Ignoring differences in light transmission efficiency, a lens with a greater f-number projects darker images. The brightness of the projected image (illuminance) relative to the brightness of the scene in the lens's field of view (luminance) decreases with the square of the f-number. A 100 mm focal length 4 lens has an entrance pupil diameter of 25 mm. A 100 mm focal length 2 lens has an entrance pupil diameter of 50 mm. Since the area varies as the square of the pupil diameter, the amount of light admitted by the 2 lens is four times that of the 4 lens. To obtain the same photographic exposure, the exposure time must be reduced by a factor of four.