[Atmos (Colin)]

I've been considering getting a 135mm lens for general photography and also astrophotography (a big part being astro) and although the Zeiss has usually been out of my "respectable" price range, but when second hand and at a reasonable price, they’re not that expensive compared to other high quality lens’.
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As a general rule of thumb (from extensive Googling) I'd say that the Zeiss is a fraction sharper and doesn't suffer from the focus breathing of the Samyang BUT the Samyang has slightly better LoCA correction (in a good copy).
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I'll be using it with a D810 for general photography and astro while I am also planning on attaching my QHY163M for astro. The Samyang goes for about $800 new and the Zeiss will probably go for twice that.

My heart is pushing me towards the Zeiss if for nothing other than the exceptional built quality and that, having been built better, is more likely to be a life long lens.
The other consideration is that if I do decide to go down the Samyang path I may get a good copy to begin with OR I could spend the next few months going back 2-5 times testing copies. Samyang QC is the luck of the draw.
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The choice for me is easy when buying a Zeiss new but when they're secondhand and a good price, in good condition and nearby... It does become more difficult. I know that a good copy of a Samyang has slightly better CA control at F/2 but I do wonder if by F/2.8 it even matters.
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So I open it up to those who have had hands on experience with these lovely sharp lens'.