Re-calibrating F-Stops : LUSENET : Large format photography : One Thread

I have a 210mm f6.3 Computar Symmetrigon mouted in a very clean Copal#1. I just called Schneider and they confirmed that the cells of a 210mm f9 G-Claron will screw directly into a Copal#1, no spacing adjustments needed.

If I were to put the G-Claron's cells into the Coputar's Copal#1 would I have to have the f stop scale re-calibrated (besides the fact that there is no f9 making on the shutter)?

Thanks in advance.

-- sheldon hambrick (, September 03, 1999


You can test it by mounting a known lens, setting it to an F stop both lenses have, opening the shutter, and measuring the light at the center of the ground glass with your meter (on reflective mode). Use a white wall, and light it with whatever light source you wish. Then mount the new lens in question, set it to the same F stop, and see if your reading at the ground glass is the same. If, not, you can use this method to recalibrate your F stops.

-- Ron Shaw (, September 03, 1999.

As you stop down the aperture, what is the first marked f-number? If it is less than f/6.3, (e.g. f/5.6) then is certainly need recalibrating.

As you may know, the f-numbers on your shutter will be for a particular lens, or at least a particular focal length. If the previous lens for that shutter wasn't 210mm, it will probably need recalibrating.

You can readily find if it is approximately correct. Mount the lens in the shutter, and stop down to a setting. Suppose this is f/16. This means that the aperture, viewed through the front of the lens, should be one-sixteenth of the focal length. 210 / 16 = 13mm. Lay a ruler across the front of the lens, and measure the aperture. Is it about 13mm? If it isn't, then it needs recalibrating. Doubtless many people can do this, but the name 'S.K. Grimes' keeps cropping up for this sort of work (in the US).

-- Alan Gibson (, September 06, 1999.

I've realised I mis-read your question. You have two lenses, same focal length (210mm) but different max apertures (f/6.3 and f/9), and one shutter. Yes, the aperture scale may be incorrect for one of the lenses, and this is likely to a certain number of f-stops out.

Do the test in the last papragraph of my previous post, with both lenses, to find if there is a difference.

-- Alan Gibson (, September 07, 1999.

Yes or No it depends.

You don't have a 210mm lens. You have 2 lenses whose NOMINAL focal length is 210mm but whose actual focal length is something else. Possibly more than 210mm or possibly less than 210mm.

If both lenses are the same actual focal length than the apertures marked on the scale that are common to both lenses are correct for both lenses.

If the lenses are not the same actual focal length then the sacles will be slightly off.

If you have a light meter designed to read through the ground glass, or a metering bellows for your camera like the Linhof Focus/Meter bellows, or an in the film plane meter like the Gossen then you can measure the light reflected off an evenly illuminated grey card and easily calibrate the lens.

If you don't have this capability, and don't want to pay a repair man to calibrate the shutter then your new lens in the old shutter may seem to be marginally faster or slower then the old lens since the scale is not calibrated properly.

-- Bob Salomon (, September 08, 1999.

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