Originally posted by theteerex I have a Pentax k-x with the 18-55mm kit lens and am thinking of upgrading my lens.What I'm interested in shooting are people walking down the street or in action poses and sports(soccer players mainly).
Some here will argue that instead of "upgrading", you should "supplement" the kit lens. Whatever. You know what you need for longer reach -- a longer lens. If you don't mind switching views at around 55mm, a 50-200 or 55-300 would do. If you don't like switching there, an 18-200 or 18-250 (what I use) is comfortable. I've tried an 18-55 and 60-300 pair and I just don't like switching there.
Quote: I want to shoot pictures of someone (my gf) walking down the street with just her in focus while the background and foreground or blurry (bokeh as I recently found out). Would the 18-200mm lens be appropriate or would I be better served with a 50mm f1.4 or f1.7mm lens instead?
Now it gets complicated. What you want, with a sharp subject and everything else blurred, is thin DOF (depth-of-field). Various methods will achieve that. The cheapest is to add a +1 diopter lens, but that only works with a nearby subject. Or, buy a very fast lens and use it wide-open. Or, buy a longer lens and shoot from a distance. I can fill a frame with a headshot with a wide-open 50/1.4 (many bucks). I can also fill a frame with a headshot with a wide-open 135/2.8 (ten bucks) from a greater distance. DOF will be about the same.
Some tricks here: If using an AF lens, set your Program Line to Hi-Speed Priority; this keeps the aperture open as much as possible when in Green or P modes, for thinner DOF. Keep your subject away from a background; the further away the background, the blurrier it will be. Keep the camera as close to the subject as possible; again, this blurs the background. If shaded, use a fill flash; this lights the subject while the darker background fades away.
Quote: Would the 18-200mm be useful for cityscapes or would the 50mm lens be useful for that as well?
Yes. I use my 18-250 for land-, city-, ocean-, village-, and mountain-scapes. I also use 24-28-35-50-58-85-100-200mm lenses for such scapes. And a Fast Fifty is useful in many situations. AF or manual, new or old, it doesn't matter. My original lens set for my K20D were the DA10-17, DA18-250, and FA50/1.4. I sometimes replace the 10-17 with a Zenitar 16/2.8 (US$170), and the FA50 (US$200 then, $350 now) with an A50/1.7 (US$50) or ST55/1.8 (US$9 then, $30 now).
Good luck!