The official 40mm f/2.8 Pancake was the smallest lens Pentax ever made for any 35mm SLR system at the time, but shockingly enough, at 110g and a wafer-thin 18mm long, it's not the smallest lens in the arsenal in 2019. Pentax went on to make two more super-thin pancakes, including the FA 43mm Limited for the 35mm frame, the DA 40mm Limited for the APS-C frame, and the razor-thin DA 40mm f/2.8 XS, specially made for the butt-ugly K-01 APS-C camera, that is less than half the weight and a third the length of the M40. Nevertheless, the M40 is the smallest lens Pentax has for the 35mm frame and by far the oldest.
Mine was purchased a few years ago, again because of its unique design and not really because I needed one. The lens is so tiny and light that when on the K-1, or any digital SLR, it's hard to tell there's a lens on the camera at all. The lens actually looks and works better on film cameras, as attested by this recent review. Film cameras tend to have flat fronts (or, another way, no pentaprism protruding over the lens), and allow for easier access to the lens for simpler handling. The lens has all the features of any other M-series manual focus lens, including an aperture ring and a focus ring. But look at the photo above; that ridge just above the aperture numbers is the focus ring. It's damn hard to grab that, and not the aperture ring, when using this on a DSLR, but the feel is much different on film cameras.
When using this lens, I typically try to keep DOF fairly wide such that fiddling with focus isn't all that necessary. At f/16 in bright sunlight, it's a great point-n-shoot lens, and at somewhat wider apertures and set at infinity, it is still a good grab-n-go street photography lens or functional for whatever genre you want to pursue that doesn't require a lot of precision.
The lens is 110g (0.24#), 18mm long (0.7 inches), and has 5 aperture blades and 49mm filter threads. The aperture of f/2.8 is on the slow end for normal lenses, but is actually fairly good for such a tiny lens. The lens bears the usual aperture ring and has a manual focus ring that's hard to find if you're not looking right at it. Minimum focus distance is 60cm (2 ft) producing 1:12.5 reproduction ratio (0.08x).
I have never had many complaints about image quality with this lens, but I guess I'm in the minority. The M40 Pancake was not intended to be a professional high quality optic. It was designed for ultimate portability, like the majority of the M-series. Whining about wide-open performance and the lack of ultra-sharp results is daft, because a 50mm f/1.2 lens will not fit in your pocket like this one will. To complain as one moron did about the fact that it doesn't work on a Canon camera is completely stupid. So read these reviews with an eye to what's important here... portability versus usability.
Here are some examples, which again I present to you in triplicate for your viewing pleasure and your own judgement. Today's series show the usual snapshot type photos one would expect to use this lens for.
Top we have a B/W shot of the Jasper "Christian Church," which I presume it the actual name of the place. I don't know how the name separates it from the other churches since I presume they're all Christian. At any rate, I set my aperture and shutter speed prior to stepping onto the center yellow stripe on Highway 7 in Jasper to get this shot. A truck and camper (or was it a motor home?) was bearing down on me from the south while I fiddled a bit with the ultra-thin focus ring to get the letters over the door as sharp as I could. I'm not a churchie, or even a Christian, but I do think small town churches are interesting places to visit and photograph.
Center is a landscape shot from our back deck, which is a familiar scene if you have perused my archives to any extent. I like landscapes with "dramatic skies," like this one kind of shows. I used the in-camera HDR mode on the "Advanced" setting, which is useful for shots like this. The camera will locally boost shadows and highlights to produce a "surreal" look that is kind of slick for landscapes. However, I took a shot of my wife at a restaurant using this setting once, and she came out looking like a coal miner after a 14 hour shift. HDR shots generated in-camera are a fun extra feature of Pentax cameras, producing single JPG images from bracketed shots on the fly, but don't work every time. I liked this shot mostly because of the swirls and lines in the sky, and kept it for your astonishment.
Finally, my boys guarding the house against some wild creature, possibly a sqrrrl out in the yard. 40mm was just wide enough to get the shot as I wanted it without having to get out of my gigantic cushy purple chair.
None of these shots show any spectacular abilities or unique features, but neither do they show, in my opinion, any significant glaring defects.
Again, click any photo to see larger versions, and comments can be left on this page or under each photo if you are so inclined.