Linhof - 150mm Lens Camming Question

[William D. Lester]

I have a Linhof Super Technika V with a cammed 150mm Schneider convertible lens which presumably came with the camera when it was purchased. My question is whether this cam will work with a new 150 Rodenstock lens if the infinity stops are reset or is an entirely different cam required even though the focal length is the same. My thinking is that if the rangefinder focus matches that on the groundglass at infinity, then all shorter focusing distances should work.

[Bob Salomon]

No. That is why the cams are ground to the specific lens and that the serial number for that lens is engraved on the cam. Once the cam is no longer used with that specific lens the cam will no longer track correctly with another lens of the same focal length, or especially, in your case a different design.

Additionally there may be a number stamped on the bottom of the cam. That would be for the specific Technika IV that the cam was cut to. It won't be accuraate on a V with any lens.

[Ed Richards]

I have been thinking about this for a while, and looking at my old view camera optics book. If we are talking about simple lenses (not telephoto or retrofocus), and the focal lengths are really the same (not just nominally, but really the same), then I cannot see why the cam would not work when the infinity stops were corrected. The range finder does not see the absolute position of the lens, only the relative position as defined by the infinity stops. The cam has to match the camera, at least on IVs, because the relative distance to the ground glass may change, but on Vs even that should not matter.

What am I missing? Optically, 150 is 150, the lenses should focus at the same points. Is the real problem that 150 is not 150, i.e., the original lens, if I remember, was 150, but I think the new lens is actually 151+. Or is the problem that the original lenses had enough variation that it would only be luck if your lens was 150.

[Michael S. Briggs]

Most likely neither the old nor the new lens has a focal length of exactly 150 mm, and chances are that the two lenses have different focal lengths. First, the value marked on the lens is a convenient label rather than the exact value that the manufacturer was aiming for. Some of the Schneider datasheets give the design focal length, which is frequently different by about one mm from the nominal. Two different 150 mm lens types from different eras might have different design focal lengths. Second, there are manufacturing variations that cause different lenses of the same model to have different focal lengths.

Bob knows what is he talking about when he says that the cams are ground for specific lenses. If you got lucky, two lenses might have focal lengths that were close enough. Or you could measure the focal lengths of numerous new lenses, trying to find one that matched the focal length of the old lens.

[Frank Petronio]

Just compare the RF and GG and make a consistent ~ quarter turn adjustment as needed...?