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Zeiss Loxia 21mm PCMagazine Review:
http://www.pcmag.com/article2/0,2817,2495555,00.asp
I used Imatest to see just how sharp the Loxia is when paired with the 36-megapixel Alpha 7R. At f/2.8 it scores 3,098 lines per picture height on a center-weighted sharpness test, which is much better than the 1,800 lines we like to see in an image. Performance is strong through most of the frame, though the center is markedly crisper (3,622 lines) than the edges (2,366 lines).
Stopping down to f/4 boosts the average score to 3,282 lines, and improves the edges to about 2,500 lines. At f/5.6 the lens manages 3,495 lines (with edges that touch 3,000), and peak performance is achieved at f/8—3,601 lines. Beyond that diffraction robs a bit of quality; sharpness dips to 3,376 lines at f/11, 3080 lines at f/16, and 2,257 lines at f/22.
There is some darkening of the corners at f/2.8, about 2EV, which is noticeable. Stopping down to f/4 dropped that to a much more reasonable 1EV—slightly dimmer than the center, but not distracting. At narrower apertures, f/5.6 all the way through f/22, the corners displayed a 1.4EV drop when compared with the center of the frame.
The lens shows some distortion, though it's not drastic. You can expect about 1.2 percent, which you'll have to look for carefully to notice. Its pattern is mustache, which resembles barrel distortion at the center of the frame, but straightens out toward the edges.
In comparison Zeiss Batis 25mm:
http://www.pcmag.com/article2/0,2817,2492704,00.asp
I tested the Bastis 2/25 with the full-frame, 42-megapixel Alpha 7R II. The Imatest resolution test shows that the lens scores 2,715 lines per picture height on a center-weighted test at f/2. That's much better than the 1,800 lines we look for in a photo, and while the edges of the frame can't keep up with the center, they're still crisp at 2,147 lines.
Stopping down to f/2.8 bumps the overall score to 2,844 lines, with edges that approach 2,500 lines. At f/4 the lens improves to 3,197 lines, with even performance across the frame, and it just gets crisper at f/5.6 (3,421 lines) and f/8 (3,664 lines). Diffraction sets in at f/11, but the lens still shows 3,468 lines there. There's a steeper drop at f/16 (3,002 lines), but it's not until the minimum f/22 aperture (2,120 lines) that diffraction really takes away from resolution.
When shooting at f/2 the corners are noticeably dimmer than the center, showing -2EV illumination in comparison. From f/2.8 the fall-off is limited to -1EV at the corners, and from f/4 through f/22 the lens shows about -0.7EV at the corners.
The lens shows about 1.2 percent mustache distortion. The lens shows a bit of barrel distortion at the center of the frame, but that gives way to a pincushion type pattern toward the edges. Thankfully it's pretty minor.
They were not tested on same cameras, but it is close enough as well as the results... 21mm might have a little more center sharpness eg at f/2.8 and more edge fall of w/ vignetting...
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