The SEL20F28 is very light and ultra compact – 2 mm shorter than the already thin SEL16F28. Autofocus is very fast when using hybrid-AF and build quality is excellent with a magnesium alloy body, although the lens feels plasticky due to the low weight. Handling on a NEX-5T is a little fiddly when using manual focus or focus override, because the focus ring is extremely thin and has nearly the same diameter as the body. The effective focal length of 31 mm on APS-C cameras makes the SEL20F28 an ultra compact lens with excellent performance for reportage photography.
Condition of my copy
Optics: Excellent. No scratches, no dust – new.
Mechanics: Excellent. Focus ring uniform, aperture clean and fast.
Exterior: Excellent. Like new.
Optical performance on NEX-5N / 5T
The following evaluation is based on uncorrected RAW images. Please keep in mind that all Sony E mount lenses are electronically corrected in camera (vignetting, distortion, CA) and JPEGs of these shots therefore look very different.
Notice: In the test charts, the left quadrant of the picture was a little less sharp than the right. This suggests either a slight decentering of the lens or a misalignment of the camera in relation to the chart.
The lens starts at f/2.8 with very good center sharpness and only slightly soft corners. At f/4, corner performance improves visibly and central sharpness reaches outstanding levels. The image stays mostly unchanged at f/5.6 and the corners finally sharpen up at f/8. This is also where diffraction starts to soften the center again, hitting it harder at f/11 and f/16. Looking at the image series, though, I might have misfocused a little at f/8.
There are no visible CAs at any aperture. Either this is an excellently corrected apochromatic lens, or the electronic retouching of CAs in-camera cannot be deactivated.
Vignetting is pronounced with about 1 1/2 stops at f/2.8, improves to 1 stop at f/5.6, but never disappears. The lens also exhibits a complex, moustache-like barrel distortion of -2.3%. Under-corrected vignetting and distortion are typical for modern, electronically corrected lenses as the designers usually trade these errors for better sharpness and less CA. The effective T-stop at f/2.8 is T2.9 (-0.1 EV), which is excellent.
The SEL20F28 unsurprisingly performs better than Sony’s kit zoom SELP1650 in pretty much every aspect – except the corner performance at f/4, where the zoom is actually ahead a notch. The 20 mm is the most compact and light lens you can get for a NEX. Combined with the fast (and silent) AF, this lens is predestined for street and reportage style photography. If you are looking for slightly better optics at half the price and don’t mind size and weight that much, you may be better off wih a Sigma 19 mm f/2.8 EX DN E, though.
The following images are pixel-level crops from the test chart. They may appear scaled in your browser window. Click on them to view the crops in full size and cycle through them easily. For more info on the test setup, visit the details page.
(Cropped areas marked in orange)