Which of the following f-stops is considered "fast"?

A fast f-stop refers to a lens opening that allows more light to pass through, enabling quicker shutter speeds which are beneficial for capturing images in low light conditions or freezing motion. The f-stop number signifies the size of the aperture, with lower numbers indicating a larger opening.

In this case, f/1.4 is considered the fastest among the provided choices because it has the largest aperture. This larger opening permits significantly more light into the camera compared to f/2.8, f/4, and f/5.6. As a result, using f/1.4 not only facilitates shooting in dim environments but also allows for a shallower depth of field, creating a stronger subject isolation effect through beautiful background blur (bokeh).

This is particularly useful in portrait photography or when you want to focus attention on a specific subject while diminishing distractions from the background. Consequently, lenses with an f-stop of 1.4 are often favored by photographers who need speed and creative control over their focus and lighting.