Angulon exposure compensation

[Scott Kathe]

I have a Schneider Angulon f6.8 mounted in a Synchro-Compur Press shutter. At f32 I am getting exposures that are at least 1 and 1/2 stops under exposed on Velvia 100. My typical shutter speeds so far have been 1/10 to 1 second. I have been using it for landscape shots so there is no bellows factor correction needed. I had Grimes do a CLA on the shutter when I first got it and the times seem to be right, besides before the CLA the slow shutter speeds were to long which would have resulted in overexposure not underexposure. I have been using the meter in my Nikon N80 35mm camera and a similar composition with the same film comes out properly exposed from that camera. Last time I used the Angulon at f32 I just used my shutter time from the N80 when the aperture was set at f22, the transparency from the Angulon shot was still just a little (~1/2 stop) underexposed. Could the f32 setting on the Angulon aperture scale be more like f45+? Any other ideas?

[Frank Petronio]

Those older shutter scales that get closer as you stop down probably contribute to part of the problem. You may have to do film tests and create your own custom scale, or have Grimes do it for you.

[Ole Tjugen]

Unscrew the elements from the shutter. Set the aperture at f:8. Measure the diameter of the opening. Stop down to f:16, measure again. Stop down to f:32, measure again.

The diameters should be at proportions 4:2:1 - exactly. If not, the aperture scale is off.

[Scott Kathe]

Ole,

I understand what you are saying but I'm not sure it's that simple. The light is coming through the circular (close enough) aperture and the area of a circle is pi R squared so I don't think I can simply go by doubling or halving the diameters to check the aperture scale. Physics was never my strong point and certainly not optics but I think that I can work this out theoretically. The focal length is 90mm divide this by 32 (f-stop) gives me a 2.81mm aperture, but that aperture would be in the center of a lens and I'm not sure if the iris in the camera lens is in the center. Anyone else know how to do this or even if it's possible?

[Bob Salomon]

On a lens as old as yours the shutter is probably long in need of a good, professional CLA. It sounds like your speeds are off.

[Scott Kathe]

I had SK Grimes do a CLA and I don't think it's an issue with speeds, the shutter actuating sounds right. Before I had the CLA the speeds were really slow which would result in overexposure what I'm seeing now is underexposure. As far as I understand shutters when they go out of calibration they get slower not faster.

[James E Galvin]

Seems to me that setting the N80 to f/22 is the wrong way, if you think f/32 on the Angulon might be actually f/45, then you should have set the N80 to f/45. The diameter of the iris can be measured, but it is the effective diameter, as magnified by the lens elements, that counts, and that is hard to measure. But if it is right at f/8, it should be half of that at f/16, and half again at f/32, and that can be measured, as Ole said. As the scale is not linear, mechanical errors will affect the small openings more than the large, so the measurement should disclose any problem with the scale.
You can measure the aperature by taking a picture of a distant small light at night, with a measured out of focus, say an inch, and measuring the diameter of the blur circle. The f number is the distance out of focus divided by the blur diameter. (doesn't work for retrofocus or telephoto lenses)

[Bob Chambers]

Ansel Adams describes this problem and the solution in "The Camera" page 66. To sum it up briefly a light meter is used to measure light on the ground glass as the lens is stopped down.

[Scott Kathe]

Ole,

After reading some other replies and rereading your post and crunching some numbers, I stand corrected. 90mm/32=2.81mm, 90mm/16=5.62mm and 90mm/8=11.25mm or 1:2:4 as you stated. I was thinking one stop not two--see, I said physics wasn't my strong point I apologize.

[Bob Salomon]

"I had SK Grimes do a CLA "

That is a start. Now have you had the shutter calibrated?
Is the lens in its original shutter? If not the aperture needs to be calibrated.

If it is in the original shutter the shutter speeds are much more likely to be the problem then the aperture. If the shutter has been changed then the aperture scale is the likely culprit.

Also your meter. Are you using the actual speeds your shutter is calibrated in on the meter or the modern equivelents?