Originally posted by WPRESTO SO--- In my opinion, don't replace the 10-20 with a fisheye. Don't expect to convert an 8mm fisheye image to an 8mm rectilinear image (get the Sigma 8-16). Expect the "fisheye look" to have limited artistic impact unless you're awfully clever with your subjects. Interiors done with a fisheye sometimes work, sometimes just look distorted.
Quite right. A FE and UWA complement each other. They have different coverages and purposes. My set are the DA10-17/3.5-4.5 (the lens that drove me to Pentax) and the Tamron 10-24/3.5-4.5 (the lens that completes my walkabout set). For coverage, the FE starts where the UWA leaves off. They can't be compared by focal length. Here are AOVs on my K20D (I'll include your Sigma and the Samyang for comparison):
Pentax 10-17 FE: ~110-175 degrees
Tamron 10-24 UWA: ~60-110 degrees
Samyang 8mm FE: ~180 degrees
Sigma 10-20 UWA: ~70-110 degrees
Note: These are numbers for the actual K20D sensor (diagonal= 28.1mm) not a nominal APS-C sensor (diagonal= 30.1mm). I'm not sure of the exact size of your K200D sensor, but you'll be somewhere in the neighborhood.
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The speed differences are hard to judge at AOVs of 100 degrees and more. My fishy Zenitar 16/2.8 has an AOV of ~100 degrees, comparable to a UWA @12mm, and it's notably faster than the DA10-17 @16mm (f/4.5) and the Sigma UWA @12mm (~f/4), and a bit faster than the Tamron UWA @12mm (~f/3.5). But we don't usually use FEs and UWAs for their speed, but for their projection effects. DOFs are immensely thick from 100 degrees on up. I like the Zenitar in darker active interiors. I like the Pentax and Tamron everywhere else.
Ah, projection effects. Cartographers have grappled for around 650 years with the problem of projecting a curved universe onto a flat plane. Rectilinear UWAs stretch the edges, FEs bend them. An FE projection is actually more accurate but our brains don't like to see that bending. I can't use an FE in my forest -- all the trees are falling on me! So as with mapmakers, we choose the projections that best serve our purposes.
There's a nifty trick that's cheaply done: Take a DA18-55 or -135 or -250. Mount on it a cheap 0.25x FE adapter -- mine (new) cost US$20 (shipped) a few months ago. The zoom @18mm gives a 180-degree full-circle fisheye; by ~30mm it's a frame-filling FE, and it becomes less fishy (with a narrower AOV) as we zoom onwards. It's easy to dial-up a projection for almost any purpose. No, don't expect the greatest optical clarity at the edges. But it's fun and educational.
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With your Sigma UWA at the wide end, you know how a slight shift in lens angle can have a dramatic effect on perspective distortion, how rectangles become acute parallelograms. FE distortion gets bendy instead. No, we usually don't want straight lines to curve too much. So, just as we use a UWA carefully, we use a FE even more carefully.
Where I use the 10-17 FE @10mm (similar to the Samyang 8mm FE):
* Inside a curved space
* Where 3 or more lines intersect
* Shooting straight up or straight down
* With centered rounded subjects where edges don't matter
* With land- and sea-scapes with the horizon centered
* For stitching 360-degree full-world panoramas
* Where I really *want* perspective distortion
That last is popular in some action sports (like surf- or skate-boarding) and fashion (really s-t-r-e-t-c-h those legs!) and 'psychedelic' shots (everything's bent!) and so on. The frame-filling FE works inside wells and tunnels and caves. Stuff like that.
There are fairly few straight-line interiors where only a FE will do. Case in point: The Venetian Casino in Las Vegas has a reproduction of the Sistine Chapel ceiling where its entry lobby merges up to a gaming level. Tis a darkened space, and the 10-24 UWA just won't capture it all. But the FE @10mm grabs it very nicely -- and those slot-machine lights at the image edges just become a necklace of shining gems.
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The Samyang is the least expensive high-quality frame-filling FE. The DA10-17 costs rather more but is much more flexible. The Zenitar is the fastest of the lot and not super-fishy. There's also a super-fishy and pretty cheap Sigma-made 12/8 fixed-focus FE; but the less said about that, the better. An 0.25x FE adapter (mine is branded Precision Design Super AF Fisheye) on an 18-whatever would be a cheap way to find if you like super-fishiness enough to spend US$200-500 on a better lens.
My recommendation: Get the cheap adapter. Test your comfort level for distortion before you spend more money.