Depth of Focus and Diameter of Field of View

Threads at 50X total magnification: focus is on the red thread, but blue and yellow threads also are in focus.

Threads at 100X total magnification: focus is on the red thread. Notice that the blue thread is not quite in focus, and the yellow thread is completely out of focus.

Threads at 400X total magnification: focus is on the red thread. Notice that the blue thread and the yellow thread are completely out of focus. At this magnification a single thread fills the field of view.