In a working microscope, the length L in the sketch above is much longer than either of the lens focal lengths f_o and f_e.

Compound Microscope

A compound microscope uses a very short focal length objective lens to form a greatly enlarged image. This image is then viewed with a short focal length eyepiece used as a simple magnifier. The image should be formed at infinity to minimize eyestrain.

The general assumption is that the length of the tube L is large compared to either f_o or f_e so that the following relationships hold.

Microscope Example

The general assumption is that the length of the tube L is large compared to either f(o) or f(e) so that the following relationships hold.