Well tonight I finally went out since it was something that resembled clear apart from the humidity that we have been having the past week. With the humidity and the the moon I didn't chase anything worth while but instead went and decided to figure out the Samyang/Rokinon 135mm f/2 UMC and put it through its paces. Before today I had just been blindly running it at 2.8 having never really spent the time to do a good and proper test.
So toady I left it out in the garage this afternoon after snowshoeing through so it could come down to temp so I wouldn't have to deal with that while out. I went out to a reasonably dark area since the almost fog has been bad and in my backyard there isn't much visible at present because of it and the fact that it is a bright bortle 8.
As usual when doing these things I started wide open with an ISO and exposure length with astrotracer that doesn't show trailing or show tons of light pollution. The resulted in the first shot being ISO 200, f/2, and 20s. From there I would stop the lens down 1/3rd a stop and increase the ISO by 1/3rd a stop. I carefully focused and verified focus using a bahtinov mask.
First impressions in looking at the first shot at f/2 is that it does have some slight coma, possibly a bit more wide open than the SMC A* 400/2.8 does wide open. The stars show very little bloat and if there is color fringing it may be 1 pixel at most so I wouldn't fret that. However it does have substantial vignetting.
The noticeable vignetting goes away once you get the lens stopped down to f/2.8. Also at f/2.8 the stars have shrunk to their smallest size. At this point the lens still shows coma for stars in the corners and along the edges. However for most cases this shouldn't be a problem as it is very small coma that is slightly less than the SMC A* 400/2.8 at 2.8
It isn't until the lens reaches f/4 that the coma fully goes away.
All in all this is a very good astro lens and going forward I will probably run it at f/2.8. Comparing it to the SMC A* 400/2.8 for astro seemed reasonable given how highly regarded Samyang/Rokinon 135/2.8 UMC is and I would put it in the same class as the SMC A* 400/2.8. I would say there are very few lenses that are this good for astro. I would still say the SMC A* 400/2.8 is better but not by much however finding one of those is rather difficult. The SMC A* 400/2.8 gets better faster and doesn't vignette wide open which is why I say it is still better.
Because I can I also pointed it back to my city, this is a 50s shot at ISO 100 and f/4 with the lens, I am pointed about 1 frame above the horizon. Also this had no edits and only used the default settings fro development from RawTherapee. It really shows with the humidity that has been providing others in the area with some great hoar frost shots over the last few days.
