How to calculate ?

[karavelov]

Hello,
I am trying remount a Super Angulon 65/8 lens that I have acquired on Sinar DB mount in a shutter. It fits in a Copal 0 that I have but the the aperture marks on the shutter are for different lens (135mm). There should be a formula for the diameter of the aperture opening for given focal length and stops but I could not find it. Please help if you know how to calculate it!

Thanks in advance
luben

P.S. The aperture in the Sinar mount is missing so I could not just measure it.

 

[removed account4]

hi luben

you could calculate your diameter by dividing your fstop into your focal length.
65mm/f8 = 8.3mm ... f11=5.9mm; f16=4.1mm; f22=3.0mm; f32=2.0mm
i rounded up, but if you want to be more exact, it is a pretty easy calculation
i would draw your circles on pieces of paper, and then put them either over or under your
lens mount. open/close your aperture until it is the same as the little circle,
and mark your scale.

good luck!
john

 

[noseoil]

Luben, as john mentioned, the lens focal length divided by the f-stop is the diameter of the pupil.

If the 65mm lens at f8 is 8.125mm, the 135mm lens at f16 is 8.4375 (close to the same, but slightly more light), or two stops.

At f11, the 65mm lens is 5.9mm, while the 135mm lens at f22 is 6.136 (or about 2 stops, but slightly more light).

At f16 the 65mm lens is 4.0625mm and the 135mm lens is 3mm at f 45 (again, slightly larger).

You can just use 2 stops, but this isn't correct math. My old math skills are too weak to fighure out the relationship (I suspect a log relationship is correct, but don't know). For color slide film, you will get in trouble a bit, but for B&W, just use 2 stops and shoot away! best, tim

 

[Ole]

65mm is close enough to half of 135mm (especially taking production tolerances into account) that simply using two stops difference should be close enough - even with Velvia.

 

[karavelov]

Thank you very much!
Yes I will use this method. Just marked f45 and f64 to the scale of the Xenar in order to know where to stop f22 and f32 with the Super Angulon.

luben