Accurately comparing FF vs APS-C sensor performance? An open discussion.

Started Apr 8, 2013 | Discussions thread
VirtualMirage
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Re: How about this?
In reply to Klipsen, Apr 16, 2013

Klipsen wrote:

On IF lenses, focusing is not done by extending the lens, but rather by changing the focal length of the lens. This is another thing you have to take into consideration - and no DoF calculator does.

My Minolta 200 mm macro has a focal length of only 105 mm at the 1:1 setting, but I know the magnification, so I can still calculate the DoF. Do you have a DoF calcultor that does this? If not, then, by your logic, all results from your DoF calculators are equally wrong ... and therefore equally right.

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Ceterum censeo soleam calidam ISO esse delendam.

Now that makes more sense.  This is related to focus breathing, correct?

So would it then be more correct to say that the DoF calculators would be more accurate if you are using the equivalent focal length the lens achieves after it locks focus and then matching that to a lens that offers the same equivalent focal length after it locks focus?

This may then cause a difference in what lenses may be needed in order to achieve the same equivalent focal length after focus has been locked.

So to say that a 60mm lens on an APS-C will give you a similar equivalent focal length of a 90mm on a Full Frame may or may not be correct. It's accuracy will depend on whether the equivalent focal length changes during focus and whether that final equivalent focal length is the same on both.  Am I better understanding this now?

So to make my observation more accurate, I should not base my equivalent focal length on the lens focal length designation (as well as crop multiplier) but, instead, its final equivalent focal length after focus has been achieved (which may or may not be the same already since it is lens dependent)?

Thus, if it is assumed the latter instead of the former, then my observation holds true?

Thanks!

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Paul

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