Exposure


Exposure explained

Exposure is the term given to the amount of light that the film or digital sensor is exposed to when taking a picture. This determines how dark or bright the final image will be. More light = a brighter image. When taking a photograph the objective is to get just enough light to the film/sensor to accurately reflect the scene that you are taking.

Compared to the scene you are taking, a photograph that is too dark is under exposed and one that is too bright is over exposed.

Under exposed

Over exposed

Correct exposure

 

It helps to understand how film and digital sensors react to light and convert it into an image:

Film: Film is made up of many layers but the photo-sensitive layer that captures the image is coated with silver halide. This is made up of many individual crystals or grains that (when developed) turn dark when exposed to light. This is a chemical reaction caused by the light energy and more light causes the crystals to turn darker.

With film, the lightest part of your image causes the crystals to become darkest, this is why, with film you have a photographic negative, where dark and light are opposite to the image that you took. This is why a negative has to be converted into a positive or print. Interestingly the same thing applies when making a print from a negative; the light sensitive chemical on the paper also turns dark with more light but of course you are passing light through the negative onto the paper, so the dark parts of the negative pass less light onto the paper and these area remain light on the paper – just like they were in the original scene. Negative becomes positive again!

Digital: A digital sensor is made up of millions of individual electronic photo-receptors. When photons of light hit a receptor it builds up an electrical charge; the more light, the greater the charge in the photo-receptor. The electronics in the camera then convert this information about the level of charge in each individual photo-receptor and create an image from the millions of pieces of information.

With either film or digital if there is too much light, either the silver halide crystal turns completely black or the photo-receptor fills completely with charge – each would represent pure white in the final image. Conversely if no light hits the film or sensor, either the silver halide crystal remains clear or the photo-receptor has no charge – each would represent pure black.

This explains why we need to get just the right amount of light onto the film/sensor to get a properly exposed image. Of course there will be some areas of the image where you want it to be bright, such as clouds and some where you want it to be dark, such as shadows but the overall image should be an accurate reflection of the scene.

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Text and images © 2013 David Preston – all rights reserved

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