Pentax alone has four ways to achieve the 17mm fish-eye effect: the f/4 Takumar, the f/4 SMC (bayonet mount), the 17-28 f/3.5-4.5 F (autofocus) and the 10-17 f/3.5-4.5 DA of current days. The first three are full frame/film friendly (FFFF?), the later for aps-c only. Other primes and zooms can cover 17mm, though I don't believe any of them are fisheye types. My K-5 bodies really trim off most of the fishiness anyway, but effectively the sides of the frame compare well to 15mm straight lenses - and the primes are very small and light, making them easy to carry along.
My plans are still to pick up a second copy of the SMC 17/4 as I really liked the last one and manual primes are fun to use. Trouble is, that seems to be the rarest of the lot! The Takumar is probably the same optics and is no cheaper, but is definitely more common on the used market. The 17-28 is a smidge faster at 17mm f/3.5 and the 10-17 a mite slower at f/4.5 maximum. The DA 10-17 has never been seen below $350 so it's the more expensive model, while the 17-28 can be found for under $250 and is the least expensive most of the time. It's slightly heavier and of course longer than the 17/4, but neither zoom is a massive beast.
I need make no decision in a hurry, thankfully - but when the time comes it will be curious what I end up with. The F 17-28 might even be fun on the Q, sitting beside the 5-15mm normal lens. ???
