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At full aperture, image quality at 10mm is quite good right in the center of the frame - not exactly crisp, but fairly good levels of detail are recorded. Moving towards the corners, things get softer though - About the central two thirds of the image is reasonably sharp, with a noticeable fall in resolution as you move right in to the corners. Results at 13mm are very similar, while at 17mm the image gets softer still - Even central details have a slightly soft focus look to them. Overall, full aperture performance is OK at 10-13mm, poor at 17mm.
Stopping down to f5.6 improves results markedly - especially at 17mm where the center field is now quite sharp, only the corners still look a bit soft. Another stop fixes even that, with good sharpness, right through the range even in to the corners. This is an extreme lens - at 10mm it’s field of view is 180 degrees, so poor corner performance at wide apertures isn’t unusual, but it’s at 17mm that this lens is at it’s weakest. Vignetting is hard to judge with a lens like this - it’s not noticeable in any of the test frames.
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