Say I have an object at 40 cm from a convex 20 cm focal length lens, then a concave -30 cm focal length lens 10 cm behind that. This would give an image at infinity, which one could take a picture of, but around what magnification should they expect (there would be some leeway in the result if since these measurements wouldn't be exact)? Saying infinite magnification like from normal lens equations wouldn't make sense.

  • 2
    I’m voting to close this question because it has almost nothing to do with the practice of actual photography, as outlined in the group's description. This reads like someone's homework assignment for an optics class.
    – Michael C
    Jan 26 at 5:50
  • It's for photographing at infinity in a closed space, no homework Jan 26 at 15:12
  • Again, is what you are doing the kind of photography defined in our description, or something else? Machine vision (not really on subject here)? Security video (definitely not on subject here)? Something else?
    – Michael C
    Jan 26 at 15:16
  • I want to check the focus of a camera at infinity in a small space. I would think adjusting the focus is part of photography. Jan 26 at 21:13
  • where is the description? Jan 26 at 21:47

1 Answer 1


If you mount this lens set like a filter, with the "infinity" side toward your camera's lens, you'd get to photograph an object 20 cm away with infinity focus on the camera's own lens.

This is a Newtonian refracting telescope, in essence, so you'd use diopter arithmetic and then convert diopters to magnification: you have a +5 (= 2.25x and a - 3.33 diopter (= ~1.8x) (diopter strength is the inverse of focal length in meters, while magnification is diopter strength divided by 4, then add 1) -- but with the negative lens acting like the eyepiece of the Newtonian telescope, its power would be the negative of its strength. That means you'd have about 4x for the combination.


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