View Full Version : calculating aperture scale - an easy way?

tim atherton
4-Feb-2006, 09:54
Is there an easy way to calculate an aperture scale for a shutter/lens?

I have my 210mm Kowa Graphic in a Copal #1 shutter with an appropriately marked aperture scale.

Say I want to put a 150mm Kowa Graphic in to try it for a while - is there a way to come up with a scale without sending the whole thing off to Grimes or somewhere? (I'm not talking about the actual little piece of metal for the scale itself - I'll just use a piece of tape or some such and mark it - rather the measurements)

Jim G - don't you add scales to some of those Polaroid shutters?


Michael Gudzinowicz
4-Feb-2006, 10:16
When going from a 210 to a 150 lens, multiply the marked value by 150/210 or 0.714 . If you use the 150 cells, then the actual aperture will be 1 stop faster than that indicated on the shutter (f/22 with the 210 is actually f/16 with the 150).


f#150 = 150/iris_diameter

f#210 = 210/iris_diameter

iris_diameter = 210/f#210 = 150/f#150

f#150 = f#210 x 150/210 for the same iris diameter (marked f-stop).

tim atherton
4-Feb-2006, 10:32
okay Mike - thanks - but what about for a focal length that isn't so conveniently close to being one stop different?

4-Feb-2006, 10:47
I think you can buy the aperture scales for copal lenses. You just need to tell them the focal length to get the correct one. Its only two screws to undo and replace. You shouldn't need to send the lens off.

Richard Kelham
4-Feb-2006, 11:08
I seem to remember that the f no is derived from dividing the 'apparent' (rather than actual) diameter of the iris into the focal length. In practical terms this means working out what the apparent diameter should be for a particular f no then move the lever until the iris is the right diameter (ie with the lens cells in place) then mark it on the scale.


Ole Tjugen
4-Feb-2006, 11:24
Two lenses of the same focal length will not necessarily have the markings in the same place. The apparent aperture at a given setting will vary with the construction of the lens.

Ernest Purdum
4-Feb-2006, 11:43
The problem with mathematical approaches is (as Richard points out) that the "apparent" aperture determines the f number, along with the focal length. Differing designs of lenses have differing "apparent" apertures. You can see this by taking the front cell off a lens. The aperture will look different with and without the cell in place. Apparent aperture sizes are not easy to measure, The usual way is by a "traveling" micrometer. This is a little telescope with a vertical guideline which is mounted on a transverse calibrated slide. You look at the front of the lens in question, move the telescope so that it lines up with first one side of the aperture, then the other, and read the slide markings in each position. People have made up devices which are crude simulations of the traveling micrometer. The accuracy of these would be adequate when one considers that the difference from one stop to another is X2.

I think the most feasible approach is to aim the camera at a plain wall and use an exposure meter at the groundglass to compare between a known lens and that for which a new scale is desired. Ordinary care to assure that all conditions remain the same between readings should give a satisfactory result;.

4-Feb-2006, 12:51

Not being a mathematician in any way, shape, or form, I decided to send my 360 in to the good folks at S.K. Grimes. They engraved the aperture scale onto the copal shutter directly.

And, might I say... they did a great job of it! :)


Michael Gudzinowicz
4-Feb-2006, 13:57

The calculation is the same. For a 135 mm lens, the factor is 135/210,
or 0.643. So f/22 on a shutter marked for a 210, would be 0.643 x 22.6 or
14.5. Using the table below, f/14.5 is 1 & 1/3 stops faster than f/22.
Going the other way, if you want to set the f/stop with the 135 to f/22,
divide by 0.643. So you'd set the aperture for 22/0.643 = f/34.2, which
is about 1/3 stop past f/32.

I've included a lookup table below based on that published in the SPSE

The most convenient approach is to mark a piece of tape with the f/stop spacing
off the Copal scale, and slide it over to one side before pasting it on. The
shift is equal to the exposure factor. For the 135 example, the f/22 mark on
the tape would be placed at f/34.5 (1 &1/3 stops) on the scale for the 210 mm
lens. If you're changing focal lengths permanently, you can just drill the scale
and move it over by the desired compensation.

First Second
Whole Half Third Third
Stops Stops Stop Stop

1.00 1.19 1.12 1.26
1.41 1.68 1.59 1.78
2.00 2.38 2.24 2.52
2.83 3.36 3.17 3.56
4.00 4.76 4.49 5.04
5.66 6.73 6.35 7.13
8.00 9.51 8.98 10.08
11.31 13.45 12.70 14.25
16.00 19.03 17.96 20.16
22.63 26.91 25.40 28.51
32.00 38.05 35.92 40.32
45.25 53.82 50.80 57.02
64.00 76.11 71.84 80.63
90.51 107.63 101.59 114.04

I hope that helps. I'm under the weather, so if it isn't clear,
drop a note and I'll reply when I'm feeling better.


Conrad Hoffman
4-Feb-2006, 16:24
It's not that tough, and it doesn't have to be dead on for a temporary test. Hold the lens about two feet away and measure the aperture diameter with a pair of vernier calipers. The elements must be in place, thus you get the apparent aperture. If it's a 150mm lens, f/11 will be a 150/11 apparent aperture, or 13.6 mm. And so on. Adjust to the sizes you want, and mark the tape.

Michael Graves
4-Feb-2006, 17:40
I'm in that same situation. I have a 360 that the shutter crapped out on. It fits perfectly into the Copal 3 that my 300 is in. I just did a different film speed test for the 360. When shooting with the 300, I use one film speed. When shooting with the 360, I use the other. Crude; but effective.

Paul Fitzgerald
4-Feb-2006, 19:24

You can measure the apparent aperture by putting the lens assm. on a lightbox and put a sheet of frosted glass/ acetate/ paper across the front lens. You can do the math to find the needed sizes and draw them out on the paper. Stop down until they match and mark your tape.

(Actually this should be done with a point-light source at the focal length for inf. but the lightbox is easier.)

Have fun.

Jason Greenberg Motamedi
5-Feb-2006, 09:02
To follow up on Paul's suggestion, I have used a laser pointer as a point light source. IIRC I focused the camera on infinity, locked it down, and then pointed it at a white wall parallel to the camera. In a darkened room I pointed a laser pointer at the center of the GG and measured the size of the light which was projected through the lens with a calipers. This should be pretty close to the size of the apparent aperture.