Light Is Information

Light is information.  In film based photography, it is the sliver and dyes (if color) on the negative or slide that store this information.  In digital photography, it is the discrete electronic units called pixels that store this information.  In both, the more information, the better.

When developing negative and slide film, the chemicals used to develop the film wash away the sliver and dyes not used to record the image.  The more silver and dye that is washed away, the less information that part of the image contains.

If photographing with negative film, shadow areas will show up as light areas on a negative and highlights will show up as dark areas on a negative.  So in a negative, shadow areas have the least amount of density and highlights have the most.  The higher the density, the more information.  Therefore, in negative film, it is the highlights that are likely to have more information.  This is why it is important when exposing negative film to 'expose for the shadows' in order to get as much information as we want in the areas that are prone to not having much.

When using transparency (also known as slide or reversal) film, the image is recorded as a positive image on the slide.  Shadow areas will show up as dark areas on the slide and highlights will show up as light areas on the slide.  So in a slide, shadow areas have the most density and highlights have the least.  As before, the higher the density, the more information.  Therefore, it is important when exposing slide film to expose for the highlights in order to get as much information as we want in the areas that are prone to not having much.

When using a digital camera, the more light reaching the sensor, the more information there is for the sensor to work with.  The light that reaches the sensor is converted from analog information to digital information.  To record as much information as we can, we want a digital image that does not clip either the shadow areas or highlight areas when viewed in a histogram and we want the histogram biased toward the right.  If we clip shadows and/or highlights, they will appear as solid black or white, respectively, with no detail.  An image whose histogram is biased toward the right had more light processed by the sensor, thus more information to work with, than the same image whose histogram is biased toward the left.

So why is all this information good?  If you are a casual photographer who just wants to take a good picture, or you use slide film and the slides, not prints, are the final product, then my suggestion is to expose for the subject.  However, if you take a critical approach to your photography, whether professional or personal, then the maximum information you can record in your image gives you more options and leeway when creating prints.

Remember, in digital photography we can still control the shadows and highlights through various techniques, such as reflectors, fill flash, neutral density filters, etc. Also, when digitally editing an image, we have additional options to control contrast and tonal range, such as Levels and Curves.  To learn more these controls, read the Levels page and the Curves page.