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Anonymous
>>278234 >the aperture measurement refers to the hole formed by the aperture blades.
Not exactly... it's actually the radius/diameter of the light ray entering the lens to form a point. Depending on the design, the aperture hole will most often be much smaller than the entry beam.
I'll grab the noctilux as an example again (hell, even Sonnars are good examples). It's a 50mm F1.0. From what you said, the aperture should be around 50mm in width. The lens, however, is around 67mm wide, giving only around 9mm in which to hide the aperture blades when wide open. If you have looked into a Noctilux, it would seem that it's a big tube full of clear glass. Looking at the back, however, you find that the exit lens is much smaller.
Explanation: A 50mm beam enters the 60mm-wide lens. This beam's width is condensed/compressed into a smaller beam, around 30mm in width farther into the lens by the first few lens elements. The aperture hole is in fact 30mm wide. In appearance, the hole will be much bigger than 30mm, but that's because of the way the lens bends the light entering through either side of the system. On the other hand, while the front element is 60mm wide, you won't have a 60mm wide beam getting to your final image because of two reasons: 1) the light entering from the same direction but outside the useful circle will bend inwards too much, not reaching the image plane. 2) any useless light scattered will be attempted to be reduced by the aperture.
TLDR: The lens aperture is that of the actual light beam -used- to form a point image at the center of the front element. The diaphragm opening doesn't coincide with this, as light does not travel in a parallel direction within the lens; it coincides with said coherent beam's diameter at that point in the system.
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