smc Pentax-M 20mm f/4
The Pentax SMC-M 20mm f/4 is one of my favorite Pentax lenses. You can find reviews online that will talk about things like "moustache distortion," and unsharp corners, etc, but these aspects are either unnoticeable or very minor in comparison to the multiple positive aspects of this lens.I've had this lens for several years now. I bought it to use on my Sony APS-C camera with an adapter to use as a wide angle lens. This, and the K17mm f/4 Fisheye, were the two lenses that drove me to experiment with film. Why? Because there was no full-frame camera available at that time, and I really wanted to try it on a full-frame.
The lens is beautiful and small, and qualifies as a pancake (width greater than or equal to twice the length) even though it doesn't bear the word "Pancake" in the name. The lens is only 150g and 29.5mm long (5.3oz, 1.2 inches), so it's easily pocketable in even a medium sized pocket. I think the only smaller lens in my collection is the M40mm f/2.8, which *does* bear the Pancake moniker. I think Pentax missed an opportunity here to put that name on this lens too.
The lens has 49mm filter threads on the front, however I only use circular polarizing filters which can vignette on ultra-wide lenses such as this. Nevertheless, UV, haze, and neutral density filters will be fine on this or any focal length when needed.
I have selected three photos from a walkabout in downtown Kansas City. Most folks consider wide angle lenses to be "landscape lenses," but I tend to strongly disagree. Wide angle fields of view reduce the size of landscape features to which the photographer might want to direct the viewer's attention. Cityscapes are another matter. Foreground interest, in my opinion, is a critical feature of most successful wide angle photographs. In a rich environment like a big city, there are many opportunities to get close to something that can provide foreground interest while capturing environmental detail in the background. Fast lenses are not required for this purpose; f/4 is plenty fast and I have never felt this was a deficiency.