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Full Aperture: Viewed at 100% images shot at full aperture are pretty soft. There is also a fair amount of distortion at the wider settings and large amounts of vignetting at all focal lengths at maximum aperture. It’s not all bad news though - images respond quite well to sharpening - there is detail there - and the increased resolution means that at normal print sizes, say up to A4 - results will actually look OK. The drop in illumination in the corners isn’t as drastic as I had feared. Although very noticeable in a technical sense - I measured it at about 2/3 stop at 20mm - in real world pictures it’s less of an issue with this lens than with some of the others I’ve been testing with the 5D, although there is a very small darker area right in the extreme corners. Overall, full aperture is best avoided.
I’ve debated for a while how much sharpening to give the test crops - in the end I’ve decided to give my standard Canon lightness channel USM to all the crops. The full aperture results can be improved with a much more aggressive sharpening but for the purpose of the test I think standard sharpening is more appropriate. I’ve also decided to try using larger test crops to make it easier to spot differences when viewing online.
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