Why are organisms that are buried rapidly more likely to fossilize than those that are
uried slowly or not at all?

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.