I'm confused

LOL

What it means Ally is that with that lens on your 20D throwing the image onto a sensor smaller than a 35mm frame then you get the equivalent field of view which you would using a 320mm lens on a 35mm camera. The actual focal length of the lens doesn't really change.... sorry. :)
 
yes the crop factor means that you x1.6 the focal length to give the effective focal length when attached to the 20D so yes 200 x 1.6 = 320mm. the lenses doesnt change but the effective focal length does and would give the same magnification as attached a 320mm lens to a film slr

as far as im aware it has no effect on the aperture.

hence why DSLR's that crop are a pain for wide angle as a very wide lens on a film slr will be subject to the crop factor of the camera and ends up not being as wide after all.

this is great for people who use telephono zoom lenses alot (as shorter telephotos generally dont cost as much as the longer ones!!!!) but not so good if you use wide angles, which leaves you with either buying specialist crop only wide angle lenses (canons EF-s range and sigmas digital only lenses) or a full frame DSLR if wides are your thing!
 
Whitewash is right - the aperture isn't affected. It is actually a serious advantage in some situations. Say you're photographing a tiger at the zoo with your 200mm lens and next to you is a guy with a full frame 1DSMk2 using the same lens.You both get the same size image on the sensor, but he ends up cropping more. :D To fill the frame on that huge sensor he needs some very expensive glass. :eek:
 
Yep - you get the equivalent of a 'longer' lens for free! 200 'appears' to be a 300 (ish) on most D-SLRs - all Nikons and some Canons.
 
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