Question

Two convex lenses of focal lengths 2f and f are arranged to from an eye-piece and are spaced by f. The first lens is to be used as field lens and the second one as eye-lens. Find (a) equivalent focal length. (b) See whether it is positive or negative eye-piece. (c) Whether it is free from spherical aberration and chromatic aberration? (d) What will be the disadvantage it f is used as field lens?

Answer 1

Correct Answer - (a) F = f, (b) positive, (c) free from spherical aberration but not chromatic, (d) eye-lens position falls as the focus of the field lens and so dust particles etc on field lens will be infinitely enlarged and whole field of view will be spoiled