Samyang 8mm f/3.5 Fisheye (Sony NEX) - Review / Test Report - Analysis
Lens Reviews -
Sony Alpha/NEX (APS-C)
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The Samyang is a fisheye lens so the extreme amount of barrel distortion is actually a feature and not a bug. However, the Samyang is still a fairly unusual fisheye - it has a near-"stereographic" projection so its distortion characteristic is actually less extreme at the image borders compared to conventional fisheye lenses. It's the first of its kind and Samyang even filed a patent for the design.
It's basically impossible to provide really valid vignetting data for a fisheye lens. We tried to do some "creative" lab testing during the corresponding EOS test and we'd estimate the vignetting to be around 1EV at f/3.5 and about 0.7EV from f/5.6 onwards. This may appear to be a bit on the high side but in the real life this is really a meaningless amount because the fisheye effect will overshadow any amount of vignetting to viewers anyway.
Testing a fisheye lens is a bit of an adventure rather than factory-style measurements. It's fairly easy to get meaningful values at the image center but the borders are already far, far, FAR out there so we had to use a quite non-standard setup here and it was really impossible to measure the extreme corners in a meaningful approach.
Anyway, the center resolution is already very good to excellent straight from f/3.5 whereas the borders are quite soft. However, the borders receive quite a boost at f/5.6 reaching very good figures here. Based on our field image we'd say that the extreme corners see a further improvement at f/8 though. f/11 is still perfectly usable but diffraction effects start to reduce the effect resolution here already.
The tested sample showed a centering issue on the far right image side.
Below is a simplified summary of the formal findings. The chart shows line widths
per picture height (LW/PH) which can be taken as a measure for sharpness.
If you want to know more about the MTF50 figures you may check out the corresponding
Chromatic Aberrations (CAs)
Fisheye lenses are extreme lenses by definition and their edge projection characteristic isn't exactly easy task for today's image sensors. Consequently there's quite an amount of lateral CAs (color shadows at the image borders) of around 2px (width) at the image borders and some more in the extreme corners. This may sound worrisome but this is in line with other fisheye lenses that we've tested so far.
However, even such an amount of lateral CAs can be corrected during post-processing (e.g. in Photoshop).