For full access to all your Canon EOS 70D's exposure controls, set the Mode dial to one of its advanced exposure modes, highlighted in the following figure.
Using these modes lets you manipulate two critical exposure controls, aperture and shutter speed. That's not a huge deal in terms of exposure because the camera typically gets that part of the picture right in the fully automatic modes. But changing the aperture setting also affects the distance over which focus is maintained (depth of field), and shutter speed determines whether movement of the subject or camera creates blur. These settings provide you with creative options that you don't enjoy in the fully automatic modes.
Each advanced mode offers a different level of control over aperture and shutter speed, as follows:
P (programmed autoexposure): The camera selects the aperture and shutter speed, but you can choose from different combinations of the two.
Tv (shutter-priority autoexposure): You select a shutter speed, and the camera chooses the aperture setting that produces a good exposure.
Why Tv? Well, shutter speed controls exposure time; Tv stands for time value.
Av (aperture-priority autoexposure): The opposite of shutter-priority autoexposure, this mode asks you to select the aperture setting — thus Av, for aperture value. The camera then selects the appropriate shutter speed to properly expose the picture.
M (manual exposure): In this mode, you specify both shutter speed and aperture. Although that prospect may sound intimidating, it's actually the fastest and least complicated way to dial in exactly the exposure settings you want to use. And even in M mode, the camera assists you by displaying a meter that tells you whether your exposure settings are on target.
Setting the Mode dial to M has no effect on whether autofocusing or manual focusing is enabled. You set the focusing method via the switch on the lens and can use auto or manual focusing in any exposure mode.
B (Bulb): Bulb mode is a variation of Manual mode. You still control aperture and shutter speed, but instead of setting a specific shutter speed, you hold the shutter button down for the length of time you want the image to be exposed.
Bulb mode is great for night photography, catching lightning in action, and photographing thunderstorms and fireworks because it enables you to experiment with different shutter speeds simply by holding the shutter button down for different lengths of time. You don't have to fiddle with changing the shutter speed between each shot.
C (Camera User Settings): Camera User Settings is another special mode. You can register (save) most camera settings (including exposure mode, menu options, and so forth) and instantly recall them by selecting C from Mode dial. To put it another way, you can create your own, custom exposure mode.