The Exposure Triangle - understanding the relationship between aperture, shutter speed and ISO

By Gary.

The Exposure Triangle.

The exposure triangle sounds daunting but it really isn't, partly because you can see the results of using it instantly with a digital camera and because when you catch the photo that you want, you quickly realise the power of the knowledge you have and how it fits into every picture you take.


What is it?

Put simply, it is the relationship between only 3 settings on your camera. The aperture, shutter speed and ISO. These 3 are the fundamental building blocks of every single image you will take and each one affects the other directly. Used correctly, they will allow you to control a scene and get that great photo almost every time. Lets look at the 3 elements of the 'triangle' one by one.



The shutter is extremely fine metal or fabric vanes that move out of the way and allow light into the camera when you press the shutter button and instantly close again. You can vary how fast they move and therefore for how long light comes in. A typical digital camera will have speeds from 1/8000th of a second, right up to 30 seconds and these are measured in what we call 'stops'. A stop is simply a notch,or a mark, it's a way of measuring the next step up or down and each 'stop' is either double, or half, the speed.


Lets explain that a bit better. The accepted standards for shutter speeds are

1/1000 s, 1/500 s, 1/250 s, 1/500 s, 1/250 s, 1/125 s, 1/60 s,1/30 s, 1/15 s, 1/8 s, 1/4 s. You can see that the numbers half each time. Each one is called a stop so if you have 1/1000th of a second set on your camera, and you change it to 1/500th of a second you've turned your shutter speed down 1 stop. Using the stop as a measurement is useful because everyone knows what it means, but also because the other two, aperture and ISO are also measured in this way, and you'll see the benefit of this later.


Your camera will probably have more speeds on than this as I said.So if you're on 1/1000th of a second and you go up one stop you'll get 1/2000th, another stop is 1/4000th and yet another stop is 1/8000th of a second. Each time you are halving the amount of time that the shutter is open.


1/8000th of a second is extremely fast, so you need a lot of light to ensure that enough comes into the camera to make a picture in that short space of time. 30 seconds is quite a long time so this is useful when it's quite gloomy as it allows the small amount of light that is there enough time to come into the camera. If your camera is set to 1/8000th of a second and you want to set it to 30 seconds, how many stops difference is that?


1/8000, 1/4000, 1/2000, 1/1000, 1/500, 1/250, 1/125, 1/60, 1/30,1/15, 1/8, 1/4, 1/2, 1sec, 2sec, 4sec, 8sec, 16sec, 30 seconds. So that's 18 steps to get down to 30 seconds, so that's 18 stops difference.

On a bright sunny day you might use 1/1000th or 1/500th of a second shutter speed, as there is so much light that you don't need the shutter to be open for very long. If it's inside on a dull day there's not much light so you might have to use 1/30th, or even as low as 1/8th of a second. There's less light around, so you need the shutter to be open longer to allow light in for a longer period. Remember of course that the longer the shutter is open, the more risk there is of blur from you moving!


Uses of shutter speed:

Lets say you wish to take photos of a surfer and you want to freeze the action. They are moving fast so you need a fast speed of maybe 1/500th or even 1/1000th of a second. You may be covering a football match so again you need a fast speed, this time 1/1000th or even 1/2000th of a second to freeze the motion of the players and the ball.


Later, you may want to take photos of a flag fluttering in the wind. You use maybe 1/100th of a second so that there is some movement in the flag. Perhaps you want to take a photo of a fast car but with background blurred? So you 'follow' the car with your camera( called panning) and use maybe 1/50th of a second. This keeps the car sharp as you are moving the camera with it, but the background is blurred. You can see then why you would need to chose a specific shutter speed rather than just choosing any old speed.


So, the shutter determines how long light is allowed to come into the camera for, and is measured in stops, each one being either half or double the one before. A slow speed allows you to have movement in the picture, a fast one freezes all movement.



The next part of the exposure triangle is aperture. Where shutter speed determines how 'long' light comes into the camera for, the aperture determines how 'much' light comes in. It's called an aperture because it is precisely that, an aperture, or hole. This aperture is in the lens and is varied by thin metal vanes which open and close, making a hole smaller or bigger. On some lenses if you take them off the camera and look through them as you change the aperture you can see the vanes moving. On most modern digital cameras you have to set an aperture, then press the 'depth of field' button(if your camera has one). Check your user manual to find out where this is. When you press the depth of field, or DoF button you can see the vanes in the lens moving.


Aperture is usually stated as an 'f stop' and like shutter speeds there is an accepted standard, these are shown as f stops, f2, f2.8,f4, f5.6, f8, f11, f16 and f22. There are often more numbers on some lenses but don't worry about this for the time being. Again just like shutter speeds, an f stop is either half, or double the one before. 

So, let's say you have f2 set on your lens. The 'lower' the number, the 'larger' the aperture, therefore the 'more' light comes in. f2 is said to be wide open as the aperture or the vanes that determine the size of the hole is very large at f2 and so lets in lots of light. Because it lets in lots of light the shutter speed can be set faster, so lenses with f2 or even smaller, are said to be 'fast' lenses. Many zoom lenses might only have the lowest number off 5.6, so they're said to be slow lenses, because you need to set a slower shutter speed to let more light in.

On a bright sunny day you might use f11 because there is so much light that you need to restrict how much comes in. If you're inside on a dull day you need to let more light in, so you open the lens up to maybe f4, or even f2.8 in order to let more light in.


Now, let's say you have our child in a park and y want to take a photo. You would like the child to be sharp, but everything else in the background is distracting. So you set your aperture at a number such as f2.8 if you have it, or f4. At these numbers the amount of distance that is in focus will be limited, depending as well on what lens you're using. This is the Depth of Field or DoF that we mentioned earlier. At f2.8 if you are focussed on the child's face there will be almost nothing behind or in front of the child that will also be in focus. The further away things are the less focussed they will be. You can use this creatively. On the other hand you may wish to take a photo of a garden where you want the whole scene to be sharp. So you choose an aperture such as f16. This time when you focus on where the child might have been, things are in focus for many metres in front of and behind where you focus. 


So, the aperture determines how much light is allowed to come into the camera and like shutter speed is measured in stops, each one being either half or double the one before.


Stopping down means making that the aperture is small so that you have lots in focus, which oddly means you choose a bigger number,like f16. Opening the lens up means making that the aperture is very small so not much is in focus, such as f2.8 or f2.


How does aperture and shutter speed affect each other?

You may have realised something here. By having a longer shutter speed you allow the light a longer time to come into the camera, so that's useful on dull days. However, on dull days you can also set an aperture of f2, which allows much more light in. If you are using a slow shutter speed, maybe then you don't need quite as much light in,so you can turn the aperture to f5.6. That's great for those slow lenses! However if you have a fast lens you can use f2, and because there's more light coming in you can set a faster shutter speed,which means less chance of camera shake right?


I hear you say but how do I determine what shutter speed and what aperture to use? Well there's two things here. Firstly most digital cameras have an exposure scale in the viewfinder. It's a lot of little upright lines going along the bottom of the viewfinder usually, but they may be at the top or even at the side. There's one taller line in the middle and a little marker which runs underneath these lines. The centre line denotes the point at which correct exposure is set when the marker is on it.


Make sure your camera is in manual mode. Set the aperture to f8,and set the shutter speed to 1/60th. Where is the little marker in relation to the centre line? If it is not on the centre line then you are not correctly exposing the image. If the marker is below or to the left of the marker, it's too dark and the photo will be underexposed. If it's to the right or above the marker, it's too bright and the image will be over exposed. 


Shutter speed determines how long light can get into the camera,so if your marker is to the left or below the centre line and the image is under exposed, you need light coming in for a longer time,so you need to set the shutter speed one stop higher, or 1/30th. Set the shutter to 1/30th which if you remember doubles he amount of light coming in and see if the exposure indicator now says that it is correctly exposed.


There is of course another way to do this. If your photo is going to be under exposed remember that making your aperture wider can let more light in. So if your shutter speed is so slow already that hand holding it might result in a blurred photo, you can open the aperture up instead. If it was f8 like we said try setting it to f5.6. Remember, this is one stop higher, so is letting double the amount of light in.


If your exposure is now correct you've learned two things: one is that you can choose to open or close the lens to let more or less light in so that you can keep a faster shutter speed to prevent blur. The other is that bother aperture and shutter speed work in 'stops' so that 1 stop more doubles the light/speed, and 1 stop less halves the light/speed. You don't need that meter in the camera. If you open the lens by one stop to let twice as much light in, you need to shorten the shutter speed by one stop to halve the length of time light comes in. So effectively, the numbers change, and yet the exposure remains correct! There are some good reasons why this is useful which we'll read about later on. 



ISO is an acronym that comes from film but there has been a new ISO standard set out for digital cameras and it is used to describe how 'sensitive' the camera is to light. The main part of the camera that responds to light is the sensor and this sits behind the shutter ready to record the light in a scene when the shutter opens. This is exactly what used to happen with film cameras except the film would be exposed to light by the shutter, not the sensor. ISO was expressed as 'speed' because if you had 'fast' film, about 800 ISO, it was sensitive to light so you could get a fast shutter speed. 100 ISO film was slow, as it was less sensitive to light so you had to use longer shutter speeds.


When the shutter opens light falls onto the sensor, the data about how much light there is and what colours there are is recorded by the sensor, processed by a chip and sent to the memory card, saved in the format of a picture file. The sensor isn't more or less sensitive to light as film was, it simply amplifies all light that falls onto it. During that amplification it introduces 'noise', which is a grainy characteristic of the image. 100 ISO is taken to be the most desirable 'speed' as it needs the least amount of amplification and therefore brings the lowest levels of noise. If you double that to 200, the light falling on the sensor is amplified by double the amount, it's 1 stop extra. If you then set the ISO to 400, the amount of amplification the sensor applies to the available light is doubled yet again, 2 stops extra and doubled a third time if you set it to 800, which as you might guess, is 3 stops over ISO 100. Some cameras can go up to 25,600 and even beyond. Again you see how ISO uses the stops system in the same way as aperture and shutter speed do. It's a simple doubling or halving each time doubles or halves the light amplification. 


This means the camera can work in darker conditions because the low light that is available can be amplified so much. However the downside is that as the sensor amplifies the light more and more, the quality of the image the light would have formed decreases. 'Noise',similar to film grain, becomes extremely intrusive and there are other problems. However despite these problems you can see how this might be useful.

We'll take a quick example as we did in aperture.


Lets say your camera meter has suggested that a correct exposure would be f8 at 1/50th. You need to leave your aperture at f8 for the DoF but as you're using 100mm lens you need to use 1/100th of a second shutter speed. If you raise your shutter speed to let light in for longer the image will be over exposed. You can't use f16 so, you change your ISO from 100 to 200. The sensor will have to amplify the light a little but only a little, but that raising of ISO 1 stop means as the light is being amplified 1 stop, you can make the shutter speed 1 stop faster, ie: 1/100th. Job done.


You can see from this example how the relationship between aperture, shutter speed and ISO is fundamental to all of your photography as it is the basis for all of your creative control whether you use flash units, daylight or studio lamps. 


How does all this fit together?

Well now we know all 3 elements of the exposure triangle work in stops and that for all of them 1 stop up is double the value and 1 stop down is half the value.


We know that if you change one, the other two will also have to change. Her are some examples:


Example 1

You have lots of light and are taking photos of birds on a bird table. Lots of light means a low ISO, or low light amplification. So set ISO to 100 and then say you want a fairly fast shutter speed to freeze the birds. So choose 1/1000th of a second. Then adjust your aperture until the meter says your shot is correctly exposed.


Example 2

You want to select one of the birds by focussing out everything else. So leave your ISO at 100 and choose f2.8 for minimal DoF. Check the camera's meter and adjust your shutter speed until you get correct exposure.


Final bits

This is a very simplified introduction. There are other things to take into consideration.

If you use a 50 mm lens, setting aperture of f2.8 might mean you get everything from the front of the face to the back of the head in focus. The depth of field is about 6 inches. If you use a 200mm lens,the same aperture would mean you only get a depth of field of about 2 inches so your focus is even more important. Depth of field is affected by the focal length of a lens and by your distance from the subject. 


Setting a high shutter speed may let light in for a shorter period but it can also mean that there is less detail recorded. Perhaps for a landscape you could try a longer exposure, by stopping down the lens little?


You may see much different numbers given for stops on your camera and in other tutorials. This is because most now use half stop numbers, and even 1/3 stop numbers. The principle of using stops is exactly the same though.


The actual practising of all of this is fun and within the normal sphere of taking photos but the basic exposure triangle is the most important primer you will need as it covers the fundamental elements of taking photos.

I hope you got something useful from this document.