I started wondering if a closeup diopter might improve corner sharpness, so I went on Ebay and bought a set of Vivitar closeup lenses, 52mm, +1, +2, +4 and +10 diopters. Made it to the pool to do some tests with a couple of the lenses. Setup: Nauticam NA-EM5, 4" WA port, no diopter, +2 diopters and +4 diopters, no flash. All posted pictures are taken at 9mm.
I had to apply some rather heavy distortion correction in Lightroom. For +0 diopters I used -10, for +2 diopters I used -26 and for +4 diopters I used -32 at 9mm. At 18mm, I used -10 regardless of diopter. In addition to the distortion correction, I did some angle and Keystone correction to get comparable pictures (all shot freehand while freediving), but that correction was rather minor.
First picture is the test chart (image downscaled), printed on A3 paper, laminated and brought to the pool. Set up against the pool wall the chart sagged a little, so the images are far from 100% quality. However, to me they look rather convincing...
Second image is a composite (1:1 crops). Top left corner of the chart, 1:1 crop. Left column: +0 diopters, center column: +2 diopters, right column: +4 diopters. From the top: f/4, f/5.6, f/8 and f/11.
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