Note: this post is something of a response to a comment. Forgive me if the part about the 50-200 doesn't make any sense out of context, because I'm not rewriting it.
Here's my thoughts on the lenses I own as coupled with the *ist DL.
smc Pentax-M 1:1.7 50mm
I'm actually not sure I've used this lens forward. I really liked it with film, so I can't see how eliminating vignetting and corner softness could hurt my opinion of it.
Reversed, this lens is about the same in my eyes that it was for film as well. The CCD seems to make softness away from center even more apparent, so there's no benefit in terms of that. The apparent increase in magnification is nice. Color rendition is at least comparable to the DA 16-45, as is seen in these two photos: Berries, reversed 50mm at something resembling f5.6 or half a stop between 5.6 and 8, and then Berries, DA 16-45 f8.
smc Pentax 1:2.8 24mm
Is this lens even an M series? I know I haven't used this lens forward yet. I'll try that this week and let the internet know.
Reversed, this lens is fantastic. You can find sample images here and here. Both of those images were taken at ISO 800, I believe. Basically the magnification is relatively extreme, so if you want to caputre something that's not moving but is very small, good on you. The alien crab demon was probably only marginally larger than the visible portion of the ball in a ballpoint pen. Wink! Downsides include: five blade aperture, which means petal surfaces tend to render as a beautiful solid color field filled with little tiny pentagons of various colors. this is only a problem at incident angles that result in these highlights occuring though, and when I took pictures of the cherry blossoms this way with film, I had no such problem. Additionally, the magnification is SO high that it is very difficult to focus on anything when the lens is stopped down (i.e. when the viewfinder becomes very dim to give a usable depth of field). The working distance is also VERY small, such that at an angle of less than about 40 degrees, the lens housing tends to contact whatever surface is holding your subject. Not very useful sometimes.
smc Pentax-M 1:3.5 135mm
With FOV crop, this lens is roughly equivalent to my old 80-210 on film. The obvious advantage to this is that it's a familiar view through the finder while at the same time also being more than a stop faster (3.5 v. 5.6). This shot was taken with said lens wide open. Here is a 100 percent crop of the frogs. The sharpness is dependent on incident light without the hood extended, as I forgot in that sample up there. You can tell that, if stopped down and used responsibly, it should be a near-perfect lens - I doubt it's easy to resolve spider webs from 25 feet away. It's nice and compact, roughly half the size as my tamron zoom. Since it's metal, it probably weighs twice as much as my DA 50-200 zoom. To say the least, it enabled me to take a shot I wasn't able to take with either of my other telephoto lenses, and that's worth more to me than anything else.
Tamron AF 80-210 f4.5-5.6
I only used this lens at full extension. For examples, roll out to my SQH state park gallery and check out the photos of the heron fishing. Color is pretty good, and though I swear that some of the images ended up looking like I had added a coat of vaseline to the lens, sharpness is good. The corner softness you might see in the lens is outside the image circle area, so all in all, it's quite useful for birding. Quite nicely, the lens has a 7-blade aperture, reducing distraction. Problems with this lens are problems regardless of what camera you mount it on: it's a bit large, has a limited zoom range, and has a noisy AF gearing. If I know I'm going to be doing primarily bird photography, this lens will be a must in my bag.
smc Pentax DA 16-45mm f4.0 ED AL
What can I say... Pentax would have had me pay $549.95 for this lens. I got it for $410 from B&H, and I'm sure it would be worth it at either price. I got used to 24mm as a wide angle on my film cameras, so I decided that the equivalent focal length would be worth more to me than the .33x magnification of the Pentax DA 18-55 f3.5-5.6 AL. Sharpness and color are good, as can be seen in images like this one and this one. Add a polarizer, and you get this. Ding. Problems are limited to the physical nature of the lens. At 16mm, the lens is almost 10 inches away from your eye. This is, coincidentally, such a length that your filter is basically making contact with whatever subject you might be minimum-focus-distancing upon. This also means the camera's flash is unusable at wider angles, as the lens barrel itself is in the way. If you're considering a Pentax digital, forgo the kit lens and spend the 300 dollar difference to get this one (as long as you actually want wider-than-28mm field of view). I have noticed some chromatic aberration (purple fringing, specifically) on some shots where there's a great deal of backlighting. This was basically a dark room shoothing through a wire chair between me and the window on an overcast day - it was, effectively, the dumbest picture I could have possibly taken. In practice, this is purely academic, though, as I haven't noticed any problems with it since. SPOILER ALERT!!! It is worth noting that this lens has an 8 blade aperture. None of pentax's literature says this. Needless to say, I was obscenely happy when I discovered that.
smc Pentax DA 50-200mm f4-5.6 ED
I'm not sure my DA 50-200 does the camera justice. The lens seems to be a bit squirrely on AF, though the manual focus is very fast compared to my tamron 80-210. I bought the new zoom for a couple of reasons. First, I decided that I needed ED glass in the telephoto range; I had already noticed some chromatic aberration and occasional softness in images I took closer to into-the-sun angles, so since I have a bad habit of getting myself into that situation, and since the after-rebate price was appealing, I said "what the hell" and went for it. Second, the physical size of the DA is amazing. You can see the difference between it and my old zoom here and here. The whole assembly becomes a lot lighter, which itself has two advantages: first, the balance stays closer to the same place at 50mm when you zoom to 200mm, and second, when I carry the camera around, I don't find the lens auto-extending to full zoom because the front element/lens hood combination weighs too much for the internal mechanisms to resist the pull of gravity.
Downsides? Six blade aperture; this is less a problem for me than most people because, as someone who takes a lot of pictures of birds, I leave the camera on aperture priority mode and have it permanently stuck to the widest aperture. This means I probably get a lot of edge softness. What I avoid are hexagons in the background -- if you don't do what I do, then you may have more of an issue with this than I do. Additionally, there's a little bit of a grinding sensation when you twist the focus ring manually. I think this is just the byproduct of a telezoom with a fixed front element, but it is a little unsettling. Finally, since the front element fits inside a 52mm filter ring with room to spare, I'm not sure how much light this lens actually lets through. It probably has some effect on flare, but I don't know what that is yet.
That's about it. Do with this information what you will.
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