The aperture tester simply measures the amount of light getting through a lens.

Starting with a known light intensity (our old familiar Adjustable Light Source) at a known distance. A record is made of the resistance readings taken through a CdS cell positioned at the focal plane with the shutter held open on B or T. The resistance at each aperture is recorded. Again, several camera/lens combinations should be compared.

The circuit is very simple. I used the CdS cell from a scrap Canon Canonet QL17 GIII. CdS cells are slow to react so give a few seconds between readings, especially when going from large apertures to small ones. (for example from f/1.4 to f/16) CdS cells can be bought at Radio Shack if you don’t have any junk cameras laying around.

I used 3/32" thick light baffle material on the tester surface that contacts the cameras focal plane. All extraneous light must be blocked.