StephenM
I know a Blithering Idiot
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ok cheers. Doesn't that change the focal length of the lens though, and consequently the size of the subject in the frame? Its a problem i have with my 5x4, trying to get the right framing of the subject while getting it in focus.
It works the other way round actually (I'm speaking of lenses in general, nothing RB/RZ specific). The size of the image is given by the ratio of the lens to subject and lens to image plane distances (strictly measurements made from the nodal planes of the lens). If you have a lens with a fixed focal length, you have to rack the lens in or out to focus, and the image size alters. If you want a constant image size while focusing, you have to change the focal length of the lens to focus (many (all?) lenses that use internal focusing do it this way; you can compare the image size at a close distance with identical (nominal) focal length lenses and see that some have a smaller image size close up). It's easy to demonstrate.