Originally posted by nomadkng I think the poll is kind of an apples to oranges to bananas comparison, it might be why you are having a tough time deciding.
A 16mm manual focus only prime f2 versus a mid grade auto focus 16-45 f4 zoom?
AND you want a lens to use for film/FF? Too many divergent criteria.
First, figure out if you want AF or MF. If even sometimes you'll want AF then the Samyang is out.
Next, where do you shoot? if you are all over the range from 16-45, then a prime is out unless you are willing to buy multiple primes.
Third, the 16-45 isn't even in the same class as the 17-50's - you can read the PF review/comparison for further evidence.
Fourth, are you a F2 one eye in focus junkie or do you shoot more f8-13. Why pay a premium for an F2 lens if you'll never use f2? I can't help but think the Samyang was designed specifically for astrophotogs and not all around everyday shooting.
If it were my $400 it would come down to the Sigma 17-50 and the Tamron 17-50 because they outclass everything else on your list. I had this very same decision to make and went with the Tamron solely because of it's better flare resistance. It's the step child in a bag full of Sigmas, but I'm very happy with it's results.
If you want a semi compromise on all fronts, find a Sigma 20-40. It's full frame and optically on par with the 17-50s, it just doesn't have the range.
Those are all very fair points... let's see if I can comment a bit on those...
First, I have to say that the 16-45 vs 17-50s discussion is far from settled. There's a few people here in this forum whom I respect very much, and they chose the 16-45 over the third party offerings, and I saw fantastic results from it. Having seen probably thousands of pictures by now and aware of what these cameras can produce, I have to say that I honestly find the Pentax colors more pleasing and I think it's just as sharp as the third party offerings. I know it's all personal opinion... the truth is that I would probably get good results from any of those if I knew well what I was doing
As for the MF vs AF thing... honestly, what *I* prefer are manual focus primes. But if an AF zoom will get me a better 16mm picture than anything that I can afford in the same price range, yeah I will have to consider it. I'm after the final result, more than what it takes me to get there. So why am I even considering the 16-45 when the Samyang 16mm exists in my price range? Simply because the Samyang costs twice as much and I could get something nice with the price difference, if the 16-45 can get me in the ballpark - which in a way I think it does. So my conundrum kind of is, do I want to pay 100% more for a 5% improvement in quality in the Samyang? Also - is the Tokina 17 3.5 going to give me images that are as pleasing at 17mm as the 16-45 will give me, for even a bit less money? Perhaps so! (I'm really only concerned with 15-24mm range, I have 28mm and up covered by other lenses, mostly primes).
The most important question you asked is about what I want it for and style of shooting... I kind of answered a bit of that, but to clarify why I'm comparing apples to oranges... my signature quote kind of describes my shooting style. Life happens, and I happen to take pictures of it. I'm not specialized in any type of photography. A lot of the time I'll be taking pictures of my family - it could be indoors, it could be outdoors, it could be portraits (have the lenses for that) or it could be a group setting (where I would use this lens). It all depends on what life presents to me
So sometimes I'll be shooting wide open or a stop or two down because I'll be indoors, other times I'll be outside and want more DOF and shoot at f8 or even f11. Also, we travel sometimes so I'll end up with cityscapes and landscapes. And I also like to walk around during lunch time and shoot whatever catches my eye. Life just happens.
Finally, the film thing is a nice to have but not necessary. I don't shoot so much film that it will make that much of a difference in my decision. The 16-45 does cover the full frame circle at 24mm and up, so that would be good. The Samyang doesn't, but I would get it for extremely good APS-C IQ that I would be able to use even when much more demanding sensors come up - I think.
In the end I'm probably over-thinking. I don't think I'm a very good photographer so I probably won't get results worthy of a magazine cover even if I had the equipment for it... so maybe what I end up with is not quite as important as just continue practicing the basics and making sure I take a lot of pictures.