![]() ![]() Similarly to the 20D, diffraction limiting sets in at ƒ/11, but the actual effect on image quality is minimal even at ƒ/16, average image sharpness is approximately 2 blur units from corner to corner. The optimal setting for sharpness seems to be 35mm and ƒ/8, where central sharpness is just over 1 blur unit, and just over 1.5 blur units in the corners. At ƒ/5.6, corner softness starts to settle down at 35mm and longer. Stopping down to ƒ/4 and ƒ/5.6 improves results for sharpness dramatically, similarly to what we noted with the 20D, but corner softness is still noteworthy. Things improve a bit as the focal length is increased, at the expense of central sharpness. True to what we noted in our tests with the 20D, central sharpness is still remarkable at 24mm and ƒ/2.8, but outside of this sweet spot sharpness falls off rapidly, reaching (and I think this is a record for us) 17 blur units in the top corners and 13 blur units in the bottom corners. ![]() If you are looking for a lens that is corner-to-corner sharp when used wide open at ƒ/2.8, this is not the lens. On the full-frame 5D, the true character of the lens is revealed. At ƒ/16 we note a small loss in sharpness (1.5-2 blur units across the frame) and at ƒ/22 it's around 3 blur units across the frame. At ƒ/8 we see minimal improvement, and while diffraction limiting sets in at ƒ/11, there's no practical drop-off in sharpness. Sharpness improves as the lens is stopped down, reaching excellent results by ƒ/5.6 - under 1.5 blur units across the frame at every focal length. Overall sharpness degrades towards 70mm, where we note better than average sharpness, about 3 blur units across the frame. On the subframe 20D, the lens shows excellent sharpness at 35mm and wider at ƒ/2.8, with a generous sweet spot of sharpness (1-1.5 blur units) in the center of the frame which quickly devolves into some pronounced corner softness (4-5 blur units). It loses its sharpness fairly quickly in the corners, especially obvious on a full-frame body. The Sigma 24-70mm ƒ/2.8 is a decently sharp lens, especially when mounted on a cropped-frame dSLR body such as our Canon 20D. The Sigma 24-70mm ƒ/2.8 HSM ships with a petal-shaped lens hood, takes 82mm filters is currently available for approximately $900. ![]()
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