The Samyang 135mm f/2 short-telephoto lens is considered by many one of the best lenses for AP out there. As I really wanted a modern day lens in that range (although I wouldn't mind a bit more FL if I can help it) I decided to bite the bullet and get one. In my incarnation it is a Nikon-fit one but I believe optically they are all the same. For the test I employed the highest resolution DSLR I have, the Canon 7Dii, via adapter.

As is for all the other tests of mine this is done with untracked pointing at Polaris on a sturdy tripod and consists of taking 60 5s integrations. Image calibration is down (in PI) by dark/bias removal, registering and integration with normalisation. CFA drizzle is then applied for the final integrated light. DBE is subsequently used to flatten the image and finally the image is colour-calibrated using PCC. The non-annotated image has been stretched is via HistogramEqualization and ArcsinhStretch, against the standard PI's STF. This stretching allows to see even small defect in the colour rendition that would otherwise be hidden by just looking at a STF dump of the image, I believe.

In short I have yet to test an f/2 lens this good, period. The PSF is a nearly constant <2 pixels across the board, from side to side. To be really, really picky there is a small tilt toward the lower left corner but this could be well due to either the camera flange or the adapter and not an easy fix either but those guys/gals using an astro-camera could easily fix this. Colour rendition is overall good if not pretty good with just a faint trace of purple which could be easily processed out. Nikon lens with multiple ED/Fluorite elements have a cleaner spectrum rendition but not this fast, at least the ones I've tested. Sure enough they aren't as cheap as this one (the 105mm f/1.4 is well over a grand used here in the UK).

In three words: A Real Gem.