C. Maximise lens aperture

Focal length (f) is usually given in millimetres (mm)

Aperture is specified as a ratio: A = f / n , where n is some number - often called the f-number

Smaller f-number means larger aperture - f/2 is clearly a larger aperture than f/10

Setting the same f-number on different lenses gives the same amount of light exposure

Most photographers know this but it can be confusing that smaller f-number means a larger aperture.