You choose both the aperture and the shutter speed. Manual mode even offers a shutter speed of "bulb" for long exposures. Because you control both aperture and shutter speed, manual mode offers great scope for expression. But choose the wrong combination and your photo will be too bright or too dark, or in other words over- or under-exposed. Keep your eye on the exposure indicator when choosing aperture and shutter speed.
01. Rotate the exposure mode dial to M.
02. While the exposure meters are on, rotate the subcommand dial to set aperture to values between the minimum and maximum for the lens.
03. To set shutter speed, keep the shutter-speed dial lock release pressed while rotating the shutter-speed dial. Use the exposure indicators to check exposure.
1: Shutter-speed dial lock release
1: 1/4000 s–1/2 s
2: 1/3 EV steps
3: Flash sync
4: Long time-exposures
5: 1–4 s
View information on functions related to Exposure Mode: Mode M (Manual).