Your basic lens is the pinhole camera. You can illustrate
a picture to exemplify how the light rays bypass through the hole to outline a picture
on the screen. A lens is a development on the pinhole, but the pinhole result
is still there, defined by the width of the lens. In a camera, the width is
adjustable and that is what the f-stop embodies. A small f-stop means a small
hole and the pinhole effect permits focus in a large range. A large f-stop
means the focus has to be more accurate, and things in front or behind the
focus point are blurred. In a microscope you have no such adjustment, so depth
of focus is always the same meaning that some objects in front of and behind
the object being viewed will also come into view in focus.
