In order to put the results of this test into perspective, let's compare the Samyang 16mm to the smc Pentax-DA 15mm f/4 Limited--a fine lens by any standards.
First, the full frames at f/5.6.
Samyang 16mm:
DA 15mm Limited:
Note that the light is different: it was getting dark and there is less daylight involved in the 15mm shots. I adjusted the colour temperature accordingly. So you cannot draw any conclusions concerning colour rendition. The Samyang has noticeably more distortion (but I had no difficulties in correcting it 100% in Hugin; once a Lightroom/ACR profile is available this can be done during RAW conversion).
Now the details: 100% crops for various f-stops. I don't post the Samyang shot at f/5.6 because there must have been some motion blur. As I am interested in the real-world possibilities of a lens rather than its theoretical performance, all samples are treated to get the best out of them: ACR's CA auto-correction and a little sharpening (a lot more would be possible without overdoing it).
The sharpening/NR setting is the same throughout:
Sharpening
Amount: 40
Radius: 0,6
Detail: 50
Masking: 0
NR
Luminance: 0
Color: 5
Color Detail: 50
Samyang 16mm, f/2, center crop:
Samyang 16mm, f/2.8, center crop:
Samyang 16mm, f/4, center crop:
Samyang 16mm, f/8, center crop:
As far as I am concerned, there is no relevant difference in the center between f/4 and f/8. Some diffraction is already visible at f/8 (or there was again some motion blur--the wooden floor in my livingroom is not very solid).
DA 15mm Limited, f/4, center crop:
DA 15mm Limited, f/5.6, center crop:
The DA Limited is remarkably sharp even wide open. I think there is no clear winner.
Now for the extreme corners. The Samyang is a little weaker in the lower left corner, so I chose that corner for the samples. This--or better than this--is what you can expect from a good sample.
Samyang 16mm, f/2, corner crop:
Samyang 16mm, f/2.8, corner crop:
Samyang 16mm, f/4, corner crop:
Samyang 16mm, f/8, corner crop:
At f/2 you see the effects of vignetting (not too bad; at f/2.8 vignetting is less than with the DA 15mm Limited at f/4). Sharpness is quite good at f/2 and very good from f/2.8 onward. It gets better until f/8 where it is beyond any doubt.
DA 15mm Limited, f/4, corner crop:
DA 15mm Limited, f/5.6, corner crop:
Ouch! The Samyang is much sharper at f/2.8 than the DA 15mm Limited at f/5.6. This is what the weight is for.
Stay tuned for more.