Can you shoot some stars tonight with this lens and a 70-200 (and a 200 prime if you have one) all at max focal length? I suspect the problem Bryan saw was in the combination of zoom and focus. Up close the optical changes required to focus the light may have masked the change in focal length. And, unfortunately, Canon's definition of "close" might be half a mile.Originally Posted by neuroanatomist
Remember, the effective focal length of a lens actually only applies to paraxial (near the optical axis) rays of an infinite source. That's not to say that's how a lens will behave at finite conjugates nor anywhere but on the optical center (just read your profile so I'm sure you're well aware of this fact - microscopy is littered with marketing half-truths - same as many diagnostics fields).
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