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I have to decide between APS-C and full frame, and one of the things to look at is sharpness. With APS-C, as opposed to full frame, you're only using the central portion of the image circle, the sweet spot: you're not using those blurry corners, and this should lead to sharper pictures. On the other hand, because the sensor is smaller but final resolution is usually comparable, you're blowing up the analog image that the lens projects on the sensor; this is bad, because every defect will be magnified. Which force is bigger? I intend to find out empirically.
For this test I will use my best 3 lenses:
* Leitz Elmarit-R 35mm f/2.8 (1978) (Leica R)
* Leitz Summilux-R 50mm f/1.4 (1976) (Leica R)
* Leitz Elmarit-R 90mm f/2.8 (1966) (Leica R)
Plus a Canon 24mm f/2.8 that is relatively soft in the corners both on APS-C and on full frame.
The cameras will be a Canon 5D Mark II and a Canon 550D (T2i). Same manufacturer, same generation, similar sensor tech, similar pixel count, different sensor size.
I look first at high resolution (stills) and then at downsampled images (2K resolution).
With every camera and lens, I adjusted the distance to the test chart so that it always filled the frame. This way, two different questions can be answered:
* Is this lens sharper on full frame or on APS-C? (compare 50mm on full frame with 50mm on APS-C).
* Given my lens set, will my images be sharper if I move to full frame? (compare 50mm on full frame with 35mm on APS-C).
Scroll down to see the results.
As a general rule, a lens will always be sharper on full frame than on APS-C, both in the center and in the corners.
One could think that some lenses might be sharper in the corners on APS-C, depending on how fast sharpness goes down from mid-frame to the very edges of the image. But with the 3 lenses I looked at the result is always the same: full frame wins. Add to that the fact that for a given depth of field you're closing down the iris further on your full frame lens (and this helps sharpness too) and I think the result will be pretty much general.
Keep in mind, though, that this is a "same lens" argument. It's much easier to design lenses for smaller sensors. But if you've got a set of lenses that will work on full frame (like I have) then a full frame sensor is what they need to look as sharp as they can.
Also, when your final image is relatively low-res (1920x1080 video), images are basically identical, except that APS-C gets softer in the corners when the lens is really soft there (like my summilux f/1.4 when wide open). If the lens is not that soft, or if it has been stopped down a bit (say, f/2.8), then there's basically no difference between APS-C and fullframe.