You can adjust aperture and shutter speed only in the advanced exposure modes on your Canon EOS 70D. To see the current exposure settings, press the shutter button halfway. The following actions then take place:
The exposure meter comes to life. If autofocus is enabled, the autofocus mechanism starts to do its thing.
The aperture and shutter speed appear in the viewfinder, LCD panel, and the Shooting Settings display. In Live View mode, the settings appear at the bottom of the monitor, assuming that you're using a display mode that reveals shooting data. (Press the Info button to cycle through the Live View display modes.)
In manual exposure (M) mode, the exposure meter lets you know whether the current settings will expose the image properly. In the other modes, the camera indicates an exposure problem by flashing the shutter speed or the f-stop value.
The technique you use to change the exposure settings depends on the exposure mode:
P (programmed autoexposure): The camera displays its recommended aperture and shutter speed. To select a different combination, rotate the Main dial. But note that your change applies only to the current shot. Also — and this is a critical point — you can't stray from the initial shutter speed and aperture when you use flash.
Tv (shutter-priority autoexposure): Rotate the Main dial to change the shutter speed. As you do, the camera adjusts the aperture as needed to achieve the proper exposure.
Changing the aperture also changes depth of field. So even though you're working in shutter-priority mode, keep an eye on the f-stop, too, if depth of field is important to your photo. Also remember that if the aperture value blinks, the camera can't adjust the aperture enough to produce a good exposure at the current shutter speed. So you may need to compromise on shutter speed (or, in dim lighting, raise the ISO).
Av (aperture-priority autoexposure): Rotate the Main dial to set the f-stop. The camera automatically adjusts the shutter speed.
The range of possible f-stops depends on your lens and, with most zoom lenses, on the zoom position (focal length) of the lens. For the 18–135mm kit lens, for example, you can select apertures from f/3.5 to f/22 when zoomed to the shortest focal length (18mm). At the maximum focal length (135mm), the aperture range is from f/5.6 to f/36.
If you're handholding the camera, be careful that the shutter speed doesn't drop so low that you run the risk of camera shake. If your scene contains moving objects, make sure that the shutter speed is fast enough to stop action (or slow enough to blur it, if that's your creative goal).
M (manual exposure): Select aperture and shutter speed like so:
To adjust shutter speed: Rotate the Main dial.
To adjust aperture: Rotate the Quick Control dial.
B (Bulb exposure): Rotate the Main dial to set the f-stop. You control shutter speed through the shutter button: The shutter remains open as long as you hold the button down.
C (Camera User Settings) mode: Use the same technique as for the exposure mode you used as the basis for your custom mode. For example, if you based the C mode on the Av mode, you can adjust f-stop by rotating the Main dial.
You also can use the Quick Control screen to adjust settings in all these modes except P. First, choose the setting you want to adjust. For example, in the following figure, the aperture setting is highlighted, and the name of the option appears at the bottom of the screen. Rotate the Main dial or Quick Control dial to adjust the setting. You also can tap the f-stop or shutter speed to display a screen with a setting scale; drag your finger along the scale or tap the arrows above it to adjust the setting. Press the return arrow or the Set button to exit the screen.
Keep in mind that when you use P, Tv, and Av modes, the settings that the camera selects are based on what it thinks is the proper exposure. If you don't agree with the camera, you can switch to manual exposure (M) mode and simply dial in the aperture and shutter speed that deliver the exposure you want.