Originally posted by sahale Are you sure about this? That doesn't make sense to me. If I zoom in and focus on someone 2 meters away, it should be in focus on the same spot when I zoom back out because I haven't changed the focus point. After all, the the 2 meter point on the manual focus ring is 2 meters whether at 18mm or 250mm. If what you're saying is true, then the 2 meter spot on the manual focus ring changes depending on the focal length.
Zooming in for the purpose of obtaining better focus then zooming out to take the photo has worked across the Nikon and Canon cameras I've tried as well. I still haven't tried that many cameras or lenses, so feel free to correct me if I'm wrong.
Yes, I'm sure. It is a lens design issue and not specific to one camera system. For one thing, I have lenses that behave this way. I can't remember if it is one or both of them, but my Pentax SMC-A 35-105/3.5 and/or A 70-210/4 operate like this. Folks on the forums who know more about lens design than I do may be able to say if the vaifocal nature of these lenses is connected to their having a constant minimum aperture across the entire zoom range.
From wikipedia (
Varifocal lens - Wikipedia, the free encyclopedia)
A
varifocal lens is a
camera lens with variable
focal length in which
focus changes as focal length (and
magnification) changes, as compared to
parfocal ("true")
zoom lens, which remains in focus as the lens zooms (focal length and magnification change). Many so-called "zoom" lenses, particularly in the case of fixed lens cameras, are actually
varifocal lenses.
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