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 fo or fe so that the following relationships hold.
In a working microscope, the length L in the sketch above is much longer than either of the lens focal lengths fo and fe.