I have already read much good about this lens before I decided to purchase one. Its main purpose in my field will be wide-field astroimaging. I plan to use it both with Canon DSLR and with my mono QHY163M camera for wide narrowband fields. It turned to be a little bit smaller and lighter than I suspected for f/2 lens with 77mm filter thread. Lens is well built, does not have plastic feeling. Both focusing and aperture rings works well, although focusing ring movement is not perfectly smooth and also could work a little bit easier. But anyway it does not cause any problem with focusing.
Focusing ring movement spans for about 270 degress and it allows to set focus point very precise, which is pretty important for f/2 lens, where focus point depth is minimal. Samyang 135 f/2.0 ED lens is fully manual, and there is no electronics that could talk with DSLR body. So if you want to have focus confirmation you need to equip it with popular Dandelion chip. That can be easily attached to bayonet mount.
After attaching to my Canon 550D body I shot the first light of this lens. It is 6 seconds exposure with full f/2 aperture and ISO400 settings. The insets are enlarged to 1:1 actual pixel scale. I spotted no CA and also sharpness is very impressive.
Next picture is daylight trees view against bright sky – common aberration test. The results are very good. Both aperture settings shows almost the same overall sharpness and also no signs of chromatic aberration at the high contrast edges.
I was pretty happy with daytime tests, but of course that is not the main reason I purchased. Few days later I was able to spot some stars, although conditions were pretty far from perfect. Seeing was quite good, but transparency was disappointing – about 4.5 mag stars in zenith. Anyway I was more interested in star sharpness and aberrations this time, so I decided to shot some images. I turned camera to Cassiopeia and made some series with umodded Canon 550D at ISO400:
- 10 times 60s exposure at f/2.0
- 10 times 90s exposure at f/2.4
- 10 times 120s exposure at f/2.8
Then I made three stacks. Here is APS-C frame corners. There is basically no visible CA. Only at brightests stars there is incosiderable blue fringe sometimes visible. Plus some almost invisible star elongation at corners caused by either coma or field curvature.
Next comparison is bright star (2 mag) about 1/3 from frame center. There is interesting phenomenon – at f/2.4 spikes are more visible than at f/2.8. I later checked it and it turned out, that at f/2.4 settings there are visible slight gaps between blades. After setting to f/2.8 and lower these gaps disappear and diaphragm becomes almost perfect circle.
And that is another bright star close to bottom frame edge. There is some nasty spikes visible that disappear after stopping to f/2.8. These effect occurs only for bright objects next to bottom frame edge, thats why I suspect it is not lens fault, but rather camera body – when mirror is up there may be some obstacles that come into light cone and causes such artifacts. I will try to confirm it with different Canon DSLR soon.
And last image so far is M31 area. Transparency was so poor I decided to give up on further tests.