When you use a handheld light meter like the Weston Master V below, it quotes an EV number which you then translate into shutter speed and aperture combinations using a rotating dial. When the camera measures the light level, it too starts with an EV number, which it then uses to decide shutter speeds and/or apertures, depending on the mode you're working in.
If you want to use manual exposure control, or take more control over what shutter speeds and apertures the camera uses, a basic knowledge of exposure values is really useful. In fact, you might not need an exposure meter at all, because it's possible to predict the EV values for different scenes surprisingly accurately. The table below gives some examples:
Once you know the EV number, either from a handheld light meter or using the table above, you can use it to work out what shutter speed and aperture you're going to use with the table below.
One thing to be aware of is that the EV values change at different ISOs. It's not something you need to worry about if you're using the camera's internal light meter because if you change the ISO the camera will allow for this automatically. But if you're using EV values from the table above, or transferring them from a handheld meter, you will need to match the EV to the ISO - you can do this with the table.
What you can see from this table is that for any one exposure value there are lots of different shutter speed and aperture combinations you can use. When you use the 'program shift' function on a camera, this is what it's doing.
