lenses for istDS: Pentax 16-40, Sigma 15-30 or what?

Hi Folks

I really can't make my mind up. I am looking for a substitute to the 18-55 kit lens and for a wider angle of view.
So far I have narrowed my choices to:
1. DA 4.0/16-40, which seems to be held in high regards
2. Sigma 3.5.../15-30, which might also prove useful with my analog cameras
3. Sigma 2.8/18-50, which wuold simply be a faster replacement
(Which brings me down to the annoying point, that "fast" lenses are obviously not on the agenda of most manufacturers now.)

I have very nice lenses to int the longer focal lengthes: Pentax 2.8/20, 4.0/20-35, 2.8/28-80 and the 2.8/16 mm fisheye (not to forget my venerable Tokina 17 mm), so I do not necessarily need anything longer than 30 mm on the Sigma. But on the other hand it is very handy, to have a "standard" zoom which is faster ans supposedly better quality, than the Pentax 18-55. I am contemplating this question now for more than a week and have read through dozens of web pages and some (few) magazine tests, but to no avail. For example I cannot really point out, how much the difference in the widest angle between the 15 mm (Sigma) and the 16 mm (Pentax) really is. One user wrote it is significant, the next notes, it doesn't matter. But how wide ist the 15 mm??

Which brings me to the point, that just calculating equivalent focalt lengthes (based on a 1.5 crop factor) is of not much vales, because especially with short focal lengthes, the manufacturers state different angles for the same focal length. This is obviously also a question of optical paths and of course of the real focal length (not the advertised one).

Please!!! Come in with your own experiences and suggestions. I am not a novice, but now I really have no idea which road to go.
Regards
Ron

Personally, I am waiting for the pentax 12-24mm, which should be due in a couple of months maybe.

Quoted focal lengths are an approximation, but the field of view for a particular length ought to be the same for all lenses. A 15mm on the *istDS for example works out at around 22.5mm equivalent and should have the appropriate field of view.

Anybody who suggests that there is a material difference between 15mm and 16mm is a nitwit, and should be ignored.

The problem with fast zooms is that they are very expensive, and the quality of fast film and 800 ISO digital is now such that lens speed is much less important than it used to be.

Furthermore, the other major advantage of a fast lens, which is the ability to create pictures with a narrow DOF, is arguably less important with wide-angle lenses than with, say, portrait lenses.

So I reckon that a fast wide-angle is not a particularly desirable thing. I would make a choice based on general lens quality, including resolution, and (like JR) I would wait for the Pentax 12-24mm zoom to appear before making any decisions.

George

George