Samyang 8mm/2.8 on the NEX-7

CornerFix requires a reference shot in order to calculate a lens profile. This should be an shot of an evenly lit neutral surface. For a fisheye lens, this proves problematic as you cannot simply point it at a neutral grey card or white wall. I set up two flashes either side of the camera. I held up a white diffusion sheet so that it wrapped around the lens and gave full coverage over the angle of view (180 degrees diagonal). I then took several shots making sure that the image was not overexposed (no clipping).
Magenta lens cast in the corners due to oblique rays causing non-uniform response
CornerFix is able to compensate for the non-uniform response and reduce the colour casts

I just received my copy of the Samyang 8mm/2.8 fisheye in native NEX mount. I’ve tried it on the NEX-7 and have some very preliminary remarks. First the good news, it’s sharp, light, compact, pretty well made, and matches the form factor of the NEX-7 quite well. The only downside so far is the interaction of the lens with the sensor leading to magenta corners.

This lens colour cast is due to the way that off axis light rays interact with the filter stack on top of the sensor and the pixels themselves. The different coloured sites in the Bayer array are affected differently leading to colour casts. This can be a problem with certain lens designs where the exit pupil is situated close to the sensor (non-telecentricity).

To fix this, you can use CornerFix, a program which takes DNG files and applies a rescaling to the data to compensate for the lens colour cast. One has to first create a reference file from which the compensation can be calculated, not exactly straight forward for a 180 degree diagonal fisheye. I have made such a correction profile and people can download the file (you may need to right mouse click and choose “save link as”, or similar). The lens was set at f/5.6 but the profile should work more or less at other apertures. The correction isn’t perfect, but for most photographic scenery it corrects the majority of the colour cast, as well as the vignetting (which can be adjusted).