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Thread: Wrong aperture blades on Grandagon 90mm

  1. #1

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    Wrong aperture blades on Grandagon 90mm

    Hello,

    the aperture blades on the Copal 1 shutter of my recently aquired Grandagon-N 4.5/90mm don't match. (I hadn't noticed it before as I did not have the time to use the lens since I have received it.) They go from 5.6 to 64, while they should go from 4.5 to 45.
    As 4.5 to 5.6 is 2/3 of a stop, will I get correct exposures if I simply set the lever to 22 2/3 if I wish to have an aperture of 22? As I am curently on a trip where I would like to use the lens, I cannot run test shots first.
    Your help would be much appreciated. Cheers

    Aender

  2. #2

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    Re: Wrong aperture blades on Grandagon 90mm

    The aperture blades are correct, as we discussed earlier, you need the correct aperture scale.

  3. #3

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    Re: Wrong aperture blades on Grandagon 90mm

    Of course you are right, Bob, sorry for my mistake. Of course, the blades are correct. I meant the scales. But since I could not get an answer to this precise question from you, I thought maybe someone here could help me out and make the lens usable until the scales issue can be solved. I just want my exposures to be correct in the next few days.

  4. #4

    Re: Wrong aperture blades on Grandagon 90mm

    In a word, No.

    If you are lucky, the 5.6 spot on the scale is the same as the 5.6 spot on the correct scale, but there is no easy way to check that without having the correct scale handy.

    However, in a pinch, if you have any way to accurately measure the scale, you can set it to a point on the scale that is marked (say f11 or something like that) and then measure the aperture.

    In an idealized lens (no real lens is ideal), the aperture at f11 for a 90mm lens will be 8.1mm. It will actually be a little different on a real lens, due to the location of the aperture relative to the center of the lens. However, if you can determine what the diameter is at f11, then you can figure out an adjustment factor to apply until you get home.

    Take the focal length and divide by the 'stated' aperture, and you should get the diameter of the opening. Suppose you set it to f11 and then measure and you get an aperture diameter of 12.25 instead of 8.1. That would be about the proper aperture for a 153mm lens rather than a 90mm lens (12.25/8.1)*90=135.

    So when the stated aperture is f11, you have an effective aperture of f7.3 (90/12.25=7.3), which is about 1-1/3 stops more than the scale is telling you. That will then give you the adjustment factor to be applying. In this case, if your meter says 1/250 @ f11, you need to stop down the aperture 1-1/3 stop to about f18 to be putting the proper amount of light on the film.

    So, the formula is:

    Focal length divided by the 'stated' aperture will be close to equalling the diameter of the aperture if the scale is correct.

    If not, take the focal length of the lens and divide by the actual diameter to get the effective f-stop. Compare that to the 'stated' aperture and determine the difference (in stops). Apply that on the aperture scale to all shots, and you'll be pretty close. If you calculate a larger aperture, then you need to stop the lens down further (higher f-stop value). If you calculate a smaller aperture (not likely, as that means the scale was from a shorter focal length), then you open up the lens (lower f-stop value).


    ---Michael

  5. #5

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    Re: Wrong aperture blades on Grandagon 90mm

    Hello Micheal,

    thank you very much for your detailed answer. I'll try to do as you wrote. Many thanks.

    Aender

  6. #6

    Re: Wrong aperture blades on Grandagon 90mm

    Aender,

    One more note, if the scale is for another 90mm lens, (the 5.6 version), it may be pretty close. If it was for a longer focal length, it will be larger, and require you to stop down, and if it was for a shorter lens, it will be smaller, and require you to open up.


    ---Michael

  7. #7

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    Re: Wrong aperture blades on Grandagon 90mm

    Quote Originally Posted by Michael Mutmansky View Post
    In a word, No.
    In an idealized lens (no real lens is ideal), the aperture at f11 for a 90mm lens will be 8.1mm. It will actually be a little different on a real lens, due to the location of the aperture relative to the center of the lens. However, if you can determine what the diameter is at f11, then you can figure out an adjustment factor to apply until you get home.

    Focal length divided by the 'stated' aperture will be close to equalling the diameter of the aperture if the scale is correct.

    If not, take the focal length of the lens and divide by the actual diameter to get the effective f-stop. Compare that to the 'stated' aperture and determine the difference (in stops). Apply that on the aperture scale to all shots, and you'll be pretty close. If you calculate a larger aperture, then you need to stop the lens down further (higher f-stop value). If you calculate a smaller aperture (not likely, as that means the scale was from a shorter focal length), then you open up the lens (lower f-stop value).


    ---Michael
    The easiest way is to calculate what the aperture should be, measure it, holding the lens at arms length (which will very closely approximate the entry pupil diameter, the f stop of a lens is the focal length divided by the entry pupil diameter) then see how far off it is, you may get lucky and it may be very close, but it only takes a min or two to do with a caliper and you'll know for sure how much correction you will need to apply. You should only need to check one point on the scale, as the error would be the same throughout the scale, but checking another as a sanity check would be prudent.

    erie

  8. #8

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    Re: Wrong aperture blades on Grandagon 90mm

    Erie,

    thank you for your help and please forgive me for being ignorant, but is it correct that I should try to measure the aperture with the lens cells fitted to the shutter, not directly on the shutter blades (which would be easier).
    I'll have to buy a caliper tomorrow morning (it's late in the evening here now) to get it right.

    Thank you very much.

    Aender

  9. #9

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    Re: Wrong aperture blades on Grandagon 90mm

    Aender, I have a heap, well, three, shutters whose scales are wrong for the lenses I've put in them. What I did was hold the shutter up with the aperture wide open and then slowly close the aperture until the blades' edges were just visible through the lens' front cell. The aperture setting at which that happens corresponds to the lens' maximum aperture. Since the aperture scale goes in one stop steps, and this doesn't depend on the lens at all, once I found the true wide open setting (for one f/5.6 lens, true f/5.6 = marked f/8, and so on) I was set.

    This is less expensive than having the shutter rescaled, but requires more thinking.

    Cheers,

    Dan

  10. #10

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    Re: Wrong aperture blades on Grandagon 90mm

    Aender, the f-value is the diameter of the image of the mechanical diaphragm at the entrance pupil, not the diameter of the diaphragm! Every lens, and in your case specialy with a WW-lens, has its own magnification factor for the diaphragm. So the way erie has shown is a quick reference. But for a proper measurement here is the method Richard Knoppow has descibed in the days of the usenet:

    Since f/stop is defined as the focal length devided by the _effective_ diameter of the stop its easy to measure. The _effective_ stop takes into account the magnification factor of the lens, usually not very large for conventional lenses. An easy way to get a usefully accurate measurement of the effective size of the stop is to place a pin-hole light at the exact focal point for infinity focus, then place a translucent screen over the front of the lens (a piece of thin paper will do). Measure the size of the image of the stop on the paper. For most older barrels and shutters the diaphragm construction is such that the diameter of the stop will vary directly with the angular distance of rotation of the stop control. That is, supposing the clear opening of the iris is a maximum of 50mm and the travel of the stop lever is also 50mm. Then moving the stop lever 10mm will change the diameter of the stop by 10mm. The stops can be calibrated that way remembering they follow a square law. So, wherever you start, two stops down will be half the diameter, etc. The scale will become crowded at the small stop end.
    There is another way of making the iris which results in a scale which is linear with stops and avoids the crowding. This form is often found on shutters with LVS or EVS scales.
    Good luck with repairing the barrel, a trip to a large hardware store may find what you need to fix it.
    Last edited by Peter K; 30-Dec-2008 at 16:13. Reason: spelling

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