Originally posted by Class A OK, but why?
Sometimes there is a technical reason, like "can be still handheld without SR", "any longer and resolution wasn't that swell anymore (in the 70's)", etc.
Is there a particular application for a 135mm on FF? I don't find the FL odd, just wondering what the primary application was/is.
That matches my liking for 70mm on APS-C. I really want that Sigma 70/2.8 Macro badly, as it doubles really nicely as a portrait lens.
Well, I'm sure you're familiar with the idea that for portraits just a slight amount of compression distortion ("telephoto" distortion) is considered flattering (especially for people with one exaggerated feature, such as a large nose). Then, of course, comfortable working distance (comfortable for the subject generally being the main concern) enters into it.
Of course, there will not be just one point where the combination of these factors is perfect and a millimeter or two off messes everything up, so we are dealing with a range of focal lengths here. That's really all there is to it. Classically you will hear slightly different variations on the portrait range for 35mm film, the narrowest generally being 85mm to 135mm. Sometimes you will see 85mm to 150mm or 70mm to 150mm (possibly even ranges including up to 200mm occasionally). Of course for APS-C this translates to ranges starting from 50 to 55 millimeters and ending from 90 to 100 millimeters (or 135mm to cover up to the 200mm mark).