okay, light enters through the barrel of a lens onto the shutter of a camera. The shutter opens for a given time and the image is recorded on the sensor(or film) behind the shutter that was blocking the light. For light to get through,it must past towards the front element, the focusing element, and an adjustable hole (much like the iris around a pupil in your eye except made of aperture blades) then out of the back element into the camera. As you adjust this opening the amount of light that can get through changes. There is a numerical value for the opening. A smaller number means it is open more, an aperture of zero is a camera with no lens. The numerical value represents a stop, or one value of exposure. For the perfect exposure, you have to have the aperture and shutter line up in a ratio. If you speed the shutter up one stop, you open the lens one stop. It will remain 1:1 ratio. To overexpose or underexpose a picture you change the ratio. As more light gets in, you can use faster shutter speed and reduce blur (more open lenses also blur the background and foreground in front and behind your subject more) Closing this hole is called "stopping down" because you will reduce the exposure stop value of th aperture(or raise the #). If you throw your camera into "P" mode, that is program AE where the camera selects the values for both shutter speed and aperture. If you compose your shot and press helfway, the camera will tell you what it is going to use. You can change these values for desired effect and the camera will keep them the same stop ratio.
For example. You see a picture you want to take. You're in P mode (aAVOID AUTO MODE, P is basically auto with control of the ratio, full auto sets one ratio rather than giving you a set to choose from) you hold up the camera and press the shutter halfway, the camera focuses, sets up a ratio for the scene, and gives you values in the viewfinder.
Let's say the camera tells you, that for this situation, you want to use an aperture of F/4.5 and a shutter speed of 1/15th of a second. Well you know by practicing that you can't handhold the camera at 1/15 and get a shaper picture, you also want the background blurry. so you turn the wheel and watch the values change to
F/4.5 and 1/15(you start with this) "ugh, no, I can't handhold this, my picture will be blurry"
F/4.0 and 1/30 "still not fast enough"
F/3.5 and 1/60 (most cheap lenses can't open up any more than this) "maybe, I'd prefer more security"
F/2.8 and 1/100"ok shutter speed's good, but let's blur that background a little more"
F/2.0 and 1/120"a little moe..."
then F/1.8 and 1/150...... "perfect, thanks canon for making the 50mm 1.8 so affordable"
-click-
and a beautiful picture shows up on the LCD screen.
Do you get it?
Overexposed 1.5 stops:
good exposure:
underexposed 1.5 stops:
(what I was saying earlier is that most lenses have the absolute sharpest subjects at F/8)