Exposure

Understanding exposure control in 35mm film and digital cameras

Exposure is the amount of light that hits the film plane

(You will often see the word "film" used when explaining different concepts in exposure. Film and digital terminology is interchangeable when discussing exposure. 100 ISO film is the exact same sensitivity as a digital camera set at 100 ISO. Film is still used by many photographers, and the word sounds much better than "digital imaging chip.")

BEF: or Basic Exposure Formula is a rule of thumb for determining correct exposure for film without using your camera meter. Every film has an ISO number associated with it. The ISO references film speed or how much light it needs to create an accurately exposed image. Every film needs just the right amount of light striking the film plane for a correctly exposed image. Too much light and film is overexposed (looks light and washed out) and too little it is underexposed (looks dark and muddy). The total amount of light hitting the film is determined by two settings on your camera, the aperture and the shutter speed.

Your built in meter (assuming your camera has one) is a 'reflective light meter' and is based on evaluating light in scenes that have a 'normal brightness range'. If your scene does not represent a 'normal' brightness range, you will notice that the resulting exposure will be darker than or lighter than the true values that originally existed in the scene – this is also why we must learn to be 'smarter' than our meters.

Meters only 'interpret' light (energy) relative to the film speed we're using. A reflective light meter would 'see' (interpret) an 18% gray card (the basis for measurement in all meters; called 'middle gray') as 18% gray; but it would also 'see' or read a white snow scene as 18% gray and it would 'see' a black building as 18% gray if that building filled the viewfinder's frame.

To obtain a normal exposure without the benefit of a light meter, we must first assess if the conditions are 'normal'. The easiest light condition to assess is a BRIGHT SUNNY DAY as most people can identify this condition accurately. It gets trickier if we try to determine a normal exposure on a 'slightly hazy' day, or a 'very hazy' day or a 'partially overcast' day, etc. Fortunately, there is a FORMULA that we can apply to determine the correct combination of aperture and shutter speed to produce a normal exposure on bright sunny days. This is called BEF or Basic Exposure Formula (some call it the "sunny 16 rule").

BEF = 1/ISO @ f16. Here's the translation. 1/ISO means put the number one over the ISO (film speed) of the ISO setting you are using and this fraction becomes the shutter speed you should use @ f16. This means that f16 is the f/stop you should use in combination with the shutter speed you just formulated. Example: ISO = 125; then a normal exposure will result from a setting of 1/125 (shutter speed) @ f16 (aperture) in bright sunny conditions.*

*The BEF formula (1/ISO @ f/16) means that if you are using ISO 100 as your beginning setting for bright sun your exposure settings would be 1/100 @ f/16 (or equivalent); if you are using ISO 200 as your beginning setting your exposure would be 1/200 @ f/16 (or equivalent); if you are using ISO 400 as your beginning setting your exposure would be 1/400 @ f/16; and so on.

However, not all cameras have these shutter speeds, but all cameras do have 1/125, 1/250 and 1/500 etc... As stated in the BEF page here in MOODLE, with ISO 100, since 1/125th is the closest shutter speed on many cameras, this shutter speed will be used, resulting in 1/125 @ f/16. For ISO 200, you would begin with 1/250 @ f16, ISO 400 would begin with 1/500 @ f/16 and so on.*