Samyang AF 18mm f/2.8 FE - Review / Test Report - Analysis
Lens Reviews - Sony Alpha (Full Format)

Distortion

Ultra-wide angle lenses tend to produce a certain degree of barrel distortions and this also applies to the Samyang AF 18mm f/2.8 FE. A native distortion of 1.8% is actually quite good for such a lens. However, most users probably prefer to have image auto-correction activated in which case the distortions are negligible.

Vignetting

Vignetting is pretty much unavoidable in ultra-wide images - it's just a matter of how extreme it gets. In this case, it's quite extreme indeed with a maximum light falloff of ~3.4EV (f-stops) at f/2.8. Stopping down helps but even at f/8 the native vignetting is very high and visible.

With activated auto-correction the falloff is reduced by about 1EV (f-stop). This means that vignetting is still easily visible at f/2.8 but it's acceptable from f/4 at least.

MTF (resolution)

The resolution characteristic of the tiny Samyang AF 18mm f/2.8 FE is pretty decent. The broader center quality is actually very good at f/2.8 but the borders and, more so, the corners are soft at this setting. This is not unheard of in ultra-wide lenses though. Stopping down to f/4 lifts the borders to very-good levels (albeit just) and the corners are good. The sweet spot is reached at f/5.6 with an excellent center and the corners have caught up with the borders. As usual, diffraction takes its toll from f/8 onwards and there's a visible drop in quality at f/11.

The field curvature is very low for such a lens. The centering quality of the tested sample was Ok.

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 Imatest Explanations

Chromatic Aberrations (CAs)

Lateral CAs are quite good with an average CA pixel width of ~1px at the image border. Of course, image auto-correction can come to the rescue here as well so it's not really something to worry about in real life.