Good Morning - and I have not factored in the time difference so it may be dinner time over there.....
Where do I start..... I have a wide selection of lenses at the wide end, including the 10-17, 8-16 and the 12-24. The focal length comparison is only valid when comparing apples to apples, e.g., rectilinear to rectilinear, not fish-eye to rectilinear. So, the comparison to use is Field of View (FoV) as expressed in degrees.
The 10-17 fisheye has a corner to corner FoV of 180 degrees, but for side to side, what is it? Pentax does not indicate (nor will they say). So, I pulled out my trusty CRC vintage 1965 and did some figuring (fingers and toes included).... The reason for this is to be able to compare edge to edge FoV with the rectilinear lenses
Bottom line is somewhere between 135 and 150 degrees. Why the range? In fisheye optics they use a spherical reference model, and what Pentax does not indicate is what are the parameters/type of the spherical projection they used for the optics. So 150 is the calculated FoV (using a nominal spherical projection) and 135 +/- is the observed. Now, just a note - I was careful, but not painstakingly exact. My method was good, but I was not writing a paper for a peer reviewed scientific journal. So, I would put the actual measurement somewhere in-between. Exactly where, depends on how radical you believe the spherical projection used actually is. Also, in the link above Rico and I were thinking that the Pentax 10-17 used some type of hybrid spherical projection in order to blend out the bend so well by 17mm. Here is an excellent reference on fisheye spherical projections....
Ok, so we are done with all of the mathematical nonsense (its enough during the week 7am to 7pm - this is the weekend). Comparison time, so here are the maximum FoV (edge to edge), and I am going with the most conservative numbers.
- 10-17 FE - 135 degrees +/-
- 8-16 - 114 degrees
- 12-24 - 90 degrees
Now, the primes. I'll digress back into optics for a minute. Regardless of the lens' focal length, they are all 180 degrees corner to corner.. Where they vary is in two specific areas....
- Are they full frame lenses - If they are a full frame lens, then on a cropped sensor, you are not getting the full 180 degrees corner to corner, thus the fisheye effect is significantly reduced. Essentially, you are just getting the center of the lens. In this case the most important aspect in terms of what view you will be getting is the next question - the projection used. So, you really need to only compare full frame to full frame, and cropped lenses to other cropped lenses. Zentier comes to mind, where its a full frame lens, and on a cropped sensor its fish eye bend is pretty mild.
- What is the type of projection they use - Depending on the projection actually used, this will determine what type of view you actually get.
Next, let's talk about sharpness and IQ. On a fisheye lens, you are going to get a tremendous field of view, to the extent that each pixel will need to represent a large amount of area. Also, remember its area - so its length x width. This is important. If you are thinking of comparing the sharpness of say a 15mm lens to say a 10mm, the amount of area represented per pixel may be actually squared (not just doubled). So sharpness as a criteria is out. Image quality, contrast and richness of color are more important.
Another aspect is the - back to projections again - sorry. The 8-16 has a pretty radical set of distortions near the edge (put the main object near the edge [especially with lots of straight vertical lines] and shoot a frame). Fisheye lenses have a completely different set of distortions - the bend. These are as different as oil and water.
Alright, so where am I going with all of this. My suggestion is the Pentax 10-17 - why, because its the zoom. The Pentax/Tokina FE zoom lens is unique, Canon / Nikon / Sony / Olympus do not offer these. The zoom adds such a range of versatility that the primes do no offer. In rectilinear lens space, zoom works against sharpness and IQ, while primes take advantage of these aspects. In fisheye space, sharpness and IQ concerns are diminished in importance due to the wide field of view, and the pixel area represented is ballooned up so much that comparisons go out the window. What does matter is the additional capability to compose the frame - that is provided by the photographer behind the camera. My opinion, is that with FE lenses, the photographer is even more important, and the ability to frame/compose the shot adds an additional dimension to the mix - and that is where the zoom comes into play. Also, by slightly tipping the camera up or down from perfectly horizontal, you - the photographer are able to move the bend, and place it where you want it, and to the degree that the bend is a bend. The closer to the edge the bend is - the greater the bend is amplified.