This is a long winded way (sorry) of just trying to clarify a simple point that started with my trying to clarify what "Sprockette" actually meant with the statement: "held close to the body (for marco shots) or further away (for landscapes)" which still seems backwards if we are referring to lens distance to the camera body. I suppose it might mess up autofocus on slrs but by the time you have your lens on a rubber plunger you have pretty much given up on autofocus. Important in some places such as movie cameras, but not so much where we focus by eye. The wiki article Sprockette linked said: "Flange focal distance is one of the most important variables in a camera, as lens seating errors of as little as 0.01 mm will manifest themselves critically on the imaging plane and focus will not match the lens marks." It is not saying you cannot focus if your lens mount is off by 0.01 mm, it is saying the calibration of the scale (lens marks) will be off. Going from 4x5 to 8x10 is trickier if you are using a lens designed for 4x5 (cheaper), you can put the 8x10 back on but the corners of the image will probably be dark as the image cone cast by the lens is not large enough to completely cover the film (forget about tilts and shifts), you might even get a circular image on the film, it depends on the lens. It is also a great place to look at coverage as you can you can change from 8x10 to 4x5 format as easily as swapping the 8x10 back for a 4x5 back and the focus does not change, you can put on a medium format back, and as long as the backs are true, the focus in all will be the same. There focus is managed via the bellows and you can focus closer and closer until you run out of bellows draw. An easy place to see this is in large format where the focus adjustment is external to the lens. The lenses for the different formats are much different for the requirements of covering the film effectively, and tend to get more expensive as you move up in format. The angle of view changes as you change format but 90mm focal length is 90mm focal length. The focal length of a lens is measured at infinity, it is more complicated in multi-element (the elements shift inside) lenses but in theory the focal distance is the same for a 90mm lens regardless of whether it is on a DX, FX, medium format, 4x5 etc. Shifting the lens away from the body loses infinity focus but increases the close focus capacity, as seen with extension tubes, which simply moves the lens plane further from the sensor plane (or film plane).
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