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Star shapes on Samyang 135mm

Samyang 135mm T2.2 VDSLR MK2 Canon EOS 6D Tilt Coma
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Haakon Rasmussen avatar
Haakon Rasmussen avatar
I have adapted the lens using the Vetec IMX571 camera. The problem is the the bad star shapes in 3 of the corners. In 1 corner it looks prolonged, another corner it has a tail. The tilt is 11 % according to ASTAP. On my EOS 6d my stars look good. I always use the lens wide open. I hope it is possible to see what the stars are like on the images. Perhaps it is a combination of the wrong working distance (not 44mm) and tilt. I have tried many different distances, but the star shapes are the worse at any other distance.  Image ist upper right corner. The other 2 corner are different, but not round.




Dan H. M. avatar
Dan H. M. avatar
There’s a couple things going on here. The EOS 6D has 6.5 μm pixels. The pixels of the IMX 571 are about 1.7x smaller, so any issues are going to be much more apparent when using the 571. Another problem is that the lens was manufactured for use with a Canon DSLR, so the corners are going to look better out of your 6D than in an astrocamera. 

These two issues notwithstanding, it’s really hard to tell if you have sensor tilt or if the lens is decentered unless you attempt tilt correction with a good device like a Photon Cage. Only by trying to adjust the problem corner can you tell if that corner is being tilted or if there’s something inherently wrong with the lens. 

I will say that my Rokinon 135 shows issues with APS-C and larger, even when stepped down. My Sigma 135 behaves a lot better.
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Haakon Rasmussen avatar
Haakon Rasmussen avatar
Thank you for your quick answer. I will try to do some tilt correction, and see what happens.
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