Reading these last few posts reminds me of how surprised I am that my 135/2 isn't larger, especially compared with my 200/2.8
135 / 2.0 = 67.5
200 / 2.8 = 71.4
Only 4mm different...
Yes now you mention it I can see that based on the same maths the 200/2 and the 300/2.8 are similar diameter.
But if the formula is as simple as focal length / f stop, why does my 135/2 vignette much more at f2 than my 200/2.8 does at 2.8 ?
The simple formula is just that.. simple and a guideline. A lot more goes on in the world that is precision optics that can produce(or not) the vignetting you're experiencing.
My questiion is
essentially, what goes on ?
So to reply 'a lot more goes on' is hardly an answer.
I suspect it has to do with the physics of passing light through a group of lenses: the closer the design comes to perfect 100% transmission of the light value the more disproportionate the fall off at the extreme of the image circle. So to avoid more vignetting the faster lenses would have to produce a larger image circle, which would increase size, weight and cost in a similar vein to the OP's original question on zooms.
So given that the 135/2 and 200/2.8 are of a similar design and price, I should have expected the faster lens to have more vignetting when wide open.