Depth of Field and the Telescope

Focus is not all or nothing

The f-number and the depth of field law

Concave lenses and virtual focus

A telescope from two lenses

A ruler is photographed from a fixed distance at f/2, then at f/4, then at f/8, with the camera position unchanged and the focus left alone. What happens to the band of numbers that reads as sharp?

A Galilean telescope must send out parallel rays when parallel rays arrive, so that the eye can view the image without straining. What arrangement of the two lenses achieves this?

A Galilean telescope is built with a convex objective of focal length $f_1 = 60$ centimeters and a concave eyepiece of focal length $f_2 = 4$ centimeters. Using $M = f_1/f_2$, compute the magnification and write the number.