f-stop controls depth of field. Depth of field is the amount of area closer or further away from the camera that is in "acceptable" focus from the point that is in the sharpest focus.If you wanted to make a photo with everything in focus from the closest object all the way to the horizon, then you'd need a very small f-stop.
This confuses people so pay attention here. The higher the f-stop number, the smaller the opening and greater the depth of field.
Opening the f-stop to a smaller f-stop number, will allow more light into the camera, but will decrease the depth of field.
f-stop and exposure time are inter-related. Closing the f-stop one full stop requires you to double the exposure time for the same amount of photon to hit the film and be exposed at the "same" light level.
By playing f-stop against film speed and exposure time, you can create motion blurs or throw the background out of focus.
This photo for an example of a photo taken with a shorter exposure and wider f-stop.
Notice that the subject in the photo is in fairly sharp focus, but the background is pretty much out of focus. This draws attention away from the background and toward the subject.
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