I just picked up a new Pentax DA 18-55mm lens and wanted to compare the image quality to that of my Pentax FA 50mm F1.4. Here are two very similar images shot with two different lenses. The top image is with my 50mm prime and the bottom one used my new 18-55mm. I have to say, it took me a while to see any differences and I don't think they are very significant. However, after some major pixel peeing, I was able to determine that my new 18-55mm has slightly brighter or bolder colors. This might be a little off because Olivia did move around as I was changing the lens and her position relative to the light might have something to do with the subtle difference.
Camera: Pentax K20 with Pentax FA50mm prime for the top photo and my new Pentax DA18-55mm for the bottom photo. To keep everything as similar as possible for both images ISO = 200, Aperture=11 and the exposure was 1/180 of a second. Lighting: Nikon SB-25 @ 1/2 power and 50mm zoom to camera left shot it into a very light khaki colored wall to camera right, about a foot from Olivia. I then added fill light with a silver reflector to camera left. Triggered with AlienBees CyberSyncs.
Thanks for reading my ramblings.