You can tighten or loosen some of the springs by turning the shutter speed knob. This adjusts the gear mechanism, increasing or decreasing the delay between the first curtain opening and the second curtain closing. When you set the knob to a very slow shutter speed, the shutter is open for a very long time. When you set the knob to a very high speed, the second curtain follows directly behind the first curtain, so only a tiny slit of the film frame is exposed at any one time. The ideal exposure depends on the size of the light-sensitive grains.
Larger grains are more likely to absorb light photons than a smaller grain.
The size of the grains is indicated by a film's speed , which is printed on the canister. Inside a manual SLR camera, you'll find an intricate puzzle of gears and springs. Manual SLR cameras have a built-in light meter to help you do this. The main component of the light meter is a panel of semi-conductor light sensors that are sensitive to light energy. These sensors express this light energy as electrical energy, which the light meter system interprets based on the film and shutter speed.