This is a very interesting and informative thread. It may help me understand some behavior of my Nikkor Micro 60mm F2.8. I was under the impression that a prime lens was at a fixed focal length. And, I was under the impression that focus was achieved by physically moving the lens (or an element group) to get the correct distance between the optical focal center point and the image sensor plane. For a zoom lens, another element group would be moved to alter the focal length.
But, I have noticed that at close subject distances focus affects image size. That implies that the focal length has changed. So do the comments regarding effective apertures in this thread. It appears to me that the focal length would get smaller as the subject distance decreases? (That is a question.)
The camera view finder data and EXIF data do show the “effective aperture” changes, but they do not show any change to the focal length (it still says 60).
I realize that there are lots of different lenses by different manufacturers, but in general… Does the lens rating (such as 60mm) for a prime lens only refer to the focal length when focused on infinity? Actually the same question holds for a zoom lens. What does the “rated” focal length mean?
Or, does the whole focus mechanism change when you get real close-up and run into that “limit” switch on the side of some lenses?