Originally posted by SOldBear This is another reason why "crop factor" concept is a mistake. It's confusing.
More succinctly, it sucks. It's not a technical term, it's a marketing term, like MACRO ZOOM or 35MM EQUIVALENT or NEW AND IMPROVED or TRADITIONAL VALUES.
Quote: The focal length of the lens stays what it always is. The minimal focusing distance stays the same. The size of the image, and thus the magnification ratio, stays the same.
Quite right. The ONLY factors affecting magnification via extension are the FOCAL LENGTH of the lens and the TOTAL EXTENSION on that lens. Subject size and distance, and frame size, do not enter into that calculation. A 50mm lens on 100mm total extension has a magnification of 1x, no matter whether the frame (film or sensor) is 110-m4/3, APS-C, 135/FF, 6x9cm, 8x10 feet, whatever. (This assumes that the subject is in focus.)
And the minimum focusing distance you mention, is the focal length of the lens. No non-reversed lens can focus closer than its focal length, no matter what camera it's on, no matter what size frame its image is projected on. The easy way to test this: put a lens on a bellows. Extend the bellows by various amounts. See how close the lens can focus at each extension.