Why does my Copal #0 shutter go past the minimum f-stop marking?
Asked 9/10/2024
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2 answers
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I bought a used Fujifilm GX617 with the 180mm f/6.7 lens in a Copal #0 shutter. The aperture scale is marked down to f/45, but the aperture lever moves noticeably past the f/45 mark and the opening gets smaller beyond that point.
Is it normal for a Copal shutter’s aperture control to travel past the last engraved f-stop, or does that suggest misadjustment or wear? If the actual minimum aperture is only reached at the end of the lever travel rather than at the f/45 marking, would that mean the intermediate engraved f-stops are also off? What’s a practical way to check whether the aperture scale is accurate before deciding on service?
Originally by Peter M. Source · Licensed CC BY-SA 4.0
Peter M
1y ago
2 Answers
2
The fact that you can continue past f/45 seems weird to me. That being said, open the back and likely you can find a way to place a small flashlight so that it is aimed back at the lens from approximately at the position of the film plane. If achieved, the light will exit the lens backwards as parallel rays. Now mount a piece of paper over the lens as if it were an optical filter. You will see a circle of light projected on the paper. You can mark the diameter of this circle and measure it. This is the diameter of the entrance pupil. Make your measurement in millimeters. Set the aperture on each engraved setting and make a list of the corresponding diameters you measure. Divide each into the focal length which is 180mm. Your answers will be a close approximate of the actual f-numbers. Not exact because but close enough. Except for the first (widest) which is not a full stop, the sequence will be 5.6 -- 8 -- 11 -- 16 -- 22 -- 45 -- 64 -- 90 -- 126. Each value multiplied by 1.4.
Originally by Alan Marcus. Source · Licensed CC BY-SA 4.0
Alan Marcus
1y ago
0
Generated from our catalog & community — verify before relying on it.
Moving past the last engraved stop is unusual enough that it’s worth checking, but it doesn’t prove the lens is unusable.
A practical test is to verify the actual aperture sizes. Open the camera back and place a small light near the film plane so it shines through the lens from behind. That sends light backward through the lens. Hold a sheet of paper over the front of the lens and you’ll see a bright circular spot representing the entrance pupil.
At each engraved aperture, measure that circle’s diameter in millimeters. Then divide the focal length by that diameter:
f-number ≈ 180 mm / measured diameter
For example, if the circle is 4 mm across, that’s about f/45. Repeat this for several marked settings and compare the results with the engraved scale.
If the measurements line up closely, the scale is basically correct even if the lever has some extra travel. If the marked settings are consistently off, then the aperture calibration may need service.
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