Apparently the 24mm has no coma wide open which is ideal for astrophotography.
First I've seen of that. I thought some of the tests I saw, showed significant coma wide open. Also it suffers from quality control issues, many are decentered. Also it's got fairly severe barrel distortion of 3%, not good for stitching or architecture. Also, wide open, the vignetting is at 3 stops. This is easily the most severe vignetting of any 24mm lens in current production.
Also, it's fully manual, with manual aperture, does not record aperture or focus info in exif data. That means for shooting landscape at closed down aperture, it will either give a very dark view through the viewfinder when manually focusing, or else you would have to focus with the aperture open wider, and hope for the best once you close it down. Also, this test shows moderate field curvature.http://www.photozone.de/canon_eos_ff/769-samyang24f14eosff?start=2
And I quote: "However, the contrast is a bit soft at these settings (f/1.4 to f/2) so the image quality is not perceived as very sharp (sharp = high resolution + high contrast). There's a significant boost in quality at f/2.8."
That does not bode well for astrophotography. Not saying it's not possible, but it does not look like the best choice at all to me. As for flare, according to Photozone, there is significant flare and ghosting in the 24mm Samyang."Admittedly, the 24mm f/1.4 suffers from slightly lower contrast at large apertures but the resolution is already pretty high here and that's including the difficult outer image regions. The results get more snappy at f/2.8 and beyond the lens is very sharp indeed. Typical for such lenses, the Samyang produces a heavy amount of vignetting at large aperture settings so it's advisable to correct the issue during post-processing or by stopping down to at least f/4. The barrel distortion is a bit stronger than average albeit not extreme. While we didn't really test the quality of the bokeh formally this time, the field results look pretty good - normally ultra-wide lenses are somewhat on the rough side here. There is some visible bokeh fringing at large apertures but this is, again, nothing out of the ordinary. The weakest aspect of the lens is possibly its rather limited protection against back light, especially sided light. We have seen some higher than average glare and ghostings so you should always mount the supplied lens hood in difficult scenes although this will not always help, of course."
Given the presence of field curvature, that usually goes hand in hand with periphery coma (and/or astigmatism) at infinity focus. This is why I bought the Sigma 24mm instead. Overall, I feel it's a better lens, autofocuses, yet costs less when new than the Samyang! It has far lower barrel distortion, too, as well as far less vignetting at its widest f/1.8. Also it doesn't seem to be subject to such low production standards as the Korean lens. My copy is very well centered, at least. Made in Japan should still mean something
. It doesn't always, I admit. And also, yes, it does have some coma in the outer 50% of the image. Has almost zero coma in the inner 50%, though, even at f/1.8. It's mostly gone in the periphery by f/4.5, as well.http://www.photozone.de/nikon_ff/756-sigmaex2418fx
Wide open, especially in this test, the border and corner do look soft. But they don't appear this soft in real world use to me, especially for daytime shots on my 6D. Keep in mind this test is on a Nikon D3x, 24 megapixels, more than any Canon body (so far...thus the CA appears larger than it would on a Canon.).
Their MTF50 test at F/8 is found to be 3558 center, 2774 border, and 2709 extreme (corner). That is not too shabby on a D3x. The above test of the Samyang, was on a 21 MP 5D2. F/9 is the real sweet spot in my experience with the lens, which is a typical setting for much of landscape shooting. They admittedly conclude it's not a good choice for landscape. I do admit it's a better choice for wide angle portraiture (and macro), obviously, especially since that appears to be what it was designed for. But it definitely can work for landscape on a 6D. On a D800? Definitely not.
Like I said, it was an easy choice for me, between this lens and the Samyang 24mm. But if the choice (and the funds exist for it) is between a Sigma 35mm Art, obviously there is hardly any comparison (other than the low barrel distortion). At around 24mm, the only real choices for Canon, for the least optical compromise, are the Canon T/S, or the Zeiss Distagon (as I mentioned above). And the Zeiss would really be better served on a D800. However, the D800 might actually out resolve the Zeiss...which is why hopefully they will make an "Otus" wide angle lens at some point. When they do, that's what every landscaper will go after if they can.
I'll admit the Canon 24mm f/2.8 IS appears to be a good choice too, however it costs a smidge more when new (and almost double what I'm offering my used Sigma for). And sometimes you want a faster aperture than f/2.8...or the ability to shoot macro wide angles. Image stabilization is a very nice feature, but if you take time with your landscapes and use a tripod, the IS is useless there.