Need help with Computar Symmetrigon 6.3/150 mm lens.

[Rudolf Karachun]

I have a Computar Symmetrigon f:6.3/ 150 mm lens in Copal Press shutter. I got it from EBAY and the shutter came with wrong aperture scale attached. If anybody have the same lens assembly, can you scan the correct aperture scale for me please. Thank you.

 

[wiltw]

If you have 150mm f/6.3 lens, the diameter of the largest aperture is 23.8mm (150/6.3 = 23.8
So if your (wrong) scale says 210mm f/whatever, really you know it is f/6.3, and you can compute the f/stop that really belongs along the rest of the scale.

For example, if the scale is labeled 210 f/11 you know aperture at that point on the scale is 19mm, and for that diameter it is f/16 for 150mm.

 

[outwest]

Wiltw, I think that would be f/8 not f/16. However, it is not that simple. The aperture used in the computation is the entry pupil which is measured with the front elements of the lens in place. Different lens designs have different magnifications of the entrance pupil so what is on the shutter for the 210's entrance pupil may have no correlation to the 150's entrance pupil. You must measure the entrance pupil of the 150 as seen from the front with the front elements in place and use this in the calculation along with the focal length of the lens. So, divide 150 by the standard f/stops - 8, 11, 16, 22 and so forth and close the aperture until one of the results is achieved and mark the shutter with that f/stop. Yes, it is difficult to measure the entrance pupil accurately but with a good caliper and a careful minute side movement of your eye you can definitely get close enough.