Originally posted by Maxington It can easily out resolve it at f2.8. The 16-50 doesn't start getting real sharp until past f4.0 in my experience.
Agreed. The 16-50 is "sharp" the same way a butter knife is sharp: only in one spot. Everywhere else it is relatively "dull." (The new Photozone tests bear this out:
Pentax SMC DA* 16-50mm f/2.8 ED [IF] SDM - Test Report / Review )
At 50mm the 16-50 is on the verge of getting a "poor" rating at f/2.8. It is not "good" until f/4 as you say.
IMO, the 16-50 is only a good choice for a lens if you need one of the following features:
1) Ability to shoot in a downpour
2) If the sound of a lens focusing bothers you.
If things like IQ are important, there are much better choices for less money. That is not to say that you can't get excellent pictures with it (especially if you only need the center sharp), but the point is that it has very disappointing IQ for the money.
Oh, to answer the OP's question, I had 3 lenses that were my biggest disappointment of 2008. All of them are the DA* 16-50.