I have both the Rokinon branded Samyang 8mm f/3.5 Fisheye and the Sigma 10mm f/2.8 DC EX Fisheye in A-mount for APS-C DSLRs and wanted to see what images would be produced by these 2 lenses when used on the same camera at the same position.
According to reviews, the Samyang and Sigma fisheyes are supposed to be designed according to different projection types. The projection type of the Sigma is equidistant while that of Samyang is stereographic. Stereographic projection is supposed to be produce more eye-pleasing images and is more difficult to make resulting usually in more expensive prices.
Although both lenses are supposed to give the same field of view of 180 degrees on an APS-C sensor, I found that the Samyang is producing a wider coverage than the Sigma in line with its focal length of 8mm compared to Sigma's 10mm.
These were taken with both lenses on a Sony A55. The aperture on both lenses were kept constant at f/5.6.
Both the Samyang and the Sigma are almost equally sharp but the Sigma has more rectilinear distortion (straight lines are more curved) and more colour fringing at the edges.