The term exposure in photography means the amount of light that reaches the film or other light-sensitive material. The mathematical formula for exposure is the product of light intensity and the amount of time that the light acts on a light-sensitive material. There are two ways a formula is presented in photographic publications. They are as follows:

E=IxT

and

H=ExT

Where:

E or H = Exposure (lux-seconds or meter-candle seconds)

I or E = Intensity or illuminance (lux or meter candles)

T = Time (seconds) Both of these formulas represent exposure. The second formula is presented in the more current publications.

As explained previously, camera exposures are controlled by the shutter speed and aperture. The shutter speed controls the time light is permitted to reach the film. The illuminance (or intensity as it is sometimes called) is controlled by the aperture of the camera. The term illuminance means the amount of light reaching the film plane. By adjusting these controls, you allow the correct amount of light to reach the film. The correct amount of light varies, depending on the film speed. Correct exposure for negative films is defined as the exposure required to produce a negative that yields excellent prints with the least amount of difficulty. Correct exposure for color reversal film produces color images in densities that represent the appearance of the original scene.