I have a Rokinon 14mm f/2.8 lens that I use as my go to for dark sky imaging with my full frame camera and it is amazing. However, I just bought a Rokinon 24mm f/1.4 lens for my crop sensor camera so it can gather enough light to allow me to reduce the iso to around 1600 and still get a bright enough image with lower noise. I have done some daytime shooting with it and it seems that even when it is at its best focus, it is still very blurry below f/6ish. However, I keep hearing what a great night time lens it is and have seems one photos that were allegedly taken at f/1.4 that were far sharper than what I am able to produce in the day time. Has anyone here any experience shooting dark sky images with this lens and do you typically shoot wide open or stop it down? If you do stop it down, what focal ratio do you find gives you quality results?
If you have the lens, make test pictures on a normal day of a very far (almost at infinity) point-like object, like an antenna. Use a tripod, use fast shutter, low ISO, compare the images. You cannot get better comparison than actually doing it yourself! :-)
Look around here:
This might give some insight on how well it does match your copy of the lens. To get to grips with the numbers I'd try to find a lens I own (and am content with) and COMPARE -number wise.