The Exposure Triangle

In days past, when I got a new camera, the first thing I used to do was have a quick flick through the user guide, familiarise myself with the everyday functions then switch it on, put it into auto mode and that was pretty much where it stayed. By keeping the camera in auto mode I let the camera decide on the difficult elements to taking my pictures. All I wanted to do was point my camera at something, zoom in or out so it looked good, focus then take the picture. This is called “point and shoot” and it works fine, to a point. The thing to remember is that as clever as modern cameras are (and they are very clever!) they often don’t make the best choice. I would look at my pictures afterwards and kick myself because if I was being really, really honest, there were an awful lot which had something wrong and which I could have done something about if only I got to know how my camera worked a bit better. That’s when I bit the bullet, put my camera into manual mode and made a decision that I would learn how to do things better. I won’t pretend it’s not difficult. It is. And frustrating. And just when you think you understand something, there’s another complication just around the corner. That said, if you’ve got this far in reading this article then I guess you must have more than a passing interest in improving your photography and I think it’s worth my while explaining a couple of the other technicalities to help you get the best out of your camera. I’ll keep things as simple as I possibly can.

In the last article I went through focal length and zoom. These dictate how close you can get to your subject. Once you’ve done that, there are three other things to consider before you take your picture. This is called “The Exposure Triangle” and it involves the Shutter Speed;  the Aperture; and the ISO.

The Exposure Triangle

In auto mode, your camera decides on these 3 settings and you don’t have to worry about them. All of these elements determine how dark or bright your image will be. They can also affect the style of your photograph and for that reason it’s very important to know what’s going on so that you can decide what style of picture you want. Let’s take these in turn.

Shutter Speed

After you’ve focused your camera, you press the button to take your picture and two things will happen in very quick succession: firstly, the reflex mirror will swing up, then the camera shutter opens then closes, exposing the camera’s sensor to the light. This animation shows the whole process in slow motion:

You can tell the camera how quickly to open and close the shutter and this is called shutter speed. If you're taking a picture of a bird flying you probably want to use a very fast shutter speed to make sure the final image isn’t blurry from either your movement or the bird's motion. In that example you would want to use a shutter speed of about 1/2000th of a second. That means the curtains of the shutter will open and close in 1/2000th of a second. So fast in fact, that it doesn’t let much light into the camera and for that reason you will need to adjust both of the other settings in the exposure triangle to make sure your image is nice and bright. If you're taking a picture of a bird sitting on a branch, you will normally be able to use a much slower shutter speed, say 1/250th of a second, maybe 1/160th of a second. This leaves the shutter open longer and lets more light into the camera so again you will need to adjust either the ISO or the Aperture to compensate to make sure the image isn’t too bright. In landscape photography, you may have pictures with a shutter speed of several seconds and so, once again, you must adjust the other settings to compensate.

Aperture

The aperture in your camera lens is formed from blades of plastic. As you adjust the aperture, these blades pull apart or slide back together again. When they pull apart they create a hole and this hole is called the aperture. The light shines through this hole as long as the shutter is open and shines onto the sensor. Adjusting the aperture means that you are adjusting how big this hole will be. The bigger the hole, the more light gets into the camera. The smaller the hole the less light gets into the camera.

Examples of Aperture. Left to Right: f/22, f/11, f/4

Aperture is measured in f-stops and confusingly a smaller f-stop number, like f/4, means a larger aperture whereas something like f/22 is a smaller aperture.

Adjusting the aperture also has another, very profound effect and that is depth of field. Depth of field is the amount of your picture that is in focus and this also determines how out-of-focus your background is. Now I am not going to get into the physics of how it works and I don’t intend to get really complicated here but in very simple terms: if you have a very small aperture, less light will get into the camera and you will have a very large depth of field – that means almost everything will be in focus, from objects near the camera to objects in the distance. This is very useful for landscape pictures where you may want everything to be in focus. If you have a very large aperture, more light will get into the camera and you will have a very shallow depth of field – that means that the objects in the background will be blurry and out of focus. This is very useful for taking nice portraits and for nature photography where you don’t want the background to distract from the main object of the photograph. Just remember, the bigger the f number; the smaller the hole; the sharper the background. The following examples demonstrate this effect in practise:

Taken at f/22

Taken at f/16

Taken at f/11

Taken at f/8

Taken at f/4

ISO

And finally there is the ISO. This is an old term that comes from the days of film photography and was used to describe how sensitive a film was to light. A film with a low ISO (e.g. 50) was not very sensitive to light and you would have to have a longer shutter speed or larger aperture (or both) to compensate. A film with a high ISO was much more sensitive to light and so could be used with a faster shutter and smaller aperture. Of course, digital cameras don’t use film, it's the sensor on the camera which gathers the light and camera manufacturers will have you believe that ISO in a digital camera adjusts the sensitivity of the sensor to the light in the same way. This is not true.

In a digital camera ISO is actually signal gain. It is applied after the image has been captured to amplify the signal, so technically it has nothing to do with exposure at all but we treat it as though it does because the effect is broadly similar. It does not add more light to your image. Only aperture and shutter speed can do that.

What you need to remember is that the higher the ISO value, the more the image, and the existing noise in that image, are amplified. Noise is something you want to minimise so a good rule of thumb is to use the lowest ISO you can get away with. I tend to keep my ISO at 100 (the lowest my camera allows) and once I’ve got the shutter speed and aperture I want, if the image is still too dark, only then do I increase the ISO. Of course, one thing I’ve learned about photography is that every rule is meant to be broken and just because this is what I do, it doesn’t make it right for you. This is exactly the sort of thing you should experiment with to see what you like best. Having some noise in your pictures can give a grainy, moody look and this might be exactly what you want to achieve. The following images (cropped to 100%) show how image quality is affected by ISO, please bear in mind this is how my camera deals with ISO, yours will be different.:

Taken at ISO 100

Taken at ISO 200

Taken at ISO 400

Taken at ISO 800

Taken at ISO 1600

Taken at ISO 3200

Taken at ISO 6400

Taken at ISO 12800

Test images like these tell me that my Camera is fine at ISO 100 to 800 in good light. I can use 1600 at a push but anything above that is a mess with too much lost detail. I would recommend you do similar tests.

The point is that depending on the circumstances, and the type of picture you want to achieve, knowing how the exposure triangle affects your pictures can make the difference between taking a good picture and taking a great one. Perhaps, the next time you get your camera out you should stop for a moment and think about the image that you want to achieve? Do you want a blurry background? If you do then having a wide aperture is going to dictate what you do with the shutter speed and ISO. Am I taking a picture of something moving quickly? In that case, you have to use a fast shutter speed which will mean you have to adjust the aperture and ISO accordingly. Do you want a sharp, clean landscape? That means using a very small aperture and a very low ISO which means you’ll need a very long shutter speed – perhaps half a second or more in which case you’ll need a tripod to hold your camera steady. You can do all of this by using the Manual mode of your camera. All DSLRs will allow you to use manual mode, a lot of bridge cameras will too but some won’t so this is something to watch for if you’re interested in experimenting.

Also, and this is really important, an awful lot depends on the available light. On a nice bright sunny day there is going to be a lot more light around than on a dark cloudy winters day (here in Scotland it’s sometimes hard to tell the difference!) but this will have a noticeable effect on your exposure triangle settings.

Most digital camera can make this a bit easier for you by having settings called “Aperture Priority” and “Shutter Priority” (please be aware that different manufacturers call these settings different things but your camera manual will help you here) and selecting either of these will allow you to choose either the Aperture value or the shutter speed then the camera will decide everything else – it’s a half-way house to going fully manual and if you’re not comfortable with manual mode perhaps you could experiment with these? Nothing beats practise and experimentation to improve.

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Cameras, Zoom and Focal Length

Here I want to talk about the different types of camera you can buy and why this matters when it comes to getting close to your subject. This is quite an involved article, so do give it some time and thought and please beware, the internet is awash with well meant but bogus information on this topic and it's taken me a long time to suss it all out for myself. I hope you find this helpful.

Before taking the leap to buying a DSLR or Mirrorless camera (we call these Interchangeable Lens Cameras, or ILCs for short), many people consider using Bridge Cameras, so let’s talk about them first. A Bridge camera is called a bridge camera because it bridges the gap between a compact digital camera (or mobile phone) and an ILC. Typically a bridge camera will have a large zoom (which I’ll talk about soon) but it will still be quite light. Bridge cameras are typically less expensive than ILCs and you can’t change the lens. Another thing to remember about a Bridge Camera is that it has a very small image sensor. Now, once again don’t worry if you don’t know what that means as I’ll explain that later. At this stage you just need to know that it’s the sensor size which is crucially important to how bridge cameras work. Bridge cameras are reasonably easy to use and are often all about their zoom range. You will hear people saying that “this camera has a 20 times zoom” or “50 times zoom”. I have a bridge camera which has an 83 times zoom but what does that actually mean? Well, once again, I will explain what it means later but for now all you need to know is that the bigger the zoom number, the closer the object you’re photographing will appear. Does it mean it will be 83 times closer? Well, sort of, but bear with me and all will become clear.

ILCs are what the serious hobbyists and professional photographers tend to use. They are expensive when compared with Bridge cameras and you have to buy lenses separately so that’s an additional expense. They are also much heavier than Bridge Cameras and when you add on the weight of bigger lenses with more zoom they just get heavier. However, the sensor inside an ILC is much bigger than inside a bridge camera. This means two things, firstly even with a huge lens a ILC will seem to struggle to get as close to your subject as a bridge camera, but the image quality you get will be much, much better.

When it comes to ILCs, Mirrorless cameras can be slightly smaller than DSLRs because, crucially, they do not contain the reflex mirror (hence the name) and they use different viewfinders. It is absolutely not the purpose of this article to explain the technical differences but mirrorless is newer technology and these cameras tend to be (but not always) smaller and lighter than DSLRs - that said, large lenses will always be heavy so the benefits of a scaled down Mirrorless camera may well be lost.

In ILCs there are also different sizes of sensor. Common sizes are "Medium Format", “Full Frame” and “Crop Sensor”. Cameras with Crop Sensors tend to be cheaper than Full Frame or Medium Format cameras and, as the name implies, have a slightly “cropped” or smaller sensor and this keeps the price down, consequently, these are the most popular ILCs for enthusiasts. Generally speaking, the larger the sensor the more expensive the camera. You cannot assume that the “bells and whistles” you get with your regular compact camera – like a swivelling touch screen for example will be present on the DSLR or Mirrorless you are interested in either but as time goes on and manufacturers have to compete with each other these features are slowly creeping into the DSLR market.

To be blunt: DSLRs and Mirrorless are for maximum image quality and Bridge Cameras are for convenience and economy.

Now let’s talk about zoom. Firstly let’s get Digital Zoom out of the way. All this means is that the software in the camera will enlarge part of the picture for you. It basically cuts out (or “crops”) part of the picture to give the impression of having zoomed in when all it’s really done is enlarge part of existing picture. Almost always this is results in very poor image quality (see the example in the last article here) and whilst it’s a handy feature for identifying a far off bird for example, it is usually best avoided for pictures you want to print.

Moving on to Optical Zoom. As I said above, Bridge Cameras tend to advertise themselves as having “86 times zoom” for example. In the world of ILCs they talk in terms of “focal length” which is measured in millimetres. Now these are just two different ways of describing the same thing but this is where things get a little bit complicated. I’ll try to explain it as simply as I can.

When the light shines into any digital camera it passes through the lenses which focus that light onto the sensor. Think of it like a projector: the lens is the projector and the sensor is like the screen. Let’s look at a very simple illustration:

Simplified illustration of light passing through camera lens

This is what happens when we take a picture of a tree. The light from the tree (red, green and blue lines) passes through the lens and projects the image (back to front and upside down) onto the sensor or film inside the camera (the electronics in digital cameras make sure you always see the image the correct way).

Now, in the picture above you can also see several rectangles all of which are representative of different film and sensor sizes found in different cameras and at their approximate scale. Let's take a much closer look at these relative sizes:

Large Format: Any film or sensor larger than 5 inches by 4 inches
Medium Format: Any film or sensor larger than Full Frame but smaller than Large Format.
Full Frame: The same size as 35mm film.
APS-C: Also known as a "crop" sensor
M4/3: Full name is Micro Four Thirds (MFT) and is four times smaller than Full Frame.
B: This is my abbreviation for the sensor found in bridge cameras and mobile phones

Thinking back to the picture of our tree, above. All things being equal, the large sensor can see the whole scene, whereas the smaller sensors can only see part of the scene. So even when everything else is identical, just having a smaller sensor in the camera gives the appearance of having zoomed into the tree - but as you can see, that's not what has happened, you are merely seeing a cropped version of the whole scene. So, depending on the camera you are using, and the size of sensor inside it, the image you can capture of exactly the same scene will vary. In technical jargon, this means that your Field of View (FoV), can vary depending on sensor size.

Now by moving the lens backwards or forwards I can make the image projected onto the sensor even bigger or much smaller. And that’s what optical zoom is. In technical jargon, that's the Angle of View (AoV). By Zooming in, I narrow the Angle of View and I see less of the whole scene, by zooming out I widen the Angle of View and see more of the whole scene. Different lenses will have a different Angle of View, and obviously the Angle of View of the lens will affect the Field of View of the camera: if a lens can see more, then so can the camera sensor. The following diagram illustrates the changing Angle of View with a zoom lens:

So, we can say that the Angle of View is determined solely by the lens you are using and this affects what your camera can see, but the Field of View is determined by both the lens you use (the Angle of View) and the sensor size in your camera.

In Compact cameras, Bridge cameras and Mobile Phones, magnification of the image is talked about in terms of "zoom factor". For example 3x zoom or 20x zoom or even 83x zoom. I posed the question above, does 83 times zoom on a bridge camera actually bring the image 83 times closer? Well that's what we're going to answer now.

Every lens has something called a Focal Length. Simply put, this is the distance, in millimetres, from the point in the lens where all the light from outside comes together (the convergence point) to the surface of the camera sensor where all the light is focused. In a lens with no zoom the focal length is fixed; it cannot move. However, in a lens that can zoom the focal length is variable; it can move. This means that the point of convergence will move back and forward inside of the lens, which has the effect of both lengthening or shortening the focal length and narrowing or widening the Angle of View. This simplified animation shows the process in principle:

A long focal length (say 600mm) gives a very narrow Angle of View (approx 3˚), and thus a lot of magnification. A very short focal length (say 10mm) gives a very wide Angle of View (approx 97˚) and thus very little magnification.

Important note. The focal length of a lens will always be the same: if a 50mm lens is on a crop sensor camera, then the focal length of that lens is still 50mm. If it's on a full frame camera, its focal length remains 50mm. All that changes is the Field of View: the crop sensor camera has a smaller sensor which equals a smaller Field of View. This introduces a concept called Full Frame Equivalency.

Full Frame cameras are basically the standard photographic format. If we deviate from that standard, by having a smaller sensor, it is helpful to create an equivalence so we can imagine how our images will be compared to the standard. To do this we need a "crop factor". The calculation for the crop factor is:

Full-Frame Sensor Diagonal (mm) ÷ Crop Sensor Diagonal (mm) = Crop Factor

For this explanation let's say this calculation comes to 1.6 for our crop sensor camera.

If I put a 50mm lens on my crop sensor camera, then the Full Frame Equivalency calculation is:

50mm x 1.6 = 80mm

Which means the Field of View of my image will look as though it has been taken with an 80mm lens on a full frame camera. Remember, the 50mm focal length of the lens has not changed. All we are doing is working out how the image would look if we pretended we had taken it with a full frame camera. This can work in reverse too. If I like a picture taken with a full frame camera at 16mm then with my crop sensor camera to get the same Field of View I need to use a 10mm lens:

16mm ÷ 1.6 = 10mm

So, with that understood we can finally give an answer to our question and that is: Yes, a bridge camera does magnify an image 83 times, but...

And the "but" here is that it only magnifies 83 times from the shortest focal length available on that camera. So, if you zoom out all the way with your bridge camera, this will allow the camera to see as much as it can - it's largest Field of View. Now if you actually look at the picture you are going to take like this and then look at the same scene with your own eyes, everything is much bigger with your eyes. This is because the smallest focal length on bridge cameras is somewhere around a full frame equivalent of 24mm whereas the focal length of your eyes is about 50mm. The larger the focal length, the larger the image seems. So, what 83 times zoom means is that an image will appear 83 times larger than how it looks at an equivalent focal length of 24mm, not 83 times larger than how you see it. It's really just a big number to draw peoples attention and means very little. Using focal length and millimetres to describe zoom is actually pretty sensible, as it's connected to a standard which is easily understood.

What’s the point in telling you all of this? Well, the point is to know roughly what a focal length is, how it relates to magnification (or optical zoom), how that’s affected by the particular lens and sensor size in the camera and how that will affect your photography. If you want to get close to a subject that is far away (or very small) you are going to need to have a large focal length. Even though bridge cameras are advertised as having say “83 times zoom” that same camera will also have the equivalent focal length information buried somewhere. In the case of that particular bridge camera, the lens only has a real focal length of 357mm but, because the sensor is so small, that gives it a Full Frame Equivalency of 2000mm. So, to get an image to appear as close with a full frame DSLR camera you would have to buy a 2000mm telephoto lens which would be, frankly, unfeasible for most people due to the breathtaking price, outlandish size and staggering weight of such a lens.

Given that, why would anyone buy a DSLR or Mirrorless camera? A couple of reasons. Those small sensors in bridge cameras (Marked "B" in the picture above) make distant things seem really close, but at the expense of image quality which can seem very artificial when looking closely (called "pixel peeping"). Because the lenses are of a small size, they don’t let in as much light as large telephoto lenses and less light means poorer image quality. These lenses are also relatively inexpensive and cheaper lenses usually means you will start to see chromatic aberration - don’t worry about that just now though, I have a separate article dedicated to Chromatic Aberration. More expensive ILCs also come with other features that cheaper cameras don’t, like the ability to take 20 pictures per second for example – when a bird is jumping around quickly, think how useful that would be to capture the perfect moment. The picture quality in DSLRs and Mirrorless is consequently much better and can also allow you to crop the final image to give a better final result.

Now, of course, everything I mentioned above about image quality is relative and depends very much on what you are going to be doing with your image. If you are printing large posters or selling to magazines then image quality is vitally important. If you are sharing amongst friends on social media and for your own use then image quality doesn’t matter quite so much – noisy images and pixelation (which I will speak about in another article) don’t matter all that much when you are looking at small versions of your image, either on the internet or printed out at standard sizes. Many people like to carry a bridge camera instead of carrying binoculars – they can carry a light and reasonably inexpensive camera to record what they see and or to identify something later if it’s very far away. As I said above, both are very good at what they do, and both have their place if you understand the limitations.

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