The Exposure Triangle
The exposure triangle or photographic triangle refers to the inter-connected relationship of the 3 elements that make up an exposure or image.
In the excellent book Understanding Exposure Bryan Peterson explains about the reciprocal relationship between Aperture, shutter speed and ISO and that if one element is altered it effects the others.
Aperture can be explained as the width of the lens opening which lets light into the camera, the wider this opening the more quickly light is allowed in to the camera and onto its sensor (or film). You can learn more about this in my article on aperture
Shutter Speed is the amount of time which the shutter remains open to let light onto your sensor, it is measured in time ranging from 30 seconds or more to 1/8000th of a second or less. To get more information about shutter speed have a look at my article explaining shutter speed
ISO is your sensors (or films) sensitivity to light. It determines how quickly your sensor can gather light.
For a great explanation check out my article which explains ISO
So how is this a triangle?
Well all these values affect one another so if you take a shot that is properly exposed at f/5.6, 1/250 and ISO 200 and then you decide to stop your aperture down by 1 stop from f/5.6 to f/8 because you want to increase depth of field that would result in half the amount of light now reaching your camera’s sensor.
In order to obtain a correct exposure now you would have to change one of the other settings to compensate. Your options are to decrease shutter speed by 1 stop to 1/125 of a second or to increase ISO by 1 stop to 400.
Now this is where practise pays off because after a while you will be able to judge which option will give a better result under the circumstances. Lets say you are photographing moving objects like sports then decreasing your shutter speed may not be an option because it might result in blurry shots. So the correct choice in this situation would be to increase ISO to 400.
But if you were taking a photo of a landscape where everything is still then the shutter speed would not be so crucial and therfore it would be preferable to change it rather than increase the ISO, because increasing the ISO would result in more noise and worse picture quality.
The Key to mastering the Triangle
The key to mastering the triangle and therfore exposure is to understand what effect each element has on your pictures and to be able to prioritise what’s important for that particular image.
Most of my work for example is landscapes and for me picture quality is the most important thing because I make large prints to sell. Therefore I very rarely increase the ISO on my camera above 100. The only times I do so are when I wish to take a shot at a certain shutter speed (perhaps to capture the movement of water in a certain way or to ensure I freeze grass being blown about in strong winds.) and am unable to increase my aperture because it would result in a loss of depth of field.
Being a landscape photographer means most of my images are shot at apertures of f/8-f/16 to try and maximise depth of field. When you look at my priorities when making images you can see that aperture is my first concern, followed by keeping as low an ISO as possible. It is only when moving elements are contained within a scene that I give much thought to shutter speed so this is the setting that is altered most freely when trying to create correct exposures.
A Wildlife photographer would no doubt replace my priority of aperture to one of shutter speed. This is because they need to be able to freeze moving subjects which requires the use of fast shutter speeds. Aperture will still have to be considered to ensure that enough of the subject is still in focus. ISO will therefore be the setting that is most free to alter in order to obtain a correct exposure. This is why pro sports and wildlife photographers really appreciate high ISO performance because it enables them to use faster shutter speeds in poor light.
If you are able to determine what your priorities are when capturing a shot then you will be able to make informed choices about which settings to use and which to alter to enable you to expose your photos correctly.