[armadsen]

To be a little more precise, f is not a camera-specific constant. It's the focal length of the lens. It's a formula that tells you the diameter of the entrance pupil. So at a focal length of 50mm, an aperture value of f/2 means an entrance pupil diameter of 25mm.

But photographers generally just say "f2", meaning an aperture value of two set on the dial of the camera/lens. It's one stop faster (twice as much light) as f/2.8. It'll give you a relatively shallow depth of field, but not as shallow as e.g. f/1.4.