Understanding optics...

Eye relief

Eye relief is the distance from the rear eyepiece lens to the exit pupil or eye point.

It is the distance the observer must position his or her eye behind the eyepiece in order to see an unvignetted image.

The longer the focal length of the eyepiece, the greater the eye relief.

See also:

- What do the numbers on my scope mean?

- What is magnification?

- What is the field of view

- What is an objective lens?

- What is eye relief?

- What is the exit pupil?

- Twilight Factor

- Relative Brightness