Optimal aperture position?

Started 2 months ago | Discussions thread
ProfHankD
ProfHankD Veteran Member • Posts: 6,809
Optimal in what sense?

E Dinkla wrote:

Optimal position for the aperture in the optical path?

Obviously, in front of the lens is optimal in the sense that it is most accessible.

Basically, there are multiple positions within, behind, and in front of most lenses that can be suitable places for an aperture stop. However, there are various differences between the different locations because rays don't get clipped the same way -- so, for example, some locations might result in better control of aberrations than others. It really takes either modeling the lens or physically testing it to determine the most viable aperture positions. A valid aperture stop position is one in which the opening does not cause vignetting (i.e., isn't a field stop); ironically, too large a hole is generally how you get vignetting.

If at all possible, I prefer the front mounting of a Waterhouse stop. It's easy to mount/unmount them on unmodified lenses and they are easy to make very precisely the size and shape desired, whereas an iris is really a pretty challenging precision build. My Waterhouse stops are generally cut on a programmable paper cutter, laser, or CNC router/engraver; larger aperture ones also can be 3D printed.

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