Question: what is the difference between f/4 and f/2.8?
Answer: The most obvious difference between an f/2.8 and an f/4 lens is in their "brightness", i.e. in the maximum amount of light each lens allows to reach the sensor. ... An f/2.8 lens would usually be capable of giving a more shallow depth of field (and therefore a bigger background bokeh) than an f/4 lens.