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Originally Posted by fingerz
No, what I mean is that my Sigma 18-50 is f/2.8 throughout the zoom. The max aperture is f/2.8 at the 18mm end and it's still f/2.8 at the long end.
50 / 2.8 = 17.857
18 / 2.8 = 6.428
So we can see that the aperture blades are capable of opening up enough to create a hole 17.857mm wide. If that's possible, why not let them open that wide at the 18mm end also? Why restrict them to 6.428mm? If they allowed them to open to 17.857mm at the wide end you'd get an aperture of f/1 (or near as dammit).
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I saw this thread when you started it. I looked at your argument and had to agree with you.
I didn't post because I knew there must be a flaw in the logic but I could not see it. I think the penny just dropped (well maybe).
The focal length is the distance from the objective lens to the film/sensor. With a long lens you can see it with a ruler held along the barrel. With a wide zoom the objective is not actually positioned anywhere near the focal length distance from the sensor so internal optics manipulate the light to make the image. Because of this and the position of the iris you don't get the theoretical aperture you calculate.
Anyways that my theory
Now I'll await the
