The exposure triangle doesn’t work!

My most popular workshop is the Switch to Manual session. It’s aimed at beginners or those stuck on the automatic settings who want to take more control of their picture-taking. Quite a few times, I am told by attendees that they have read books and watched videos, and it just doesn’t make sense. In many cases, they have told me that they have heard of the exposure triangle, but it doesn’t work when they try to change the settings on their camera. It’s not surprising because I’ve never thought that the exposure triangle works to teach good exposure.

What is the exposure triangle? It’s thought that Bryan Peterson devised the concept of the exposure triangle in his book Understanding Exposure. He identifies (as is true) the three elements which make up exposure. They are the three sides of the triangle: aperture, shutter speed and ISO. Each line on the triangle shows that these three things can be changed between higher and lower values – that is also true. The exposure triangle then summarises the effect of each of these things. Again, it’s all true.

The reason the exposure triangle doesn’t work is because it misses one key aspect of exposure. That’s why I never teach the exposure triangle on my Switch to Manual workshop. Instead, I use a different concept, which I think helps to visualise the exposure triangle in a way my clients have told me makes far more sense.

When we take a photograph on a digital camera, we are trying to balance the amount of light that comes in through the lens and ends up on the sensor. As the exposure triangle states, three things will impact how much light is recorded on the sensor. The way I think about this is that they work like a very basic seesaw. When manually exposing a photograph, we are trying (in most cases) to balance the seesaw.

Just like a seesaw, we have two weights on either side. I will call one of these weights aperture and the other shutter speed. We can adjust the weight (or amount of light) by changing the settings to either add or take away light to try and get the seesaw to balance. However, sometimes, because of what we are doing creatively, it isn’t possible to balance the seesaw by adjusting these two weights on either side. So, the other thing we can do is move the point where the seesaw pivots, which will also allow us to balance the seesaw. Let’s call the pivot point the ISO.

So, let’s examine each of the three parts of the seesaw to see how they work individually. We’ll start with the aperture.

The aperture is the size of the hole in the lens that allows light in. When the hole is big, we call this a large or wide aperture. Make the hole smaller, and it becomes a small aperture. Photographers describe the hole size using something called an ‘f’ number. When I was at school, I hated physics – in fact, I told my physics teacher on my last day that it had been a waste of time learning physics and I would never use it again. How wrong I was. It’s physics that gives the f-number naming to apertures. However, I still think physics people are a bit odd (sorry if you are one of them), but to prove it, when the hole is big, the f-number is small, and as we make the hole smaller, the f-number gets bigger! For the physics boffins, this is the equation that makes it happen, and if you want to know more, head to Wikipedia and type f-number in the search bar. Those 4760 words will clear everything up for you. For the rest of us non-physics people, you don’t need to know why it happens; remember, big hole, small number. Small hole, big number.

The thing that is worth understanding is the numbers the camera displays as you toggle through the aperture values. As you can see, they aren’t changing in what seems to be a logical order. There are decimals, whole numbers and the gap between each numbers feels quite irregular (you can blame physics for that). You can also see that I have coloured every third number in green. Some lenses may even have these green numbers printed on the side of the lens. Photographers refer to the gap between these numbers as a ‘Stop’. So, what does a stop actually mean?

If I start with an aperture of f/5.6 and then change it to f/8, the hole size will shrink as the number increases. However, the hole at f/8 is half the size of the hole at f/5.6. If we go three numbers down the way from f/5.6 to f/4, the hole size will double, and we will have twice as much light coming in through the aperture.

It isn’t just the green numbers that cause the light to half and double. It’s the gap of three numbers. If I start at f/5 and move to f/7.1, the gap is also three numbers, and the light will half if I move to the right or double if I move to the left.

So, fairly obviously, if I make the hole bigger, we are going to get more light coming in. Make the hole smaller, and there will be less light. Every one of the three things that change the amount of light being recorded on the sensor affects the picture, and the effect of changing the aperture changes the depth of field or DOF.

Let’s use this setup to show how the depth of field changes when we change the aperture. These two photographs show two different depth of fields. They both have the same amount of light hitting the sensor because the table looks the same, and the glass at the front looks the same, but they are two very different pictures. The one on the left is what we call a shallow or small depth of field. The definition of depth of field is the area around the focus point that is acceptably sharp, and in this shot, the focus point is at the front lip of the front glass. The acceptably sharp area is just in front of the glass and just behind it. The rest of the image then blurs away. We might see this kind of DOF shooting portraits or wildlife.

Now, let’s look at the second picture. I am still focusing on the same point, but this time, the acceptably sharp area has moved toward the camera and away from the focus point. So, we now bring the glass at the back of the image into the acceptably sharp area. It’s worth knowing that the sharp area usually extends from about one-third in front of where we are focusing to two-thirds behind our focus point.

We achieve these two effects by changing the aperture. To get a small depth of field, we are going to use a small f-number aperture. Therefore, a big depth of field uses a big f-number aperture. Remember, this is the number describing the size of the hole and not the actual hole itself. So, a big f-number equals a big depth of field; a small f-number means a small depth of field.

Now, let’s think about light. In the picture on the left, we have a big hole letting lots of light in. We have had to change the aperture to make the hole smaller to get the second shot. If we took the shot after adjusting the aperture, the second photo would almost be pitch black. So, we have to change something else to re-balance the seesaw. That something else in these photos is the shutter speed. When the hole was big, I used a shutter speed of 1/10^th of a second. After making the hole smaller, I increased the shutter speed length to six seconds, and the seesaw balanced again.

So now we come to shutter speed. This one is a lot easier to understand as it is the length of time that the shutter stays open. This is how a shutter works.

We are now in the body of the camera and we have the sensor. In front of the sensor are two things, a front curtain and a rear curtain. When you press the shutter button, the front curtain drops down and starts exposing the sensor to whatever light you have allowed to come in through the aperture. Then, as long as you have set the shutter button, the rear curtain drops down and prevents the sensor from being exposed to any more light.

Shutter speeds are expressed as either fractions of a second or full seconds. The shutter speed is faster when the number under the one is bigger. So, for example, 1/4000^th of a second is faster than 1/400^th. Shutter speeds then go slower and slower until they become full seconds, and usually finish around 30 or 60 seconds. As we have seen, the faster the shutter speed is, the less light comes in, and as we slow the shutter speed down, it increases the amount of light being recorded. Just like there was an effect on the picture by changing the aperture, there is also an effect of changing the shutter speed, which is motion.

Let’s go back to the glasses. We have two shots again with the same amount of light hitting the sensor. This time, I have decided that I want a small depth of field in the image. You’ll remember that a small depth of field needs a small f-number. I have set it at f/3.5.

The picture on the left is a slow-ish shutter speed. 0.4 seconds, the picture on the right is a faster shutter speed of 1/125^th of a second. Because the right-hand shot is faster, it is freezing motion. You can see all the bubbles in the glass. There are waves on the top of the glass where the water is being poured into the glass. The picture on the left shows the shutter speed blurring motion. These lines in the glass are the movement of bubbles over nearly half a second. The waves at the top of the glass look almost flat. They were moving exactly the same as the shot on the right, but they have been blurred to where gravity pushes the water straight down.

Now, let’s think about the light again. The shot on the left has a balanced seesaw. To take the shot on the right, we need to make the shutter speed faster, which will reduce the amount of light. If we took the shot at that point, it would again look almost dark. Last time, we changed the other weight on the seesaw. However, this time, we have decided that we are going to keep the depth of field the same, so we can’t change that to balance the seesaw. Instead, we are going to move the pivot point, the ISO. When there was lots of light coming in through the shutter speed, I could use an ISO of 100. When I reduced the light through the shutter speed, I increased the ISO value to 1250, and the seesaw balanced again.

Before we look at ISO, let’s look at the values we see when changing the shutter speed. My fastest shutter speed is 1/8000th. If I change it by three values, it becomes 1/4000th. Three more, and it becomes 1/2000th. Every third number either halves or doubles the amount of light coming in, again a stop of light.

This brings us to the final element, ISO. The first thing to get over is the name ISO, which has nothing to do with photography. ISO stands for International Standards Organisation and is the international organisation that defines how stuff is measured. If you remember buying film in the old days, it would have had an ASA rating. ASA stood for American Standards Association, the American precursor to ISO, so the name has never had anything to do with photography!

So, let’s see how ISO actually works. For all the keyboard warriors, I will caution that this is a very simplified version to try to make the explanation easier to understand for beginners.

Our sensors are made up of lots of tiny individual sensors. We call these individual sensors pixels. At a very high level, the pixel’s job is to record light hitting it, and light is either red, green or blue. Once the pixel has recorded the light, it sends a message to the processor in the sensor to say how much red, green and blue has been recorded, and it does this through a metal wire by passing an electric current through it. This process goes on every fraction of a second, backwards and forwards.

Our cameras have between 15 megapixels and 60 megapixels, depending on the make and model. So, in our black boxes that we take pictures with, there are between 15 million and 60 million bits of wire passing current backwards and forwards every fraction of a second. When we put wires close together and pass current through them, it causes interference, which I’ve displayed here in orange.

Let’s imagine that in the first photo I took with the Aperture of f/3.5, shutter speed of 0.4 seconds, and ISO 100, one pixel has recorded a value of 85 for red light, 50 for green light and 70 for blue light. We also have a small amount of interference shown here in orange. I am now going to take the second photo, and I will first change the shutter speed. That will reduce the amount of light, but it will reduce the amount of colour in the same proportion. The shape will stay the same, but the amount will be reduced. The red becomes 25, green becomes 15 and blue 20, and we still have the small amount of orange caused by the wires being close together. When we add ISO, we don’t allow any more light to come in through the lens. Instead, a process in electronics called gain will be applied. Imagine it like a volume button for the sensor. So, if we add the same amount of ISO back in, as we took out through, in this case, the shutter speed, the red will go back to 85, the green back to 50 and the blue back to 70. However, as the volume button affects everything equally, the interference will also get amplified. This is what causes the effect of ISO, which is noise.

When the ISO value is small, there is less noise in the image, and as we increase the ISO, we also add noise. So, the obvious next question is, ‘What is noise?’.

Here are two photos, again, with the same amount of light being recorded on the sensor. I have used a nice middle-of-the-road aperture, f/11. The shot on the left has a 100 ISO, so it should have less noise. The one on the right has an ISO of 25600 and should have much more noise. I have balanced the exposure by changing the shutter speed. Seeing the noise in these two pictures might be difficult because I am using a relatively modern camera. However, if I show you a small crop of the two images, you can see the noise, particularly in the darker areas.

The ISO values on my camera change like this: I’ll start with an ISO of 100. If I use the dial to change it by three values, it becomes 200. Three more, and it is 400. Once again, every third number increases or decreases the amount of light added to the sensor by a stop of light. Some cameras, usually entry-level, only have ISO values of 100, 200, 400, and so on.

So, we now know the three things that affect the light recorded in the sensor. What we don’t yet know, and what the exposure triangle doesn’t tell us, is when the seesaw is balanced. To do that, we need to use a light meter.

We used to use a light meter like this, which would take a reading to tell you the aperture to use based on a certain shutter speed and film speed. We generally don’t need to use these any more, as they are built into the camera. Light meters all work in a similar way.

We have a grid with some positive numbers, some negative numbers and zero in the middle. There is an indicator that shows that the seesaw is balanced if it is at zero. If the indicator is on the positive side, the camera tells us that too much light is being recorded on the sensor, and we need to reduce it. We now know the three ways to do this. We can make the aperture smaller by making the f-number bigger; we can make the shutter speed faster or reduce the ISO value.

You can see that the light meter has two small indicators, then one big one. Each of these indicators represents one click on your camera settings, and three clicks are one stop. So, in this example the light meter is showing it is one stop over exposed. That means you could change the aperture by three clicks, the shutter speed by three clicks or the ISO by three clicks and it will move to zero.

If the indicator is negative, the camera is saying that there isn’t enough light coming in. We can fix this by making the aperture bigger by using a smaller f-number, slowing the shutter speed down, or increasing the ISO.

By using the seesaw, this more visual representation of the exposure triangle makes things easier to understand.  The aperture changes the depth of field. The smaller the f-number, the smaller the depth of field. A bigger f-number gives a bigger depth of field, but it’s the reverse in terms of light. A small number is more light, and a big number is less light. Shutter speed is all about motion. Faster shutter speed freezes motion and reduces light. Slow shutter speeds blur motion and increase light. If we can’t get these two things to balance with what we are trying to do creatively, we can increase the ISO, but this will also add noise.

On my Switch to Manual workshops here in Edinburgh, I discuss exposure and some other principles, and I show you the six repeatable steps that help you get great photos every time. If you are interested in attending, you can book by using this QR code.

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