Basic Photography Series: Exposure Triangle — final part
What is this ‘Exposure Triangle’ thing?
How it all comes together
This is now the last part of the series about that ominous thing called ‘Exposure Triangle’. It helps if you understand Aperture, Shutter Speed and ISO to get the most out of this article, as I will not explain these concepts in detail, but talk about how they all three contribute to the common thing we are looking at here, that is the “proper exposure” of an image.
What means “properly exposed”? That is easy to say, but complex to calculate for a camera (and for humans to understand). As a human you would say an image looks “properly exposed” if it is neither too dark nor too bright. Bright and dark parts are still detailed. I write “properly exposed” in quotation marks as proper exposure for a subject might be highly subjective. For simplicity, we assume “proper” in the sense that it is not over- or underexposed.
But for the camera, not that easy. It cannot make subjective decisions, it needs something to calculate and decide by hard-wired rules. It can measure the incoming light with a light meter, and it can control how long that light will hit the sensor (I will disregard film exposure here) and how big the hole in the lens is to let in more or less light. And it can control how sensitive the sensor reacts to the amount of light hitting it.
So how does the camera know what a correct combination of the three variables (aperture, shutter speed, ISO) would be to make the image properly exposed?
Well, first it uses its inbuilt light meter to measure how much light the subject reflects, considering what your viewfinder is displaying. It then calculates what exposure value (EV) it needs to get the exposure to a 18% grey (neutral grey, the middle between black and white) value. To make this a bit simpler, I will disregard the different metering modes of your camera. That would be a topic for a separate article. So in this simplified model, the camera looks only at the average brightness of the image and tries to get it to a middle grey.
Let’s build a simple model. Say for the camera a light meter value of 0 is black, 10 is white, and 5 is middle grey, dividing the scale into 10 zones, each zone we assume is double as bright as the previous, representing one ‘stop’ of light. I didn’t make this up, this zone system was invented by Ansel Adams and Fred Archer.
For our simple model, we also assume that the camera thinks that our subject, e.g. a wall painted in a middle grey, would be properly exposed with a combination of aperture f/8, shutter speed 1/250s and ISO 100. That’s our baseline or EV 0. If we now point our camera to a different subject (after all a grey wall is not that interesting), the light meter might now proclaim: “This is on average in zone… 4!” (EV -1). That means it is only half as bright as our grey wall (remember, from zone to zone it is one stop of light). If the camera would keep the original set of values (f/8–1/250s — ISO 100), our new subject would be underexposed (too dark) by one stop. To make it properly exposed, the camera could do:
- open the aperture from f/8 to f/5.6 (one stop) or
- slow the shutter speed from 1/250s to 1/125s (one stop) or
- increase ISO from 100 to 200 (one stop)
Each of the above would make the image one stop brighter, compensating for the one stop darker subject. The result should again be a middle grey (EV 0), the goal for the camera.
As this is a model, I would also not deal with a fourth option, adding light (e.g. flashlight), that would make the exposure triangle an exposure rectangle. We leave that for a more advanced discussion.
So the exposure triangle is the concept of juggling aperture, shutter speed, and ISO to get the exposure right. It is an analogy to help the human brain to understand the dependency of these three variables. There are hundreds of combinations to choose from, and the camera exposure system only wants one thing: get it to middle grey. If you don’t want the camera to choose for you in an automatic or program mode, you should gain control and select the camera mode for the variable you want control over. Choose aperture priority mode if you want to control the aperture, and select shutter priority mode for the shutter speed control. Find out how you can select the ISO manually to control ISO. Or go to full manual. With full manual mode you gain full responsibility for the exposure outcome, and you have to monitor a lot of things at the same time.
All three sides of the triangle (aperture-shutter speed-ISO) have different side effects, and that’s where the artistic decisions come into play. Do I choose a combination with a slower shutter speed, but smaller aperture? Or the opposite, a high shutter speed but more open aperture, to get the same exposure, but different side effects, like more or less depth of field?
To be honest, I never really think about the exposure triangle as a concept. I try to think of what is important for the subject and try to keep it as simple as possible. I keep ISO as low as possible to avoid noise. That gets that out of the way, and it reduces the complexity to two variables. Then I choose aperture priority mode most of the time to control depth of field, and I let the camera choose the shutter speed to get a ‘correct’ exposure, but I keep an eye on what it selects. If what the camera thinks is correct does not match what I think should be correct, I use the exposure compensation function to over- or underexpose with intent. If I need to make sure that I have short shutter speeds, I increase ISO to a still acceptable level, but I stay in aperture priority mode most of the time. That will decrease the shutter speed the camera automatically selects. It depends on your type of subjects. You might choose shutter priority mode as your primary choice if you are a sport or bird photographer.
So to throw in something resembling a triangle, you can think of applying the exposure triangle this way:
You have the three components on each side of the triangle. Each of the crossing points inside the triangle are of the same exposure value, and they are one ‘stop’ apart. Moving from one cross point to the next means you keep one variable the same, one increases one stop, the remaining decreases one stop.
Here is one example: We start with an arbitrary spot in the middle (the dark violet dot), with a corresponding aperture of f/8, a shutter speed of 1/60s, and an ISO of 400. If we keep ISO the same (we are moving alongside the blue line of ‘400’, but narrow down the aperture to f/16 (two stops less light come in, we move two red lines up in the graphic), then we can see that the shutter speed now moved to 1/15s. That is two stops more light coming in (from 1/60s over 1/30s to 1/15s), compensating for the two stops light gain of the aperture setting. The result is the same exposure as the original value set:
f/8–1/60s — 400 is the same exposure value as f/16–1/15s — 400
Another example is this:
We move from f/8–1/60s — 400 (the violet dot in the middle) one blue line to the right (we gain one stop of light by increasing the ISO from 400 to 800), and we move one red line down (we gain another stop by opening up the aperture one stop to f/5.6. The million dollar question now is what needs the shutter speed to do to compensate for the two stop gain? Right, we need to compensate with a two stop loss, which means we have to move from 1/60s over 1/125s to 1/250s.
Translated to a real world example: It could be a scenario where I have a moving object and I want that sharp in my image. Or it is windy, and I can’t hold 1/60s steady enough to avoid camera shake. So I look at what the camera suggested (I’m in aperture priority mode with f/8 set as that is the sharpest for my lens, and I had ISO 400 selected because it might be an overcast day). The camera now selected 1/60s to get a proper exposure (middle grey). But 1/60s is too slow for a sharp image in my example scenario, I want it to go to 1/250s (two stops). So what do I do? I open up the aperture one stop (depth of field is a bit shallower now), and I increase ISO to 800 (one stop gain, and still acceptable noise level). Exposure stays the same, but I have now two stops shorter shutter speed, at the expense of a bit more noise and shallower depth of field.
Keep in mind that the same EV does not mean the image would look the same, they would only have the same average brightness value. They could have vastly different depth of field or motion blur or noise levels, because of the different side effects of the three variables.
If you are familiar with aperture, shutter speed and ISO, and all their side effects, then you normally don’t think about the artificial concept of an exposure triangle. It might initially help with learning the dependency of the three variables, but once you have that ingrained, you can forget about the exposure triangle, until a beginner photographer asks you about it.
Hope that helps.
Have fun taking pictures, and practice your shutter speed, aperture and ISO setting skills to master the exposure triangle!