As I briefly mentioned in my article on the Exposure triangleAperture 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).
If you want to allow light in more quickly then you simply use a larger aperture. Aperture is measured in f/numbers generally ranging from f/1 -f/22.
The only confusion comes because the larger the aperture the lower the number, so f/4 is larger than f/16 for example.
Light is measured in stops and common apertures are as follows:
Increasing your aperture (eg f/4 to f/5.6) will result in the halving of the amount of light entering your camera and decreasing your aperture (eg f/8 to f/5.6) will result in the doubling of light being let in to your camera.
So which apertures are good in which situations?
Larger apertures like f/1-f/4 result in narrower depth of field (the depth of a scene in focus) which can be useful for throwing the background out of focus for portrait shots where you want your subject in sharp focus and the background blurred so it does not distract from your model.
Middle apertures like f/8 and f/11 often offer the best lens performance so if you do not care about depth of field because everything is at the same distance or you just want the best performance you lens can offer then these middle apertures are a great choice.
Small apertures such as f/16 and f/22 offer the largest depth of field and therfore are popular with landscape photographers who want everything in a scene to appear sharp.
However a word of caution, although these apertures will often give the largest depth of field the issue of diffraction can actually undermine image quality so you may actually be better off using a larger aperture like f/11. A quick explanation of diffraction can be found over on Luminous landscape
Another element of the photographic triangle which I talked about in my article The exposure triangle is Shutter Speed which relates to 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.
Again shutter speed can be measured in stops and changing your shutter speed from one stop to the next will result in the doubling or halving of the amount of light let into your camera.
Faster shutter speeds allow you to freeze fast moving action like sports or wildlife and capture sharp shots. Slower shutter speeds are useful in poorly lit scenes if you need to allow more light on to your sensor to get a properly exposed shot. They are also great for artistic shots of scenes which you can see in a number of my images in the gallery
The best way I have ever heard to explain ISO is to imagine that your camera is a bee hive and that ISO represents the number of bees and light is represented as honey.
When there are just 100 bees they can collect x amount of honey in a certain amount of time. If you double the number of bees to 200 hundred (equivalent to increasing ISO by 1 stop) they could collect the same amount of honey in half the time or double the amount of honey in the same time. Now if you have 400 bees they could collect the same amount that 200 bees could in half the time or collect double the amount of honey in the same time and so on and so on.
So for every stop increase in ISO your sensor is able to gather light twice as fast.
Here are the common stops for ISO:
50, 100, 200, 400, 800, 1600, 3200, 6400, 12500
Increasing your ISO doesn’t come for free. Everytime you increase your ISO your images will become more noisy, which means they will contain little bits of what almost looks like graininess. This is due to the way that increasing ISO works. Basically when you increase your ISO you are increasing the voltage running through your sensor, this in turn results in electrical interference in your images in the form of noise.
Camera manufacturers have made great improvements in this area in recent years but it is still better to use a lower ISO if possible to achieve the best image quality.
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.