Originally Posted by tbm
Actually there is a hard and fast rule. IR focal length is about 1/500th of a given FL longer than visible light, therefore in a 50mm lens the lens must travel 0.1mm farther away from the film plane for the sharpest focus. In an 85mm lens its 0.17mm etc., etc.
The name on the lens has nothing to do with it, itís the properties of light and glass here that count, Leica, Mamiya, Canon, Nikon et al.
How this applies is in determining how much to twist the lens to achieve the extra 0.1 length.