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Thread: Some Questions and Observations About Making an Aperture Scale

  1. #1

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    Some Questions and Observations About Making an Aperture Scale

    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.

  2. #2

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    Re: Some Questions and Observations About Making an Aperture Scale

    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

  3. #3

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    Re: Some Questions and Observations About Making an Aperture Scale

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

    Kumar

  4. #4

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    Re: Some Questions and Observations About Making an Aperture Scale

    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.

  5. #5

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    Re: Some Questions and Observations About Making an Aperture Scale

    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

  6. #6

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    Re: Some Questions and Observations About Making an Aperture Scale

    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 thread.

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

  7. #7

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    Re: Some Questions and Observations About Making an Aperture Scale

    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.

  8. #8

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    Re: Some Questions and Observations About Making an Aperture Scale

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

    Kumar

  9. #9

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    Re: Some Questions and Observations About Making an Aperture Scale

    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.

  10. #10

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    Re: Some Questions and Observations About Making an Aperture Scale

    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

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