Originally posted by bossa Does this mean that my FA*300 F/4.5 is really an 450 F/6 (?) on APS-C?
I just can't help myself. Sorry, Ash
The lens doesn't magically change properties, but effectively - read, effectively - it is like shooting a 450 f/6.3 on FF, as far as you, the shooter are concerned.
I think when people say "a 300mm lens is a 300mm lens, period!" and leave it at that, it actually confuses the situation more, and is not helpful, because
it doesn't really answer the question the person is probably asking. It's almost always really a question about equivalence, and how that 300mm lens will behave, for them, the shooter, on either format.
So, yes, if you shoot that 300 4.5 on FF, then move it to an aps-c camera, it will effectively (in FOV/DOF) be like shooting a 450 f/6.3 lens on FF. Or, if you're shooting it on aps-c, and get a chance to shoot it on a FF camera, that 300 f4.5 will have, on FF, the FOV/DOF equivalent of about a 200mm f/2.8 would on aps-c.
Not with regards to actual light gathering - your shutter speeds won't be affected, it's still exposing at f/4.5. We're talking FOV/DOF.
Now, if you're shooting a FF camera with enough pixels, you can simply crop that image to cover the same FOV as the aps-c image, and it will basically be the exact same image. The reason that this isn't universally done is because you'd lose IQ over simply shooting with an aps-c camera in most cases - you'd need about a 38MP FF camera to match the K-5's resolution after cropping, for example. I sometimes shoot aps-c lenses on my 12MP D700 in 'DX mode', which superimposes an aps-c sized frame in the viewfinder and crops the image to aps-c FOV for you, and it only gives me about 5MP on the target. (still good enough for 11x16 prints.)
So - I don't know, it's just a pet peeve of mine when the real question being asked here is answered with "No - lens doesn't change. Full stop." It's almost never helpful, almost never answers the real question.
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