Isn't it that the 52mm front thread lens (Tak Bayo) is also a the shorter lens, making the light path less long? Not so much light bending to go on, and the same effective aperture can be physically less wide, in a shorter lens assembly. So I have gathered from what I've seen over the years...
A long, fat f/2.5 and a shorter, narrower f/2.5 are essentially just different ways of reaching the same place. keith whaley Fred wrote: > > > Difference between f2.5 and f3.5 is significant - you will > > appreciate it in low light levels! > > ...except that the Takumar Bayonet 135/"2.5", with its not overly > generous 52mm filter threads (unlike the K 135/2.5, which has 58mm > threads) would seem unlikely to be much faster than f/2.8. In any > event, by my measurements, the Takumar Bayonet certainly has a much > narrower front element than does the SMC K 135/2.5 - > > SMC K 135/2.5 : measured 52 mm front element diameter -> f/2.6 > > Takumar Bayonet 135/2.5 : meas. 49 mm front element diam. -> f/2.8 > > http://www.cetussoft.com/pentax/135's/135252.jpg > > The above is a photo of two supposedly f/2.5 135mm lenses - the SMC > K 135/2.5 is on the left, while the Takumar Bayonet 135/"2.5" - the > one that shows my reflection (<g>) - is on the right. It would seem > that both lenses do not fit the same specification. > > Of course, the original point I quoted from is still basically true > - there is still a significant difference between an f/2.8 lens and > an f/3.5 lens. > > Fred
