On Tue, 2002-12-17 at 12:20, Fred wrote: > > Fred wrote: > F>> [The SMC K 135/2.5 is on the left, while the Takumar Bayonet > F>> 135/"2.5" is on the right.] > > > Fred, Unfortunately I was referring to the SMC K 135/2.5 . :o( The > > front lens diameter as it appeared to my limited measuring > > capabilities is around 52mm, that translates to an aperture of 2.6 > > or so. Worse even - the meter agrees. > > Hi, Alin. I see. You know, before sending my post (that you > quoted), I tried to find an older post of mine, where I had stated > my measurements, but I couldn't find it, so I'll have to measure > again. I do remember that the SMC K seemed a little smaller than > f/2.5 (by measuring and calculating), and that the Tak Bayonet was > smaller still. Hmmm... OK, where are those lenses... > > Well, here's what I just measured and calculated: > > SMC K 135/2.5 : 52mm front element diameter -> f/2.6 > > Takumar Bayonet 135/2.5 : 49mm front element diameter -> f/2.8 > > Of course, I would probably come up with slightly different > measurements each time I tried to measure them (since I don't have a > measuring caliper), and I'm also assuming that both lenses truly > have 135mm FL's. > > On the theoretical side, it has been pointed out here before that > the entrance pupil of a lens is not necessarily equal to the clear > diameter of its front element, but I myself just can't see how a > 135/2.5 lens could ever have a front element diameter less than 54mm > and still be an f/2.5 lens.
The FL is probably a bit shorter than 135mm, giving a boost to the relative aperture. According to photodo, the real FL of the new FA 135/2.8 is 130mm, and coincidentally 130mm / 52mm = exactly 2.5. -Scott
