I was working with both lenses and was shocked to see that angle of view of 28-135 at 100mm position and 100/2.8 makro is not the same. 28-135 at 100mm position is definitely wider. Can somebody check their lenses?
Guna�rs
All lenses that are IF or use any focussing means other than moving the whole lens further from the body change focal length as they focus. I don't know which 100 macro you have, but the USM also changes focal length. Most modern zooms change focal length drastically to get into their 'macro' range. In the past, some zooms went into 'macro' range by setting them into their shortes focal length, turning a section of the lens barrel and then using the push-pull zoom section to focus closer. In this latter case the whole lens assembly _was_ moved away from the camera, and the lens did not change focal length.
Another example: the 180 macro would have to move 180mm further away from the camera to focus at 1:1 if it didn't have IF and change (shorten) focal length.
Only at infinity can you expect the correct focal length, within the self-imposed limits on accuracy that the optical industry sets (around 5% max. deviation from marked focal lengths).
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