Originally posted by builttospill I see your point, but I respectfully disagree. Maybe your point has more merit with certain subjects in photography, but there's a lot more to this and the proof is in the photos, even resized for the Web.
For example a landscape photo taken with both the kit lens and the DA 15 Limited at f/8 will show the difference in optics, especially when it comes to flare and colors. You can't tell me there's just a marginal difference when comparing the two. Maybe the two will show marginal to no difference in a brick wall test, but I don't make a practice of shooting brick walls. In real-world use I find a noticeable difference taking portraits with the FA 43 and FA 77 instead of using an 18-55 or 50-200.
The discussion was about sharpness.
MTF from Photozone, both on the K-5 so we keep the same sensor.
DA 70 (closes to the 77 on the same sensor):
Da 18-55 WR:
Go under each column and compare the f/8 data.
Of course these tests were all done on a tripod.
With regards to flare the SMC coating is a factor for all Pentax glass. Distortion and vignetting are, of course, other optical factors (so to be fair look at the MTF for 28mm on the 18-55WR). CA and distortion of course both have controls either in LR or in-camera,so both can be made non-relevant.
Resolution goes way up when stopped down and using a tripod to the point where both lenses are comparable at f/8. You can look a other tests and see pretty much the same thing.
The sensor doesn't care what lens you use for resolution which is why they do these tests. MTF is MTF using iMatest. Photozone calls the centre resolution on the kit lens at f/8 excellent. Even at the border and extreme it's very close where even the brick wall test would give you fits at 200%.
It's off the centre that lesser glass starts to fall apart, especially at the edges and at max aperture. That's what you're paying for with primes an DA*s. The 18-55 edges start to look smeary at f/5.6, but even at 18mm the kit lens gets very respectable MTF at the centre.
I'm not saying anything revelatory here. Many test sites, pro photographers, advice columns, etc. have long said exactly the same thing: you can get terrific results with OEM basic glass with a tripod and stopping down to f/8 or f/11.