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| Re: Medium Format FOV Calculation |
The field of view does change between sensors. The lens focal length does not. People calculate an "equivalent focal length" (which causes confusion, because the focal length is a fixed property of the lens) because they are familiar with a particular sensor size (e.g. 24x36 "full frame 35mm") and want to know how lenses will look on a different sensor size. Since your familiar "frame of reference" is already the 24x36mm sensor of your 5DIII, no conversion is necessary to understand how lenses will look on it.
On the other hand, if you want to know how wide an 80mm lens will look on a 45x60mm Mamiya 645 relative to use on your 5DIII, then you can figure the "full-frame equivalent" factor the other way around: along the width of the picture, the Mamiya 645 will see the 80mm lens as "equivalent to" if you were using a 36mm/60mm*80mm = 48mm lens on your 5DIII (e.g. an 80mm lens on 645 format is "equivalent" to the "normal" 50mm focal length on 35mm format).