Sigh... this is one of the reasons that I've been avoiding camera suggestion threads.
FWIW... about lens mount history...
Canon's changed theirs about four times
Olympus changed mounts at least twice, new digital mount
Minolta changed mount when going from manual to autofocus
Pentax changed mount once, still offers an adapter from their web store for old lenses on new bodies (also a medium-format lens adapter)
Nikon has never changed their lens mount, however most of their cheaper DSLRs won't play well with older lenses
Pentax, IIRC, has making camera's and lenses longer than any other camera company out there (they've been making optical lenses since 1919). They are also the only camera company that is making dSLRs where ALL their lenses will mount and meter (and auto focus if the lens is an auto focus lens) on all the dSLR's in their line up. Though it could be limited to just the K mount lenses which were first introduced in 1975. Groucho can probably correct me with this. Though I think I recall him saying he bought a bunch of used manual focus lenses that are from the 1960's and he has used them on his Pentax dSLR.
Pentax has the friendliest backwards compatibility, certainly. Every lens gets nearly full functionality and image stabilization. The only thing you really lose is, when using screw-mount lenses, they all behave as preset lenses (ie, you set the aperture and see the difference live), because it can't press in the pin that will stop down the blades when taking a photo on an original screw-mount camera. You do still get focus confirmation and metering (thanks to preset, you can just leave the camera in Av mode and fire away!)
Here's a shot of a lens sold from 1957-1961 mounted on my K20D... a 135mm F3.5 Takumar preset, to be specific.
The lens is quite high-quality, too - the build quality is superb and the image quality is awfully nice, too. This particular one even has 8 aperture blades for nice round bokeh. Some of the old lenses have 15 or even more aperture blades, if you want things
really round.
I used some of these old lenses a lot last weekend at a vintage racing event and was very pleased with their performance. I wouldn't recommend older zoom lenses, but quality old primes can still compete nicely with new lenses of all types.
I know that Canon's dSLR's will only be able to use EOS lenses. EOS was introduced in 1986, but there are limitations with their dSLR bodies. The made for digital lenses wont work with full frame digital bodies, only crop bodies. Canon changed mount on their lenses so pretty much nothing earlier than 1986 will work with dSLR's.
To be fair... Canon has one of the most adaptable lens mounts, along with Olympus. You can get adapters to mount nearly anything on a Canon, however the cheap ones won't give you focus confirmation or any metering. It's the same story with Olympus except that I think it's a little more forgiving on metering, plus I
think you can get stabilization with them. The downside is that their adapters are very expensive!
Is Pentax going away? Of course not. Hoya owns them now and is a pretty successful company and has committed to continuing to improve the breed. (I wouldn't be surprised to see their PnS cameras dwindle, though - though I suspect their waterproof models will soldier on.) Their alliance with Samsung gave them the cutting-edge 14.6mp sensor found in the K20D, keeping the profit "in the family" rather than paying money to Sony like Nikon always has (and Pentax used to). They've been releasing new lenses at a faster rate than probably anyone else, including brand-new primes. (Like a beautiful new 35mm F2.8 1:1 macro lens (
glowing minireview here), complete with DoF scale - who else makes a lens like that? Most new lenses from companies other than Olympus have been mild updates of old film lenses or, in Nikon's case, old lenses with new motors.) Market share reports I've seen show Pentax pretty consistently in third place - this is probably on a worldwide sales basis, I could believe that their share is less here as they're not sold in wholesale clubs and office supply stores. Still, they definitely sell - look at how quickly the K100D and K10D sold out when production ended. Meanwhile, I still see local stores flogging 1-2-generation-old models of other brands that are still lying around. That's the downside of higher saturation.
Furthermore - if you refuse to buy something because of a lower market share - well, that's certainly not going to help! Buy what you want, and the more people who do, the more successful the smaller companies will be.
But really, it boils down to what camera is the best fit for you. The D90 certainly offers a very, very strong package. So does the 50D. Both will easily beat the K20D in some areas, like normal frames-per-second shooting if that's a priority for you, but the K20D can beat them in other areas. Overall, as I said, I do think the K20D does position itself above them, with the full weathersealing, controls, and customization options. But I wouldn't blame anyone for picking any of these. The Sony and Olympus are compelling bodies too though
IMHO I think their lens options are an issue - both are
very expensive for the "good stuff", there's not as much of a selection, and the Olympus especially suffers from poor third-party support. (Sony and Pentax are pretty similar for Sigma/Tamron/Tokina lenses, Pentax has a few more high-end oddballs like manual-focus Zeiss, Voigtlander, etc lenses.) Olympus is now also facing making a new set of "mini-4/3rds" lenses for their new mirrorless designs, instead of concentrating solely on normal 4/3rds lenses. Olympus has, for better or for worse, painted themselves into a corner with their 4/3rds sensor. They don't have the option of going full-frame like everyone else is doing, so they're going smaller; but that takes new lenses to get the full benefit. Nothing against them, though; if your needs fit into what they have to offer (just like with Pentax), I'm sure you'll be very pleased.
Whatever you choose, I will almost guarantee that you'll love it.
(Those 21 fps shots add up very quickly!)
