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Eye relief - what is it?

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An eyepiece forms an image of the objective. The size of the disk of light when you look into a pair of binoculars or at a distance from the eyepiece is called the Exit Pupil or Ramsden Disk.

It's size is Aperture/ magnification. ie a 200mm scope with a x50 magnification will give an exit pupil of 200/50 = 4mm

When we consider the whole field of view, light coming from near the edge also forms the exit pupil and where these light beams cross you get the best position to place your eye to "see" all the field. The distance from this position to the eyepiece is called the "eye relief" It can be as much as 18 -20mm or as short as 2-3mm depending on the design of the eyepiece.



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I have a question which seems to fit into this discussion.

What distance is the eye focusing at when looking into an eyepiece ?

It is obviously not the distance to the Exit Pupil or Image Plane so must be somewhere between that and infinity ;)

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