Photographer use “fast” and “slow” to describe the light gathering abilities of lenses at their maximum apertures- think about these terms as describing the shutter speeds they let you use. Larger apertures such as f/1.4 or f/1.2 take in more light, so that even in dim light conditions you can set a faster shutter speed (maybe letting you avoid using a tripod). Another reason to try a fast lens? Apertures up to f/2.8 can introduce a beautiful defocused effect to your backgrounds. This becomes more apparent with bigger sensors or film size, as the full-frame (35mm) . The best time to shoot at f/1.4 is when a background is cluttered to blur it and then it is no longer a problem. Or, if the overall lighting is flat, blur the background to pop your sharp subject forward. Finally, when there are specular highlights in your background, like the sun-dappled leaves in let say in a park behind a subject, set a wide aperture to produce soft, glowing orbs of light.
People often refer to such background blur by the Japanese word bokeh, and the prettiest bokeh is produced by the rounded diaphragm blades in the lens. To finesses the effect, fine tune the distance between your subject and the background: if they are too close to one another, even f/1.4 would not create enough blur. Conversely, if the background is too far off, even brilliant highlights back there will lack luster