Originally posted by stevebrot I am pretty sure that I don't know. I don't own the Samyang 20/1.8 or either of the Pentax 20/2.8s. How about you? Do you own any of the three?
I own both the FA 20mm and the Samyang 20mm, so I'll chip in with my limited experience with the two.
The Samyang is better optically, but like you say there is more to it than optical performance. I picked up the FA20 on a whim, when one showed up at a local second-hand store for a very good price. This was well before I bought the Samyang, and while I still shot with APS-C (before I got my K1). When I bought the K1 I picked up the Samyang brand new and got lucky with the copy.
Straight away (i.e., within the return period of the lens) I compared image quality for sharpness, and the Samyang was very much the sharper lens, with the FA20 being weaker on the edges and corners - not by a massive margin, but certainly noticeable. I had the FA disassembled by a technician, as one side of the image was worse than the other, he found a bent/squished shim that indicated that the lens had potentially taken a knock at some point, and although the lens improved it still wasn't up to the level of the Samyang, even stopped down where I tend to use it.
I popped into Lightroom to see if I could find the test photos I took (this is back in 2017, and my cataloguing wasn't very good then), but I haven't found them, so I'm going on memory only.
Like you said though, there are other considerations other than pixel peeping (which was what mattered most to me at the time ). Sample variation is a big problem with Samyang wide angle lenses (the 14mm especially), so if you were looking at buying one I would only consider lenses that I could test beforehand or return. I would also keep in mind that the FA lenses are now getting a bit older and may have similar issues simply from misuse, like mine did. 25 years of service is bound to see some wear. The A 20 is probably built a lot tougher due to not needing to keep the focusing group lightweight for the focus drive motor??
I still have the FA, but I never really used it that much even before I got the Samyang ( hence my limited experience ), so it's sat in a box ever since. I'm too much of a collector/hoarder to part with it, but I'll probably revisit it at some point as corner resolution doesn't matter to me as much these days for landscape photos. The Samyang has a permanent place in my bag, but my overall opinion on it wavers. Some times I am astonished at what it's capable of, and other times I'm a little underwhelmed. Overall my opinion of it is very positive. Unfortunately it's not the lens it used to be. I had an incident where my unattended tripod blew over and the camera landed lens-down on a rock, equipped with the Samyang 20mm. It's now decentered on one side, which shows up in wide-open photos: especially badly in astro. Stopped down it's still good, and aside from it vignetting when stacking filters I have had no other issues with it. If I want to travel light I'll still bundle it with the DFA 28-105 as a two-lens combo.
I am eyeing the Pentax DFA 21mm with some curiosity, as a smaller weather resistant prime would be better for hiking, as long as I can stack filters and the IQ is good stopped down a little. When the price comes down or some second-hand units start popping up I may look at picking one up. If you can find a good copy of the Samyang for a good price, I personally would have no qualms grabbing one again.