What F-Stops? Talk English Please

F-stops are a measure of the aperture of a lens. In other words, f-stops tell us how wide open the iris of a lens is. Specifically, they express the ratio of focal length to apparent lens aperture, so they have no units. The smaller the number, the wider the effective aperture, and the more light will go through the lens. Hence f2.0 is a wide aperture, whereas f11.0 is a narrow aperture.

In practice f-stops have discrete values; in other words, lenses have a finite number of f-stops available, with no intermediate options. The standard f-stops are as follows:

1.0 | 1.4 | 2.0| 2.8 | 4 | 5.6 | 8 | 11 | 16 | 22

Armed with this knowledge, using the Nikon Teleconverter TC-20E II with the Nikon AF-S 300mm F/4 lens would simply allow you to shoot at 600mm (900mm at 35mm format) F/8 (two stops down from F/4). Not brilliant but otherwise, still useable.