I just purchased a speed graphic camera with a top mounted rangefinder. It comes with a graflex optar 135mm f/4.7 lens. I was wondering IF (still haven't received camera yet) it has a matching cam would I be able to swap out the lens for an apo-sironar s 135mm lens and still be able to use the rangefinder?
Almost a hundred cams are listed on http://graflex.org/speed-graphic/top...nder-cams.html. Perhaps the factory selected the cam from these that most closely fitted the focal length of the installed lens. On this list the focal length is spaced at about 1.5mm in the vicinity of 135mm. On my MPP camera the lens and the cam are both marked 135mm, although with different serial numbers. The rangefinder tracks perfectly.
Marked focal lengths are approximations. Actual focal lengths typically differ from that by at least a bit, in different ways across different models.
To be able to swap directly, both the flange focal distance and the focal length need to match exactly. You can reset the infinity stops on the camera to match a different flange focal distance, but if the actual focal length differs, the indicated focus will still be accurate at only one point. Depending on the kinds of pictures you want to take and on your working aperture, it may or may not be close enough for you at other distances.
Unfortunately, unless you have an optical bench to measure focal length, they only way to find out is to try it. I'd say there is a 50/50 chance it might be "close enough for government work."
Good luck.
P.S. the Optar is a much better lens than is admitted by some ultra-snob LFers. It may be all you need.
Get youself a small machinist's hammer, a flat steel surface to work on and and a fine file. Mount the Sironar and set the lens infinity stops with the camera track back on its stops. Now look through the RF at a distant target. If it has focused beyond infinity, the cam is too high on the high end. If the RF does not yet coinside, the cam is too low on the high end. Use the file to lower high spots. Tap with the hammer along sections of the cam that are too low. This will stretch out the metal, adding height, and you can later smooth out the curve with the file. Use this proceedure at several more focus points closer to the camera. You can make a very accurate cam from hobby shop brass stock with this method. You can also adjust orphaned Linhof cams to a new lens this same way.
Infinity height on the cam is always infinity and should need no adjustment unless the cam has been re-ground or is plain defective. It's past the infinity mark, on the curve, that things could go wrong.
I've used quite a few different lenses of the "same" focal length on my Supers with no problems with the cams and RF. Of course it all depends on a correct infinity stop setting in the first place as that may change slightly between different make lenses.
As far as changes in the focal length of any particular 135mm lens? Well how about the distance between the lens and film compared to distance to subject...
Using the old formula of: (FL * FL) / (distance to subject - FL)
For a 135mm lens at 3.5 feet, the shortest distance the RF will work we get 19.6mm
Move up to a 140mm FL and we get 21.1mm
Down to a 130mm FL and we get 18mm
Not all that much of a difference unless it's something very critical and you don't stop down... Although I'm sure many will disagree with me
TRF Graphics lenses were often factory cammed on "soft stops" (RF will continue to diverge beyond infinity focus) unlike Linhofs which are set up on hard stops. I don't like soft stops myself, and I find that if I cut my own cams, RF and GG focus will be the same on all my lenses on hard stops. I'm pretty critical about accurate RF focus and I want accuracy at open stops. It can be done.
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