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When you stop down and look through the lens, the iris (circle of light) gets smaller. Does this mean that light only passes through the middle of each lens element behind the aperture blades? What would happen if you cut out a donut and stuck it on the rear element. Would that change the picture?

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    This question should probably be reworded. As it stands now the main question and the body of the question ask the same thing in an opposite way. If the answer to the headline part of the question is negative, it means the answer to the body of the question is affirmative and vice versa. – Michael C May 25 '16 at 9:23
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The answer to the first part of your question is yes.

The answer to the second part of your question is dimmer, increased flare, and poor focus if you use white material and with slightly less flare with darker material. The size of the hole will determine the amount of light fall-off and the annular size will determine the amount of focus degradation due to chromatic aberrations if the diameter is smaller than the clear aperture of the rear element. If you put the donut on the front of a refracting lens, it will act as a poor Barlow lens and vignette the image.

The blades of the iris are normally placed somewhere between the two nodal points centred on a compound lens axis, where the light rays cross from upright to inverted, so as to not vignette the image when at a minimum.

Placing an iris, fixed or variable, elsewhere in the light-path will vignette the image. The amount of vignetting will vary and may affect the image area if pronounced enough.

  • Does this mean that there is a donut such that the size of the hole means that no light would be blocked, therefore would not actually affect the image if stuck on the rear element? (since the light only passes through the middle of the rear element) – Alex May 25 '16 at 4:14
  • You are asking if one can find a fixed aperture that will not interfere with the minimum aperture of a typical barrel lens. Sure, that's what the lens mount is, in effect. We refer to the "clear aperture" of a lens system as being the largest unobstructed diameter of the system. Usually, the rear aperture is it. Your question is whether changing the diameter of the rear lens changes the lens formula. Yes. How much needs be ray-traced depending on the formula. – Stan May 25 '16 at 6:16
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When stopped down, does light still pass through all of a glass element behind the aperture blades?

No (Assuming the rear element is large enough to pass through all of the light allowed through by the aperture when it is wide open).

Does this mean that light only passes through the middle of each lens element behind the aperture blades?

Yes. (Again, assuming the rear element is large enough to pass through all of the light allowed through by the aperture when it is wide open).

What would happen if you cut out a donut and stuck it on the rear element. Would that change the picture?

It depends on which is larger: the cone of light allowed through by the aperture of the lens or the hole in the middle of the donut. If the cone of light is larger than the hole in the donut then the donut would block, and possibly reflect, some of the light allowed through by the aperture. If the hole in the donut is larger no light would be blocked or reflected.

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