Effect of shifting position of aperture

[Kevin Crisp]

In a classic design like a double protar, what is the effect of moving the iris forward or back a bit, say something like a mm or two? Does this induce focus shift or have some other dramatic effect? Assume I'm using a completed lens in the 160mm range or longer. Second question, if you have a single cell behind the aperture, what is the effect of moving the aperture forward or back? Thanks.

[Donald Qualls]

What I'd expect (in a completed lens) with the aperture away from the node is slight vignetting as you stop down. The diaphragm would cut into the light cone in a location that doesn't affect the whole image plane evenly, and result in darkening the corners and edges more than the center; the further the iris is from the node, the more pronounced this effect would become. The same thing will occur more slowly with only the rear cell (from the same lens), because any movement is smaller relative to the focal length and thus "cuts into" the narrower light cone more gradually.

Focus shift only occurs when a lens has uncorrected spheric aberrations -- that is, the center and edge have different focal lengths (generally, the center focuses longer).

[Kevin Crisp]

That makes sense, thanks.