How to figure out F/stop scale on convertible lens when in different shutter?

Discussion in 'Large Format Cameras and Accessories' started by User Removed, Jun 21, 2006.

  1. User Removed

    User Removed Guest

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    I'm having abit of problem here trying to figure this out. I just picked up a Perigraphic Convertible Gundlach 11x14 lens, 16", 25", 34". Its a barrel lens, but came with a special adapter ring to mount each element into a copal #3 shutter. Here are the following F/stop scales for each different focal length

    16" (both elements together) 5.6, 8, 11, 16, 22, 32, 45, 64.
    25" (Rear element only) 11, 16, 22, 32, 45, 64.
    34" (Front element only) 16, 22, 32, 45, 64.

    I have a Copal #3 F9, which has the scale of 9, 11, 16, 22, 32, 45, 64, 90, 128.

    I want to be able to screw the 34" (f/16) into my F/9 copal 3 shutter. When doing so, I have the f/16 lens in a f/9 shutter...what F/stop is it really?

    Same thing if I screw the 35" (f/11) into my f/9 shutter...what would it really be at?

    How can I figure out what F/stop's scale match up with each element of the lens in this shutter?

    I tired all lens elements in the copal #3 shutter and all focus correctly and cover my 11x14 camera very well. I am shooting with just the lens element in the shutter...not the extra barrel with the aperture scales.

    Thanks so much for your help!
     
  2. jnanian

    jnanian Advertiser Advertiser

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    ryan

    sounds like you will have to measure the diameter of the iris-opening of your shutter and divide it into you focal length to figure out what your fstops actually are.

    it shouldn't be too hard, kind of like a math puzzle.

    sounds like fun :smile:

    -john
     
  3. Donald Miller

    Donald Miller Member

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    Quote "I have a Copal #3 F9, which has the scale of 9, 11, 16, 22, 32, 45, 64, 90, 128."

    The Copal three is only an F9 in relation of the apparent maximum aperture divided into the focal length of the lens that was originally installed. ( I would guess that it is probably approximately in the 450 mm length) For that reason, you will be most accurate to measure the apparent apertures of each of the cells and divide this measurement into the focal length of that particular lens/cell.

    If I am accurate on the 450 mm focal length as having been installed in the shutter originally, your 16 inch would have a maximum aperture of F8...your 25 inch would have a maximum aperture of F12 3/4...and your 34 inch lens would have a maximum aperture of F 17 1/3.

    Perhaps from this you may be able to work this backwards if you have knowledge of the focal length of the lens that was originally installed in the shutter.
     
  4. Ole

    Ole Moderator Staff Member Moderator

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    Single cells are actually the "simple case", as the entrance pupil is equal to the physical aperture opening. This of course requires that the single cell is used behind the shutter/aperture; which it should be in any case.

    So it's simply a question of dividing the focal length by the aperture, which gives the aperture number.
     
  5. Jim Jones

    Jim Jones Subscriber

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    First, calculate the diaphram opening for a 25" lens at f/11, 16, 22, etc. Then do the same for a 34" lens at f/11, 16, 22, etc. Now set the diaphram for the 25" f/11 diameter and mark it on the scale. Do the same for the remaining stops. Now repeat the process for the 34" diaphram diameters. This should create functional scales for both lenses. To make a neater scale, scan the one you just made and clean it up in an image editor. Print it on an adhesive label and attach it to the shutter in the same position as the drawn one. Remember, a single cell from a triple convertable is usually used behind the shutter.
     
  6. jimgalli

    jimgalli Subscriber

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    Ryan, is it a modern Copal with equidistant spacing between all the "stops" on the scale? (except 9 and 11 of course) or the earlier one where 11 and 16 are far apart and 64 and 90 are jammed together?

    I'll assume it's the modern version. It's helpful to calculate where f16 falls for each of the possibilities. I've converted to mm. You have 400mm 635mm and 865mm. An f 16 aperture for each of those is 25mm for the 400, 40mm for the 635, and 54mm for the 865. Once you measure and locate f16, your other stops are equidistant from each other on that shutter, so they're easy to find. 54mm is real close and probably just beyond wide open on that shutter so you may have to fudge just beyond where it can't quite get to.

    Confused?
     
  7. User Removed

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    Eeek! I think I just confused myself even more, but thats okay. I'm going to have to re-read everything that you guys are saying a few times and hopefully start to understand what I need to do.

    The shutter I have is the newest Copal 3 with equal spacing between all stops. Its from my Nikkor 450mm.

    Anything else you guys have to say to help me out more would be great.

    Thanks so much!
     
  8. User Removed

    User Removed Guest

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    Also, can someone please tell me the reason why you put the lens element BEHIND the shutter, and not in front. When placed in front, they seem to work just fine, but there must be a reason to put them behind.

    Thanks.
     
  9. Jim Jones

    Jim Jones Subscriber

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    For one thing, I believe the field is flatter. Problems that might not be noticable on the ground glass can be conspicuous on film.
     
  10. photomc

    photomc Member

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    Don't feel too bad Ryan, I have 240 f9 that I was planning on getting a Copal 1 for but was looking at using the Copal 1 that I have for my 210 f6.8. So started making measurements and they did not seem to jive, so compared what I got by dividing 210 by the opening as marked (for the 210) and could not come up with the right f-stops as marked. My understanding is you take the focal length (say 210) divided by the iris opening (say 11.27 for f/16) which gave me 18.6 not 16.

    So tonight I was reading this thread and tried again, but left the lens cells on the shutter (took them off the first time) and got 210/13.53 = 15.5 which is a lot closer to f16 than 18.6 was. Not sure if you are supposed to leave the lens cells on or not...???
     
  11. Ole

    Ole Moderator Staff Member Moderator

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    No, you're supposed to take the focal length divided by the diameter of the entrance pupil of the lens. Only when there is no glass in front of the aperture will that be identical to the diameter of the iris opening.

    Think of it like the glass in front of the aperture is a magnifying glass, so the entrance pupil is larger than the physical opening.

    Placing a single lens ell behind the aperture gives better field flatness and less distortion. Try the same cell in both locations, you will be able to see the difference on the negative!
     
  12. Tony Egan

    Tony Egan Subscriber

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    I believe I have a shutter aperture scale for a 210/370 convertible lens which is incorrect. Scale is too constricted and aperture should be smaller at each of the marked stops. As a picture paints a thousand words please see two attachments.

    I would appreciate confirmation from the experts here that my conclusion is correct and that I will have to create a new, more expanded, scale for this lens and shutter. I am getting quite overexposed negs with this lens at the smaller marked apertures and assume it is mainly due to this scale problem?

    Thanks in advance for your help.

    Tony
     

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  13. Ole

    Ole Moderator Staff Member Moderator

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    That looks like someone has put a "linear" scale on a shutter with "non-linear" aperture. If the opening is 36mm at f:5.6, it should be 18mm at f:11, and 9mm at f:22.

    That the aperture can stop down well beyond the last marked stop is not uncommon and doesn't have to be wrong.
     
  14. Tony Egan

    Tony Egan Subscriber

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    Thanks Ole - My instincts and the arithmetic told me the scale was wrong but it's helpful to get a second opinion. Coming from 35mm and MF you don't watch the aperture blades move very often so this is another aspect to LF which takes you back to the fundamentals of exposure!
     
  15. Tony Egan

    Tony Egan Subscriber

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    Ole - yes it's a non-linear aperture. See picture of new hand-written scale compared to old. Not pretty but effective enough for the time being. Not surprising I was getting 2x exposure at smaller apertures on the original scale.

    Cheers, Tony
     

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