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Comments about Sigma Sigma 18mm - 50mm f/3.5-5.6 DC Autofocus Aspherical Zoom Lens for Pentax Digital SLR Cameras:
This simple and inexpensive lens has for the price pretty good performance, similar to the Pentax kit lens. For general use I would recommend stepping up to the 17-70mm or 18-50mm 2.8ex dc that have much better performance.
There is one alternative use for this lens: Infrared and Ultraviolet Photography. For IR (using a Hoya R72 filter) you will not get hotspots and the Pentax k100d is very sensitive to IR (K10d far less sensitive -> 15 second exposures on sunny days). This lens only has 8 elements and simple lens coating, so it actually can be used for near UV photography as well (with a B+W 403 filter). I switched to Pentax from Canon for my UV-photography. The Pentax k10d has an effective IR blocking filter on the sensor (B+W 403 has some IR leakage), making it very suitable for UV photography, and is recorded on the blue and red channels.