I'm considering getting a K-1 sometime down the road. I've got a number of Pentax digital lenses which include, a 10-17 fisheye, 150-500 Sigma, 50 F 1.4 normal, 50 F 2.8 Macro, 55-300, 40 Ltd., 70 Ltd....which from reading reports I think I can use these lenses on a full frame and they will work fine.
The 10-70 fisheye, that I;ve used with my ASP-C camera bodies...I've generally used at 17mm only. I find it I work with it at 17mm...I can get a nice extra wide angle...that doesn't have too much noticeable fisheye distortion. With a K-1 body I'm assuming that if I just use at 17mm the effect would be the same.
I have a 16-45, 12-24 (love this lens), 18-135, 21mm Ltd and I believe these lenses will only work well, when the K-1 is on the crop setting.
I've read a lot about the Pentax (Tamron) 24-70 F 2.8 and it seems some really like this lens and others are just so-so about it. I like the focal length and the speed (F 2.8) ...but it's not cheap and if pay the kind of money asked, I want something completely top notch.
I've been thinking of alternatives and the latest option to this lens, to occur to me, is the Pentax 31mm F 1.8 Limited. I'm pleased with my other Limiteds, everything I've read about the 31 indicates that this is a sharp, well constructed (metal), robust, top quality lens from every angle.
The 31 is only a couple hundred dollars less than a 24-70...so cost isn't that much of a factor to me.
Of course it's a prime, doesn't have the zoom focal lengths like the 24-70...but I started photography nearly 50 years ago, just using primes...a 50, 35mm and a 135mm. They were Takumars, except for the 35...it was a Bushnell. So although I really like the convenience of zooms, I figure I can myself move in and out
with a prime. When I want a portrait lens, I can pop on my 70 Limited.
Also the 31mm on a FF of course won't give me as wide an angle as the 24-70.
Anyways, I would ask opinions out there. Maybe there is something I haven't considered, or maybe the 24-70 is better than my impression.