurate speed or three, and mark f-stops that work. Keep it simple.

[Paul Kinzer]

I measured the apparent aperture coming out of the front of the lens in two ways: Just eyeballing it while looking down into it from a couple of feet above, and also by placing the camera, with the lens set at infinity focus, on top of a light box with a pinhole centered in the middle of a piece of cardboard covering the ground glass, and a piece of thin paper over the front of the lens. Both had very close to the same result, and since I know that I don't need to be spot on, I'm going to call it good enough. I have not yet put marks on the shutter; this was just for testing.

Thanks for all the help!