Originally posted by dipo 1 Have seen everything from 11mm to 48mm with f stops all over the place.and I just don't know enough about photography to even know if I really need one.
First consideration- at 18mm is your present lens wide-angle enough to meet your needs? At that setting, the maximum aperture available will be f/3.5, so is that a large enough aperture to meet your needs (smaller number = larger aperture), or do you find you must often crank up the ISO sensitivity really high to achieve the necessary shutter speed for hand held shots in low light, or for moving subjects? It won't be easy or cheap to find a WA lens that can bring a larger aperture than that. You'd need the DA* 11-18mm f/2.8 to get a little more shutter speed, (a very fine, premium WA zoom lens) or go to a 3rd party MF prime lens. If you do many shots that include linear subject matter, like architectural exteriors, interiors, cityscapes, etc. you'll want a lens having low linear distortion, so the edges do not bow or bulge. (Perspective distortion is a different matter, and with WA lenses far more prominent, having to do with the angle of the shooting position to the subject, as well as distance. Example- buildings appearing to be leaning.) As to liner distortion, a good WA zoom lens can achieve near absent distortion by using settings in the middle portions of their zoom range.
One problem is the more reasonably-priced WA zoom lenses for Pentax, if you don't need the f/2.8 aperture of the DA* 11-18mm lens), have been recently discontinued. But you could possibly still find a Sigma 10-20mm f/3.5, or a less-aperture Sigma 10-20mm f/3.5-5.6, or a Pentax DA 12-24mm f/4 used in fine condition. With these lenses, you'd get superior image quality compared to your 18-55mm lens, a much wider angle, as well as virtually absent linear distortion at 18mm, where your present lens exhibits considerable distortion. This distortion may not be evident in landscape nature shots without prominent buildings or other linear objects.