Testing Apertures

[Two23]

I think I've figured out how to make some washer stops for my Petzval lens. It has no slot to tell me where to position the stops. Looks like they should go right in the middle between front & rear elements. I'm making some out of black soft foam that should work. Lens is native f3.5. I focussed at infinity and measured from the GG to the center of the lens and came up with right at 6 inches. I measured the diameter of the front lens group and came up with 1.69 inches. To make one stop tighter, I divide 1.69 by 1.416 (square root of two), and keep dividing by that until I get to f16? (I want stops of f8, f16, f32.)

Now for the tricky part. To test whether the apeture is sized correctly, couldn't I measue the light with a meter? Here's what I have in mind. Set up my camera in a long dark room. Set up a flash at one end, my camera at the other end of the room. Focus on the flash, insert stop. Remove GG. Replace with a rectangle of the black foam and seal it tightly against the back opening of the camera. Cut a hole in the center of the black foam and squeeze fit the sensor dome of my Minolta flash meter. Trigger the flash*. The reading on the meter should tell me the actual effective aperture, shouldn't it? Wouldn't this work, or is there an easier way that doesn't involve shooting a bunch of film? I could double check flash output by using my Nikon D300 in manual mode, set to same exposure settings.


Kent in SD

*Using a flash for light will remove the variable
for shutter time, since lens has no shutter. Flash
output is constant, and a flash meter will only
measure the flash output.

[Leigh]

Hi Kent,

You can measure the diameter of the hole much more accurately than the amplitude of the light.

The area of an aperture differs from the area of an aperture one full stop larger or smaller by exactly a factor of 2, so the diameters will differ by exactly the square root of 2, which is 1.414214.

- Leigh

[c.d.ewen]

I won't take the time to explain why this works, but here are two methods:

Quickie, semi-accurate method: Put the lens on a camera and focus at infinity. Put an opaque mask, with a small hole in the center, on the GG. You want light coming through the hole, so do this outside, or put a lamp behind the GG. Look through the front of the lens and measure the visible aperture. Do arithmetic.

Better, more accurate method: With the lens focused at infinity, put a piece of photo paper in front of the lens, expose it by shining a light through the small hole (smaller = better) in the mask, develop it and measure the dark spot. Do arithmetic.

Charley

[ic-racer]

I use a Horseman film-plane meter, but one can reliably get only a relative aperture. For example, first mount up a lens with known correct factory aperture scale. Set it to what you think the waterstop should be. Then swap to the lens/waterstop combo and see if the meter nulls to the same spot.