[Marc Sabatella]

Do the math. Aperture is defined as focal length divided by diameter. The inner diameter of a 49mm filter is about 47.5 mm, but realistically the front element diameter of a lens that can accept a 49 mm filter is about 45 mm, therefore, this diameter would define an inverse curve that would be close to focal lengths like 85mm at F2, 135 mm at F3.5 and 200mm at F4.5 (edit note. I initially typed 400 not 200mm)

And to be specific, the "M" series lenses come in at pretty much these exact points. The M85/2, M100/2.8, M120/2.8, M135/3.5 and M50/3.5 all take 49mm filters and are the fastest lenses easily available in these focal lengths that can make this claim (except for the other 100/2.8's as mentioned).

Actually, since 49mm is kind of a standard in the Pentax world (largely on account of the M series, I would imagine), and f/2.8 is often held as a sort of standard for what constitutes "fast" in the telephoto range, I do wonder if the M120/2.8 is actually the longest f/2.8 lens that takes 49mm filters.

[lytrytyr]

49mm is kind of a standard in the Pentax world (largely on account of the M series, I would imagine)

From the 'sixties, the Spotmatic "kit lenses" (55/1.8) had 49mm filter threads.

[SuperK5]

PENTAX - smc PENTAX DA 50-200mm F4-5.6 ED WR

[DaveHolmes]

PENTAX - smc PENTAX DA 50-200mm F4-5.6 ED WR

Yep. Mine's on a 49mm too.

Should be able to get a definitive answer if you're prepare to use the lens database... It's a really bloody good resource!

[littledrawe]

I think this is the longest one that takes 49mm filters:

https://www.pentaxforums.com/lensreviews/SMC-Pentax-A-Star-1200mm-F8-Lens.html

[Jean Poitiers]

I think this is the longest one that takes 49mm filters:

SMC Pentax-A* 1200mm F8 ED [IF] Reviews - A Prime Lenses - Pentax Lens Reviews & Lens Database

"Mort De Rire" ... = LOL !

A+, John Frog

[top-quark]

I was wrong about the 50-200 WR being the longest lens with a 49mm thread.

It is edged out by the F (or FA) 70-210 mm.

Don't think I'd be swapping though.