My understanding of the formal way to measure aperture is that it is the ratio of the diameter of the _perceived_ aperture at the film plane to the focal length of the lens in use*, and is thus at least as dependent upon the effects of post-aperture elements on the bundle of rays which form the image, as upon the diameter of the front element. This is not to say that the diameter of the front element is irrelevant, just that you cannot expect to measure it to support arguments that a lens has an effective maximum aperture <4% smaller than it is rated.
John Coyle Brisbane, Australia *Source: M. J. Langford, Basic Photography, Focal Press 1973 ----- Original Message ----- From: "Scott Nelson" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Thursday, December 19, 2002 12:19 AM Subject: Re: SMC Takumar 135/2.5 > 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 > >
