Summary: I've had this lens for several years, and have used it on both a K200D and a K-5. In my opinion, it's a "must have" lens for any serious photographer using the Pentax system. The lens is usable on both full-frame film and the current Pentax digital cameras. With the smaller sensor size Pentax currently uses on the digitals, this lens is equivalent to 75mm in 35mm film equivalency.
Summary: I have a Pentax K-5 16.3 MP Digital SLR with 3-Inch LCD (Black Body Only) with Pentax SMC DA* Series 16-50mm f/2.8 ED AL IF SDM Wide Angle Zoom Lens for Pentax Digital SLR Cameras and Pentax SMC DA* Series 50-135mm f/2.8 ED IF SDM Telephoto Zoom Lens for Pentax and Digital SLR Cameras lenses. I think this is a great set, as I get a huge range out of two quality lenses.
Summary: I received perhaps a bad copy. Nevertheless, the lens would not focus properly on any subject. Even adjusting in-body had *no effect*. I do not mean I was not able to adjust it *enough*. I mean AF tuning *had no effect at all*. The lens would find a random point and focus. Switching to manual, I was not able to focus the lens more than one of four shots. This indicates a mechanical rather than electronic problem. Perhaps transit damage? Who knows?
Summary: This is my 3rd lens in this range. The other two are M50 f/1.7 and K55mm f/1.8. While I like the f/1.7, it is manual focus only. Even though I still do MF for static subjects, there are many other situations that are better served with AF. The 55mm has a beautiful glow for OOF which can be really special for some subjects. The f/1.4 is sharp wide open but resolution is reduced. But the OOF background is nicely diffused and not busy.
Summary: I just got this lens to replace or add to my Pentax M 50mm F1.7. The M 1.7 is a nice quality lens taht produces some fantastic results, but without Auto Focus, it's a little hard to take pictures of children. Many people say this FA f1.4 is a "little soft wide open" or "Pentax F/FA 50mm f1.7 is sharper wide open"--because of those rumors/posts I was debating on FA 1.4 or F 1.7 I am glad that I bought this 1.4.
Summary: I recently picked this lens up about 3 months ago for my Pentax K-x. Previously, my "workhorse" lens is my Tamron 28-75mm f2.8. The low-light performance of this lens compared to my Tamron is impressive. With two additional stops (1.4 vs 2.8), I am now able to capture low-light pictures of my baby that were previously impossible to capture with my Tamrom (due to such a slow shutter speed).
Summary: Bought this lens due to the price hike of the beloved DA*'s and the love I have for my manual 1.7 and 1.4. Some say the IQ isn't as great as DA70 and 77 and I happen to agree with them thumbing through pages and pages of reviews. However for the price you pay you really get an excellent tool. If you have the luxury of controlling your conditions in a studio, well this lens is a no brainer. If you want to point and shoot- this thing focuses fairly fast and pretty spot on.
Summary: This lenses takes extremely sharp pictures. I've found out they work best at f/1.8 and f/2.0. Bokeh is amazing - like a work of art, I just love the way it blends all colors in the background. It actually does a better job than my Tamron 70-200mm f/2.8 DI LD (IF) Macro. I love the compact size, my one and only complain is not the price, but the lack of lenses hood.
Summary: I got one of those rare lemons, so I cannot speak about the lens quality. Despite having everything set correctly, it would not give me an F-stop reading and would not focus on AF or MF on my Pentax K-5. Amazon was wonderful as always about a return and refund. I would not consider buying electronics and camera products from any other retailer.