Pentax SMC Pentax-M 135mm f/3.5 (K)

Everybody wants the 135mm f/2.5. Faster, dreamier, the one that shows up in the YouTube portrait videos. So nobody wants this one, the f/3.5, which is the actual mistake. The 2.5 is a big front-heavy chunk of glass that throws your whole camera off balance. The SMC Pentax-M 135mm f/3.5 weighs almost nothing, takes a 49mm filter like the rest of the M-series primes, and renders close enough to the faster lens that in a print you would struggle to call it. You give up just under a full stop and a sliver of background blur. You get a lens that disappears in the bag.

The optics are a straightforward telephoto design from the late M-series push toward small and light, and they behave like it. Wide open at f/3.5 the center is already crisp with the corners going soft, a glow at the edges that flatters skin in the middle of the frame without smearing the detail you care about. Stop to f/5.6 and it snaps to fully sharp corner to corner. Bokeh is roundish and falls off gently behind the subject, though shrinking the optics to M-series size left a little outlining on the discs and a slight cat-eye toward the edges rather than a perfectly clean wash. Contrast sits a touch lower than modern glass, which on color negative reads as roll-off in the highlights rather than weakness. The SMC multicoating is the real argument here. Shoot it into a window or a backlit head of hair and flare stays controlled where a single-coated 135 of the same era would veil and lose the blacks.

Who reaches for it: anyone shooting tight head-and-shoulders portraits on a 35mm body who does not want a brick around their neck. It is a head-shot lens and a candid-across-the-room lens, the thing you bring to a recital to flatten the background and lift one face out of the rows. On the street it pulls a face out of a crowd from across the road without you stepping into the frame. The 135mm length on full frame gives you that compressed, slightly formal look that flatters most faces and quietly tidies up a messy background by shrinking it.

The honest weakness is the maximum aperture, and it is not subtle. f/3.5 on a 135mm is slow. In a dim room or at dusk you run out of shutter speed fast, and there is no image stabilization on a 1977 manual lens to save a handheld frame. This is a daylight and good-window-light tool. Push it into a candlelit interior and you will be hunting ISO 800 film and a steady elbow. Meter wide open at f/3.5 in that low light with Zone Light Meter so you are reading the scene at the aperture you will actually shoot, not stopped down where the meter lies to you about the shadows.

Today it is one of the cheapest genuinely good telephotos you can buy, routinely changing hands for less than a nice dinner. People skip it for the f/2.5 and the skip is your gain. It mounts on any K-mount body, with stop-down or manual metering on modern digital Pentax, the SMC coating still earns its keep, and the size means it actually comes with you. A 135 you can forget you are carrying is a 135 that shoots more rolls than the heavy one ever will.