A camera lens uses refraction to focus light on the film, or in a digital camera on light-sensitive array.When a ray of light passes from a less dense to a more dense medium it slows down. If it strikes the glass surface at an angle, it is also bent a little, and this is called refraction. When it passes back into air, it speeds up again, and is again refracted if the surface is at an angle. You can use one flat surface and one parabola, this is a plano-convex lens. When you use a magnifying glass to focus the sun's rays and set paper on fire, that's the effect you are using.For closer objects, you need to move the film a little closer to the lens. That's what happens when you focus a camera. So, camera lenses have a second control, the iris or f-stop, that changes the size of the lens by masking the outer bits of it. If you use a very small lens, lots of things will be in focus.