The triangle of exposure

The exhibition is an important technical parameter in the success (or not) of a photo. It “designates the action of light radiation on the sensor (or on the film)”. Which does not help us much  This article will take stock of what exposure is, the effect it has on your photos, and the parameters that influence it and that you can control.

Exposure, overexposed or underexposed photo: definition

So we said that exposure was the action of light radiation. In other words, the more action is large , more exposure is important: the more light will be captured. Imagine a room lit by a 20 watt or 100 watt bulb: the amount of light perceived is not the same.

An overexposed photo is a photo where too much light has been captured: it is “ too bright ”. Conversely, an underexposed photo has captured only a little light: it is “too dark”.

Be careful, I did say “normal” and not “correct”. Indeed, you can voluntarily decide to underexpose or overexpose a photo, slightly or even strongly, in order to create an effect: a dark atmosphere will logically ask to underexpose your image for example. We will see an example at the end of the article, but for the moment, let's stay on the simplest case.

So how does the camera get normal exposure on its own?

As you will have noticed, in automatic mode (don't hide, we all used this mode!), The photos are normally exposed: the camera manages on its own to make an image that is neither too bright nor too dark, it is the principle. But how does he do it?

Without going into too many technical details, the device has sensors to measure the brightness of the image. It even has several different modes: it can either be based only on the center of the image, or on the entire image, or an intermediary between the two.

Depending on these measurements, the device decides to let in more or less the light, or even to trigger the built-in flash (boo, not good, as we said in the basic advice !).

How to modify the exposure?

Exposure is influenced by 3 parameters:

The ISO sensitivity  : the sensitivity sensor (or film) in the light
The opening of the diaphragm  : this is the opening diameter of the diaphragm when triggered
The shutter speed  : this is the time the diaphragm opens when the shutter is released
It all seems obscure, I know, but we're getting there, don't worry. First of all, let's think for ourselves a bit about what happens when we increase or decrease these parameters:

When you increase the sensitivity, logically there is more light captured. A bit like comparing Jack, a seasoned legionary and Jennifer, a sensitive and tortured 15-year-old girl, both watching a love movie: the more sensitive the person, the more they will cry. No need to tell you who is the more sensitive of the two! (I grant you, the metaphor is quite improbable: D)
When you increase the aperture, more light can enter: more light can pass through a window than through a keyhole. And vice versa.
When we increase the opening time, more light can enter (so we decrease the shutter speed , because the slower the speed, the longer the time). Again, more light enters a room if you open the shutters for 1 minute rather than 10 seconds. And vice versa.
To sum up, the increase of the sensitivity of the opening or the time made back more light and therefore increases the exposure. And conversely, you will have understood it.

The exhibition triangle

Where it gets slightly more complex is that these 3 elements of the exhibition are interconnected  : you can never really isolate them from each other without changing something in your image.

If you change one but you want to keep the same exposure (the same brightness of the photo), the others are affected. This is why they can be grouped together under the notion of the exhibition triangle . And since a diagram is always better than a long speech

Understanding the exhibition triangle in concrete terms

Well, that's all well and good, but I don't get it!

They are far from perfect but they help a lot to understand that there are different ways to modify the exposure by playing on 1, 2 or all 3 parameters at the same time.

The sensor of your camera exposed to light is like your skin exposed to the sun
ISO sensitivity is like your skin type: if you are blond or even red-haired, you have a high sensitivity. In other words, you will easily get a sunburn if you are exposed to the sun. This is the equivalent of a high ISO sensitivity.

Conversely, if you are brown, or even Mediterranean or even black skinned, it will take you longer to sunburn or tan, or not tan at all 😀 This is the equivalent of a low ISO. Shutter speed is like how long you stay in the sun. The higher the speed is going to be, the less time you are going to be exposed.

Whether you have brown hair or blonde hair, you are likely to get sunburned if you stay in the sun for 4 hours: this is equivalent to a slow shutter speed and therefore a longer exposure time.

Conversely, even if you have red hair, if you stay 15 seconds in the sun, you are unlikely to get sunburned! (Although :P) This is the equivalent of a higher shutter speed (and therefore a shorter exposure time).

The aperture is a bit like cloud cover: if there are a lot of clouds, less light gets through, so you're less likely to get sunburned (think of your last holiday in Brittany! :P). This is the equivalent of a small aperture.

On the other hand, if the sky is blue like Isabelle's eyes, a lot of light comes through and you are more likely to get sunburned (think of your last holiday in Corsica). It's the equivalent of a wide opening.

Personally, this is the metaphor that speaks to me best and that has allowed me to understand this triangle well. If you still have a bit of trouble, and it's quite normal, here's another one:

The window metaphor

Imagine that your camera is like a window with shutters, and that you are behind this window with sunglasses (yes it's a bit strange, but let's admit it :P).

The aperture is like the size of the window: the bigger it is, the more light comes in (the brighter the room).

The exposure time is like the time you open the shutters: the longer you open them, the more light comes in.

The ISO sensitivity is a bit like the opacity of your sunglasses: if they are more opaque you will be less sensitive to light, i.e. you will perceive less light (equivalent to a low ISO). Conversely, if they are clear, you will be more sensitive to light, i.e. you will perceive more light (equivalent to a high ISO).