I was never very good at trigonometry…
The Exposure Triangle
Let’s go back to basics! What is the Exposure Triangle? Why do you need to know about it? How can it help you?
In this article, I’ll answer all those questions - and more!
What is the Exposure Triangle?
Your first job as a photographer is to make sure that your images are well exposed, in other words, not too dark or too bright.
Photographers talk about the 'Exposure Triangle', but that's just a complicated way of saying that how dark or light a photograph is depends on three things: the shutter speed, the aperture and the ISO.
However, the triangle metaphor is relevant in the sense that it reminds us of the trade-off involved in choosing our exposure settings: just as the angles in a triangle always add up to 180°, so the shutter speed, aperture and ISO ‘add up’ to the final exposure.
If you add to one angle of a triangle, you must subtract from one or both of the others to make sure the total stays the same, and it’s the same with exposure: if you choose a longer shutter speed, that means you’ll have to have either a narrower aperture or lower ISO to get the same level of brightness in the final image.
This is crucially important, and it means that almost every one of your exposure settings ends up being a compromise.
In an ideal world, you might want to have a shutter speed of 1/8000 of a second, an aperture of f/22 and an ISO of 100, but that’s just not going to be possible - unless you’re floating in space a few miles from the surface of the sun!
So what can you do?
The only option is compromise. You have to decide what’s most important, keep that setting at its optimal level and adjust the others accordingly.
Yes, the resulting image might be softer or noisier or blurrier than you’d like, but your job is to make sure you make that trade-off in the best possible way.
For example, if you’re trying to freeze a cheetah chasing a gazelle at 70mph, then getting rid of motion blur is much more important than getting rid of noise, so you can afford to choose a faster shutter speed at the expense of a high ISO.
Making those kinds of decisions well in the heat of the moment is one test of a good photographer.
Photography is a fun pastime, but it does require a lot of thinking. I happen to like that, but you have to know what you’re doing to get the most out of your equipment.
Anyway, back to the basics…
The level of exposure is measured in 'stops' or Exposure Values (EV), but what is a 'stop'? Well, if you increase your exposure by a stop, the light is doubled (and vice versa). For example, if you lengthen your shutter speed from 1/200 of a second to 1/100 of a second, your shot will be twice as bright.
They try to use round numbers, though, so the gap from 1/60 to 1/125 is obviously not quite right! The Maths gets a bit more complicated when the gap is only 1/3 of a stop, but the idea is the same.
The built-in exposure meter in your camera will work out what the best exposure should be, but it has to make assumptions about the world that may not always be true.
To judge the 'best' exposure, cameras need a starting point, so manufacturers program them with the knowledge that, on average, the world reflects 18% of the light falling on it. If the camera assumes that to be true, then it can set the exposure accordingly.
However, anyone who's ever taken pictures of polar bears on the Arctic ice will know that's not always the case!
If you simply follow the automatic
In order to make sure the camera is not fooled by very bright (or very dark) conditions, you need to use exposure compensation.
If the scene is especially bright, you can dial in up to one or two stops of positive compensation. If it's especially dark, you can do the opposite.
It might take a few test shots to get it exactly right, but that's better than coming home with lots of shots of grey bears!
What should the shutter speed be?
In the old days, cameras used film, and the shutter speed controlled how long it was exposed to the light in order to take the shot. These days, cameras are digital and have electronic sensors at the back, but the principle is the same.
The longer the shutter speed (or ‘time value’ if you’re a Canon user), the more light reaches the sensor and hence the brighter the image (all other things being equal). The shorter the shutter speed, the less light reaches the sensor and hence the darker the image.
The shutter speed is measured in seconds and can be anything from 1/8000 of a second to 30 seconds or more.
There are two factors that determine the correct shutter speed:
Camera shake
Motion blur
The amount of camera shake increases with the focal length, so the rule of thumb for general photography is to make sure your shutter speed is no less than the inverse of the length of your lens, eg if you're using a 400mm lens, you should be using at least 1/400 of a second.
Lens technology such as Nikon's 'Vibration Reduction' or Canon's 'Image Stabilisation' means that you might be able to get away with a couple of stops slower - ie 1/100 of a second - but that's about it unless you have very steady hands!
The other reason why shutter speed is an important setting is that it controls how much (if any) motion blur there is in the image.
The faster the subject is moving, the faster your shutter speed will have to be.
I’m a wildlife photographer, so I’ll generally set my default shutter speed to 1/1000 of a second. That’s fast enough to freeze most big animals in motion, but I’ll often boost that up to 1/2000 for a cheetah hunt or even 1/3200 for birds in flight.
Whatever kind of photographer you are, it’s always useful to have a rule of thumb for the best shutter speed for a particular subject.
However, it’s also an artistic decision.
Some people like shots of kingfishers catching a fish that look like they're frozen in time, with every single water droplet sharp as a tack.
Other people prefer shots of waterfalls shown with creamy torrents of water cascading over them. There isn't a 'right' or 'wrong' answer. Just try both and see what you think.
I’m a big fan of the ‘slow pan’, which is when you use a slow shutter speed and follow the animal across your field of view in order to produce blurred legs and a blurred background while keeping the head reasonably sharp.
This is a great way to introduce energy and a sense of motion to your shots, but it’s very hard to pull off successfully!
What should the aperture be?
The aperture is simply the size of the hole in the lens through which light passes on its way to the sensor, and the principle is similar to that of the shutter speed.
The bigger the aperture, the more light reaches the sensor and therefore the brighter the image (all other things being equal). The smaller the aperture, the less light reaches the sensor and therefore the darker the image.
The only difficult thing about it is the numbers, which often have a decimal point in them like f/5.6 or f/7.1.
The reason the aperture is not always a nice round number is that it is what you get when you divide the focal length of the lens by the diameter of the ‘entrance pupil’.
This is where the iris or aperture appears to be when you look from the front of the lens rather than where it actually is.
As a result, you can make a lens ‘faster’ (ie increase the maximum aperture) by adding lens elements in front of the aperture to magnify it.
It would be nice if the aperture numbers doubled or halved with every stop like the shutter speed and ISO, but that’s never going to happen, so let’s not worry about it!
Anyway, leaving aside the awkward Maths behind it, the aperture is measured in f-stops, which typically start at f/2.8, f/4 or f/5.6 and continue up to f/22 and beyond.
A 'fast' lens is one that has a wide maximum aperture such as f/1.4. Photographers like fast lenses as they allow pictures to be taken in low light and offer great flexibility.
The reason why the aperture is such an important setting is that it controls the depth of field, which is the amount of the subject that is ‘acceptably sharp’.
The laws of Physics dictate that lenses will only focus on one particular point, so the words ‘acceptably sharp’ are used to refer to what is good enough for most people.
The depth of field extends from the focal point in both directions - both towards and away from the camera - but two-thirds of it will be beyond the focal point, and this is quite useful to know when you want to focus on more than one subject.
Again, the human eye is drawn to things it can see clearly, so making sure the subject is sharper than the background is an ideal way to focus the viewer's attention on an animal, say, but landscape photographers might want their images to be sharp all the way from the boat in the foreground to the mountains on the horizon.
For situations like that, there is a useful concept called the ‘hyperfocal distance’, which allows photographers to keep as much of the scene as possible in focus.
Simply put, if you focus on a point at the hyperfocal distance, everything from half that distance to the horizon will be acceptably sharp.
It’s very difficult to calculate manually, but there are plenty of photography apps that handle that for you easily enough.
Again, there is no right answer when it comes to depth of field; the important thing is to experiment and find what works for you.
What should the ISO be?
ISO doesn’t actually stand for the International Standards Organisation - or anything else, for that matter!
It’s just a term that was invented to measure the light sensitivity of a digital sensor. It’s exactly the same as ASA, which measured the same thing in the days of film cameras - a 'fast' film with a high ASA being more sensitive than a 'slow' film with a low ASA.
Every digital camera has a ‘native’ ISO, which is simply the lowest that is achievable with the sensor inside. This is usually 100 (or 64 with my Nikon cameras).
Conveniently, the ISO doubles as the sensitivity doubles, so 200 ISO is exactly twice as sensitive as 100 ISO - which means you’ve increased the exposure by exactly one stop.
You might think that extra sensitivity is a good thing - and it is - but it comes at a cost.
The higher the ISO, the 'grainier' or 'noisier' the image, in other words, the less smooth it is.
The highest value might be 12,800 or more, but the image quality at that value wouldn't be acceptable to most professional photographers.
All that means that you have another trade-off to make between the smoothness of the final image and the shutter speed and aperture settings that are going to be possible.
Photographers approach that in different ways, with some preferring to set their ISO, shutter speed and aperture manually and others preferring to set their ISO manually and using either shutter or aperture priority as their shooting mode.
Personally, I split the difference and use Auto ISO.
That means I can leave the camera to set the ISO and concentrate on choosing the right shutter speed and aperture in Manual mode.
These are the two most important settings from an artistic point of view, so it makes sense to have as much control over them as possible.
I do sometimes switch shooting modes, but that’s only in exceptional circumstances when the light is very low or when I’m trying to take slow pan shots.
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