What must be the focal length of the lens used as simple microscope of magnifying power 26 The final image is formed at least distance of distinct vision?

Derive the formula for angular magnification of a compound microscope, when the final image is formed at least distance of distinct vision. Draw the required ray diagram. 

The angular magnification of a compound microscope is the ratio of the angle subtended by the final image at the eye to the angle subtended by the object at the eye, when both are placed at the least distance of distinct vision.

This is the required expression for angular magnification.