Opticallimits tested the Samyang AF 18mm f/2.8 FE and concluded:
The Samyang AF 18mm f/2.8 FE isn’t the silver bullet in its segment but given its size and also price tag this can’t be expected anyway. If you are aware of its limitations, it can still be an excellent tool. While it isn’t the last word on corner sharpness at f/2.8, the question is whether it’s required for on your usage pattern. It’s not ideal for astro photography but sharp corners aren’t really needed for shallow depth-of-field photography. Stopped down, the image quality is actually very decent across the image frame so applications such as landscape photography or city scenes are certainly possible with pretty impressive results. The amount of lateral CAs is about average but can easily be corrected in post. Image distortions are moderate for such a lens even in RAW format. An unsurprising weakness – given the small front element – is vignetting. It’s very heavy at f/2.8 and not brilliant at medium aperture settings either. Auto-correction reduces this a bit but some light falloff remains. We didn’t run our formalized bokeh tests but based on the field images, the quality is in line with other ultra-wide lenses – meaning that it can be a bit on the rough side.
We aren’t totally convinced by the build quality of this little Samyang lens. Formally there’s nothing wrong with it. There’s nothing shaky about it and it doesn’t change size during focusing. The focus ring is smooth, too. It just feels a little … cheap. The AF speed is Ok for such a lens but it takes a while before it finally settles.
It has to be pointed out that the Samyang AF 18mm f/2.8 FE is – by far – the most affordable option ultra-wide AF lens in Sony E mount. And it’s not only affordable but it also represents a great value proposition.