The Sigma 70-300 APO DG Macro mentioned above is considerably better optically that the FAJ 75-300 mentioned. It's not at it's best at 300mm (most of these types of lenses will experience a quality drop at the very long end) but it's still useable especially if the subject is central in the frame.

The Pentax 100-300 isn't great either so I'd also give that a miss - some folks rate the 80-320 but I've never used one so can't comment.

Most of the time I use a Tokina 80-200 F2.8 with a 2x TC which is optically pretty good apart from noticeable CA in some conditions. Even that's a fairly expensive option though as are the likes of the Sigma 170-500 or 50-500 (both of which are supposed to be pretty decent if you stay away from the very longest end).

If it was a decent performer on film it most likely also will be on digitial, especially as the APS-C sensor size will result in the corners/edges of the image circle not being used - usually the weakest point. CA might not be well controlled however that can often be fixed in post-processing.

Any idea if this would work on the K10D?

That was also my first thought!

My 85mm F1.4 should be just about the worst case scenario as far as AF accuracy & speed at max aperture is concerned however my experience with the K10D is that it's both very fast and very accurate.

It's probably worth noting that I only use the centre AF point though.

Unless size is everything I'd suggest forgetting the DA 40 because optically it's nothing special - neither was the original pancacke, the SMC-M 40mm F2.8.

As George suggests the 43 Ltd or the FA 35mm would be worth a look. Also if you're willing to consider non-Pentax lenses there are a couple of interesting Sigma options - the 28mm F1.8 (about £200) & the 30mm F1.4 (£260). I've never used the 30mm F1.4 but I used to have a 28mm F1.8 and it was a nice lens - I only sold it after I got an FA* 24mm F2 and I regret it now as it would be useful on APS-C.

Also use as large an aperture as possible.

Very nice shot. I'd agree with Lilly that the cropped version is better however I'd have been tempted to extend the crop further down e.g. go with pretty much all of the right half of the original shot.

The FA* 24 was a good lens on film but doesn't IMHO quite click on digital which is why I sold mine.

At the risk of courting controversy IMHO, excluding the older FA lenses still available (35/2, 50/1.4) and the macros, then they're either overpriced (FA Ltd's, DA 14) or compromised optically and/or for speed (DA Ltd's) in order to make them as small as possible...