When I was a kid, what most people had to their SLR was just three lenses: a 28mm, a 50mm and a 135mm. For that reason, there are both more copies of these lenses, and more versions from more brands, than any other focal lengths. You still notice this on the 2nd hand market: as long as they are not extremely fast, you can get many 28mm/f2.8, 50mm/f1.7 or 135mm/f2.8 for almost nothing.
We have several 50mm clubs, there have been whole threads on the different 28mm's, for example the Vivitar 28mm's. On the APS-C digitals we have learned to use the 28mm wide angles as inexpensive normal lenses, the 50mm's as fine portrait lenses, but what about the 135mm's. It appears that people quite often say that they don't find a use for this focal length on a digital cropped DSLR.
But is that fair? Back in the old full frame film days, the 2nd prime tele you got was either a 85mm for portrait, or a 200mm. If we found that usefull back then, why would not a 135mm with a view angle on APS-C close to the 200mm on full frame be usefull? And you could easilly get it at a speed that would have been a dream for most in a 200mm. 135/2.8 = 200/2.8!
I find my 135mm's usefull on digital for street shooting, stage shooting, sport, my kids, some more or less domestic animals, etc. Of course, I still use them on film cameras.
To restore the 135mm lenses to their proper place, I'd like to invite you all to the 135mm club. The club is for any 135mm that can be mounted on a SLR or DSLR. Playing with converters, tubes or bellows are also OK. Just make use of your old forgotten 135mm's! Pictures by the 135mm lenses and pictures of the 135mm lenses. But primes only.
Let me start by introducing the SMC Pentax 135mm f2.5 used on the MX with Ilford XP: