Nothing is normal when shooting below 24mm - but that's why ultra-wide lenses exist! Their exaggeration of perspective makes for dramatic or curious images like no other lens provides. Add in a fisheye distortion curve and you're well off the map for images that non-techie shooters would consider 'traditional'.
In my film days I recall my first view through a 24mm lens on my Pentax ME Super. It was a distinctly different look even from the 28mm in the early 1980s and I really enjoyed it. In recent times I've tried 8mm and 17mm fisheyes and the DA10-17 fisheye zoom. And as noted in recent posts, I'm suddenly in a spot with two very different and very wide lenses.
So how can one compare a full-frame-friendly 17-28mm fisheye to a rectilinear 10-24mm lens? Plenty of ways - but whether they are 'fair' comparisons is debatable. I must start somewhere so here goes!
Overall size = 17-28. Small and light at 260g, though the 460g Tamron is not as bad as many.
Lens Speed = no difference, both f/3.5-4.5.
AF speed = 17-28 by a comfortable margin. The Tamron's minfocus is partly a factor.
Filter choices = 10-24. While 77mm filters are spendy the 17-28 cannot use filters at all.
Min. focus =10-24, it's ~half the 17-28 minimum (0.8' vs 1.4'), .2x 'macro' vs 0.08x.
Sharpness = ? Ratings give the 17-28 much better scores but my copies look equally very good.
Color = both look excellent in flash images of a box of many colors.
Distortion = ? The 10-24 shows some barrel distortion, not a major thing though.
Lesser information worth knowing -
micro43 range = 10-24. The 17-28 (34-56mme) has very little fisheye effect on the smaller sensor.
micro43 convenience = 17-28, which has an aperture ring. The Tamron needs a special adapter.
36x24 versatility = 17-28 is entirely film-friendly - though the 10-24 is useable above 14mm or so.
Resale value? Not sure, perhaps a slight lean to the 17-28 now that the K-1 can use it fully.
It appears that this is the Tamron's game to lose. It's tough though, as the F17-28 is a talented lens and very fun to use. Its overall aspect is wider than the SMC 17/4 prime but its close-focus skills are a bit less. I will continue the tests, but that's where it stands right now. Neither one is really a factor on other cameras; I have a film body but a K-1 is not in my near future, and I didn't plan to use these with my GX1. I do have one of the stop-down K adapters around here somewhere though..