Convert Aperture of G-Claron 210mm f/9 using Compur #1 with f/5.6

[Jason Tian]

Hi,

I am new here but have been reading threads here. I just got a lens of Schneider G-Claron 210mm f/9. The thing is, I just have a compur # shutter with max aperture of f/5.6, and I don't have the exact correlations or converting matches between the real aperture of the aperture showing on the shutter. By roughly comparing, it seems a one-stop correlation that 5.6-9, 8-11, so and so on. But I am not sure whether I am correct and wondering if there is any strict formula or table to follow. Thanks, guys.

[ridax]

F-stop numbers and corresponding round iris opening diameters for the 210mm f/9 G-Claron: -

f/11 = 15,4 mm
f/16 = 10,9 mm
f/22 = 7,7 mm
f/32 = 5,5 mm
f/45 = 3,85 mm
f/64 = 2,7 mm

[Jason Tian]

F-stop numbers and corresponding round iris opening diameters for the 210mm f/9 G-Claron: -

f/11 = 15,4 mm
f/16 = 10,9 mm
f/22 = 7,7 mm
f/32 = 5,5 mm
f/45 = 3,85 mm
f/64 = 2,7 mm

Thanks ridax. The thing is, what I should measure when the aperture blade is not a circle? Thanks again.

[Dan Fromm]

Not to be a complete idiot, but there's an easy way, when it works. Open the shutter. Open the aperture as wide as it will go. Hold the lens at arms length, rear towards you, front towards a light source. Slowly close the aperture until the diaphragm blades become visible. Open until the blades disappear. Check where the aperture pointer is on the aperture scale. This is f/9 with your 210/9 G-Claron.

This approach doesn't work when the shutter's largest opening is smaller than the lens' exit pupil. In your case, the image of the exit pupil's diameter is 210/9 = 23.3 mm. The #0 shutter's largest opening's diameter is 30 mm. So you're out of trouble.

[Michael_qrt]

Dan has the method to do it without any calculations or equipment. As an extra point if you have a later compur #1 with the chunkier design of the speed setting ring then there is a mechanism to adjust the aperture scale to the lens. I don't have one of those shutters on hand at the moment but if you study how the aperture setting ring works on the rear side of the shutter you should be able to see how it works, there's a series of limit stop screw holes and an adjustment for the connection between the aperture setting ring and the aperture mechanism.

[ridax]

what should I measure when the aperture is not a circle?

Strictly speaking, your should measure the area of the aperture opening and compare it to the area of the corresponding circular aperture. In practice, a rough approximation is OK for an accurate enough exposure. And for the depth of field as well as for the out of focus rendition, an angular aperture is just never equivalent to a round one.