Exposure in photography: how does it work?

Since I started this travel and photography blog, I have always wanted to write about this subject and help people learn photography, because it is definitely not something you understand instinctively. Some people prefer taking photography lessons or formal training; here on the blog, I will try to explain a few key concepts so you can learn photography online. Some time ago, I wrote a first article on the basics of photography, about the definition of focal length in photography. With hindsight, I realize I should probably have started with the real foundation: exposure in photography.

In my opinion, along with the more complex idea of composition and framing, this is the most important concept to understand if you want to make progress in photography. Exposure alone has a major influence on the final result and on the look you want to give your image. It is a crucial point. In this article, which I hope will not be too technical, I will explain what exposure is, how it works, which parameters influence it, and how you can control the exposure of your photos.

In the meantime, three more detailed articles will follow to clarify the three concepts that influence the exposure of an image: aperture, ISO sensitivity and shutter speed. I will tell you a little more about them later in the article.

Please note that the standard unit used to measure exposure value in photography is EV, for “exposure value”. You will also often hear photographers talk about “stops”.

Simply put, exposure is the amount of light received by the sensor of your digital camera when you take a picture. In the past, it was the film that received this light. To understand the concept simply: the more light your sensor receives, the brighter the image will be. Conversely, the less light it receives, the darker the photo will be.

• We speak of underexposure when the sensor has not received enough light. The result is a photo that is too dark, with a loss of detail in the shadows or black areas. You will often hear these referred to as “shadow areas”.
• Conversely, an overexposed photo means that the camera sensor has received too much light. You then lose detail in the bright or white areas of the image. Photographers often call these “blown highlights” or “clipped highlights”.

Here is an example, taken from my terrace: on the left, the photo is underexposed; in the middle, it is correctly exposed; and on the right, it is overexposed.

In any case, it is important to remember that exposure is not about judging whether a photo is “good” or not. It is only about its exposure, or, in a way, how faithfully it represents the light of the scene in front of you. You can deliberately overexpose or underexpose a photo to emphasize a particular mood or atmosphere.

Low key and high key techniques are good examples of this. They deliberately push a scene towards underexposure or overexposure to bring out a subject or create a specific atmosphere. These are the essential ideas to keep in mind about exposure in photography.

I hope you have understood that exposing a photo correctly is not an absolute truth. It depends more on your choice and on the way you want to represent a scene. As photographers, we can control the exposure of an image ourselves by adjusting the three main parameters that make up the famous exposure triangle.

Definitions:

• ISO sensitivity: this is, in simple terms, the sensitivity setting of your camera. At the same shutter speed and aperture, the more you increase ISO, the brighter your photo will be.
• Aperture: this is another very important parameter to consider when exposing a photo. It is also the one that allows you to blur part of a scene or, on the contrary, keep everything sharp. This is the notion of depth of field. Technically, aperture refers to the opening of the diaphragm inside your lens when you take the shot. It is expressed as “f/”, for example f/2.8 or f/11. For now, just remember this: a large aperture means a small f-number.

On the left, the background is blurred (f/2.8 = large aperture), while on the right, the background is in focus (f/11 = small aperture).

• The last element of the triangle is shutter speed: this is the length of time the shutter stays open when you take a picture, for example 1/200.

The basic idea is not complicated. What is more difficult to grasp is that these three exposure parameters are inseparable and constantly interact with each other. It is impossible to change one of them without affecting at least one of the other two.

To properly understand exposure in photography, we need to see why and how these 3 parameters are connected. In the 3 cases below, the exposure is increased by one stop, meaning twice as much light is recorded:

• When you open the aperture, for example from f/4 to f/2.8,
• When you increase ISO sensitivity, for example from ISO 100 to ISO 200. Strictly speaking, this does not make the sensor receive more light; it amplifies the signal after the light has been captured. The visible result, however, is a brighter image.
• When you increase the exposure time, for example from 1/100 to 1/50. As you can see, the phrase “increase the exposure time” can be misleading, because the number displayed after 1/ actually gets smaller.

It is important to remember that each of these parameters affects exposure, but not only exposure. For example:

• Changing the aperture affects the depth of field of the image, meaning the area that appears sharp compared with the blurred area, often called bokeh,
• Changing the exposure time, or shutter speed, affects how much motion blur appears in the image. A tripod may be needed to maintain sharpness at slow shutter speeds,
• Increasing ISO sensitivity makes the photo more grainy or noisy.

I know there are a lot of technical terms here, but they will all be explained later in more specific articles. In other words, there are a few habits to keep in mind when choosing your settings depending on the shooting conditions, such as keeping ISO as low as possible on a sunny day.

For example, the following 3 combinations give the same exposure, but not the same visual result.

f/11 – 1/500 – ISO 400
f/8 – 1/1000 – ISO 400
f/8 – 1/500 – ISO 200

Keep in mind that shutter speed, aperture and ISO all use standard values. Each time you move by one full stop, the exposure is doubled or halved. Here are the values:

I will explain exposure value, or EV, in more detail below. In any case, if you increase one of the exposure parameters (ISO, aperture or exposure time), more light will be recorded and the final image will be brighter. The opposite is also true. Keep in mind that every time you move by one stop, the exposure doubles or halves. For example, going from f/5.6 to f/4, from 1/1000 to 1/500, or from ISO 100 to ISO 200 each adds one stop of exposure.

Here are 5 concrete cases explained in detail below

The purpose of the exercise is to obtain the same exposure by modifying one or more parameters of the exposure triangle. The starting values are shown on the left of the table, while the modified values are shown on the right.

• The blue value is the one deliberately changed for the exercise,
• The value(s) in red are the values changed to compensate for the blue value,
• The values in black remain unchanged during the compensation.

You might be thinking: all this is fine, but it still sounds very technical. Let me explain it more concretely. You have 4 possibilities:

• You can use automatic mode and let the camera handle everything itself, but you did not buy a DSLR or mirrorless camera for that, did you?
• You can use aperture priority mode (“Av” on Canon / “A” on Nikon). In this case, you set the aperture, the f-number, and the ISO, while the camera chooses the appropriate shutter speed,
• You can use shutter speed priority mode (“Tv” on Canon / “S” on Nikon). In this case, you set the shutter speed and ISO, and the camera chooses the appropriate aperture,
• Or you can use manual mode. In this case, it is up to you to choose the values of all 3 parameters. You will get there once the concept of exposure is clear.

To see whether a photo is “correctly” exposed for the scene in front of you, you can use the histogram. It is visible on your camera for each photo and allows you to see at a glance whether your image is normally exposed, underexposed or overexposed. Here are 3 examples of histograms.

Histogram representation of the 3 pictures shown at the beginning of the article, taken from my terrace. On the left, the histogram shows an underexposed photo; in the middle, a photo with normal exposure; and on the right, an overexposed photo.

In this article about the histogram in photography, I explain in more detail how to read it and how it can help you improve your photos and your exposures.

To summarize, remember that the histogram shows the distribution of tones in your image. If your histogram is stretched to the left, as in the first picture, your photo contains a lot of dark tones, or even pure black if the histogram is pushed completely to the left. A “properly exposed” photo will often have both light tones on the right and dark tones on the left, as shown by the central histogram above. Conversely, the histogram on the right is pushed towards the right, showing that part of the image is very bright, or even completely white.

To finish this section on exposure in photography, here is one last technical point. I mentioned in the introduction that EV is a unit used to measure exposure value. In the paragraphs above, I explained that there are standard values where exposure is doubled or halved. On DSLR and mirrorless cameras, EV is usually adjusted in 1/3-stop or 1/2-stop increments when you turn the dial to change a setting. Three 1/3-stop clicks, or two 1/2-stop clicks, are therefore needed to move by one full stop.

Here is an example of intermediate 1/3-stop values, as on my Canon:

For aperture: 2.8 – 3.2 – 3.5 – 4 – 4.5 – 5 – 5.6 – 6.3 – 7.1 – 8 – 9 – 10 – 11 – 13 – 14 – 16 – 18 – 20 – 22

For shutter speed: 1s – 0.8s – 0.6s – 1/2s – 0.4s – 0.3s – 1/4s – 1/5s – 1/6s – 1/8s – 1/10s – 1/13s – 1/15s – 1/20s – 1/25s – 1/30s – 1/40s – 1/50s – 1/60s – 1/80s – 1/100s – 1/125s – 1/160s – 1/200s – 1/250s

Example in manual mode (M), using 1/3 stops: you set the aperture to f/16, but you want a brighter image while keeping the other settings fixed, so you decide to open the aperture. You turn the dial by one third of a stop and move to f/14. The image is now slightly brighter. Move one more click and you reach f/13, then f/11 on the next click. By going from f/16 to f/11, you have doubled the amount of light reaching your sensor, adding one full stop of exposure.

If you are in aperture priority mode (Av on Canon), the principle is the same, except the camera compensates for this extra light by automatically shortening the exposure time.

Example: f/16 – 1/1000 – ISO 100. After your change, the camera compensates at f/11 – 1/2000 – ISO 100. The camera has halved the exposure time, meaning it takes the picture twice as fast.

The result in terms of exposure is the same, but your depth of field is reduced, so the image may be less sharp overall.

I want to show you a real situation I experienced a few years ago while photographing an orangutan in the jungle in Sumatra, Indonesia.

To give you the context, the place was very dark and completely in the shade. Increasing ISO was necessary. I wanted to isolate my subject, the orangutan, and blur the background. To do that, you need to open the aperture as much as possible, depending of course on the maximum aperture of your lens. Mine only opened to f/4. I therefore found myself limited by ISO 1600 on my camera body and by a maximum aperture of f/4. I was using Av mode, or aperture priority, and the camera was displaying shutter speeds that were too slow, below 1/200, to take a sharp photo. What does this mean in practice?

The result was that many of my photos were blurry because the shutter speed was not fast enough in those conditions. I had reached the limits of my equipment: both my camera body and my lens.

I am telling you this to help you understand why there is such a price difference between a Canon 24-70mm f/4 ($900) and a Canon 24-70mm f/2.8 ($1,900). The price is doubled, that is the reality. But looking back at my orangutan example, if I had had a lens with an aperture of f/2.8, I could have taken the photo twice as fast. Remember the table above: shutter speed doubles between f/4 and f/2.8. Now imagine I also had a camera body that could go beyond ISO 1600, for example to ISO 3200. I could have doubled the shutter speed again, which would have made the photo sharper. I have since switched to a full-frame camera, and it is much better in this respect.

If you understood that, you have understood the essential point. It explains very concretely why some lenses and camera bodies are so expensive, and above all how the 3 exposure parameters are connected. The choice of your camera equipment is therefore important too. Did I lose you with my orangutans? 🙂

If you have understood my explanations, you can see that there are endless possibilities when you modify these three parameters to control exposure in photography. Depending on what you want to photograph, how you want to shoot, the shutter speed you need, the type of image you want, and so on, you will have to choose the right settings to emphasize your subject.

You should now start to understand how difficult it can be to choose the best compromise between these 3 parameters. For now, exposure may still feel a little vague, but I invite you to read the detailed articles on each parameter as soon as they are available: aperture, ISO sensitivity and shutter speed. To summarize, you can increase the exposure of a photo in the following ways:

Be careful, and remember that:

That’s it, I am done with exposure in photography, and I hope you now understand the general idea and the three concepts that influence it. Later, I will also explain the importance of RAW format and post-processing in relation to exposure. You will not become a professional photographer after reading a few articles, and neither will I, but you will soon know how to shoot, capture and freeze a particular moment according to your equipment and its limits. That is already a great step towards better photos.

Was I clear? Too technical? Do you need more detail? Do you have any questions? Feel free to leave a comment and we can discuss it together. To continue learning, don’t hesitate to read the article on aperture in photography, or the one about ISO.

Written by Sylvain PONS

I've been passionate about photography since 2010, learning as I went along. Today, I dedicate myself to guiding others in their choice of camera gear and sharing a variety of tips to improve their photography skills.

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