23 November 2014
Do your photographs always turn out just right or are they sometimes too light or too dark? Discover what photography exposure means and why it is so important to get it right.
When we look at something, our eyes adjust very quickly so we can see it properly. With exposure in photography, things are very similar.
There are two functions that matter here. One is the ability to adjust to the brightness of the subject and the other is adjustment to its distance from us. Adjustment to distance will be dealt with in the next article.
Here we will address the brightness of a subject and the way a camera’s image receptor (film or digital sensor) receives the right amount of light - Exposure.
Most modern cameras work in a similar way. They have an automatic exposure meter which reads the amount of light entering the camera and this information is used to make an adjustment to it’s aperture - a device which is very similar to the iris in our eye.
The aperture is just behind the camera’s lens and it opens to the correct size to allow just the right amount of light to pass through to the image receptor. It’s a bit like a water pipe - a large pipe would let more water through that a small pipe.
At this point, there is a key difference between our eyes and a camera. No matter how long we look at something, it stays looking the same - no lighter, no darker. If a camera receptor is continually exposed to something, it will eventually receive so much light that it will just burn out - not literally, but it will record an image as completely white.
To overcome this, the camera’s image receptor needs to receive light through the aperture for a certain length of time. Any longer is too much, any shorter is not enough.
Using the water pipe analogy - if we are trying to fill a bath with water, we need to turn the water off once the bath is full to avoid a flood, but not earlier because it won’t be full enough!
The camera controls this time using its shutter. Depending upon the type of camera, shutters take various forms but they all achieve the same result - control of the time for which light is received by the image receptor.
There is just one more simple factor - the sensitivity of the image receptor. In a film camera, each film has a particular sensitivity, which is the speed at which it responds to light. This is helpful because it enables us to take shots in less light if we use a faster film and brighter light if we use a slower film.
With a digital camera we can decide upon the best sensitivity of the sensor and set it for each individual photo. This is more flexible than with a film camera where we are committed to the film’s sensitivity for the whole roll of photographs.
To Summarize - The camera’s automatic exposure meter controls the camera’s aperture and shutter combined with the film or digital sensor’s speed setting, to ensure that the image receptor receives the right amount of light.
This article is an extract from the original which covers the Golden Rule of Photography Exposure and further suggested reading.
Previous Articles in the series -
1 - "Photography Technique - Holding the Camera"
2 - "Photography Technique - The Essentials"
3 - "Photography Technique - Composition"
4 - "Photography Technique - Lighting"