APS-C IMAGE QUALITY VS fULL fRAME AT TESTING

Started Feb 24, 2013 | Discussions thread
Legion5 Senior Member • Posts: 1,047
Re: APS-C IMAGE QUALITY VS fULL fRAME AT TESTING
1

Mannypr wrote:

Can someone explain why is it that a lens that is just so so at the center and borders on a APS-C sensor , the same lens on a Full Frame is sharp . By what I have understand the center that is soft with a APS-C sensor , that same lens on a Full frame , the center should also be soft but I have seen lens when tested for sharpness having a sharp center with FF but soft with a APS-C sensor . Whats happening here ? I'm I missing something ?



The reason is because of simple math. Imagine a len's resolution as curve that is high in the center and expoentially falls towards the edge.

This is a realistic graph for a theoretical 1.41x crop sensor (resulting in an 50% smaller area, and 200% difference in crop, 1.41x makes the math easier).

The green in this graph symbolizes the resolution of a full frame lens, and the red (which is dotted) symbolizes the resolution of a crop lens. The blue is what happens when you view the red section on the same scale as the green section. You'll notice that it is remarkably lower resolution than full frame, EVEN THOUGH you are looking through the better part of the center of the lens.

Every single lens behaves in this way and it is an inescapable law of simple physical conversion. Though if the extreme corners of a lens are very bad on full frame they may be better on crop. Additionally, this graph represents a realistic conversion from full frame to crop with a realistic resolution curve.

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