Exposure in photography: How does it work?
Since I started this travel and photography blog, I’ve always wanted to write about it and to be able to help people learn photography, because it’s not something innate that’s for sure… Some people prefer to take photography lessons or training, but in this blog, I will try to explain to you some notions to learn photography on the web. Some time ago, I had begun a first article on the basics of photography, on the definition of the focal length in photography, but with hindsight I realize that I should have simply started with the basics: exposure in photography.
This is in my opinion (along with the complex notion of composition-framing), the most important element to understand to be able to progress in photography. This notion alone influences the final result and the desired result of one’s photography. It is a crucial point. In this article, hopefully not too technical, I will explain in detail what photo exposure is, how it works, what parameters influence it, and how you can control the exposure of your photo.
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In the meantime, if I may say so, three more detailed articles will follow to clarify and explain the three notions that influence the exposure of a picture: the aperture of the diaphragm, the ISO sensitivity and the shutter speed. I’ll tell you a little more about them later in the article.
Please note that the standard unit for measuring the exposure value in photography is Ev (Exposure value), IL (Illumination Index) or stops.
Exposure in photography: what is it?
Simply put, exposure is the amount of light received by the sensor of your digital camera when taking a picture (in the past, it was the film). To simply understand the concept, the more light your sensor receives, the brighter the picture will be. Conversely, the less light towards it, the darker the photo will be.
- We therefore speak of underexposure when the sensor has not received enough light. This results in a photo that is too dark and lacks detail in the shadows or black areas. You’ll hear the term “dark areas”,
- Conversely, an overexposed photo means that the camera sensor has received too much light. You then lose detail in the bright, white areas of the picture. The term “burnt or burnt areas” is used for this type of photo.
Here is an example (taken from my terrace!): on the left the photo is underexposed, the middle one is correctly exposed, the right one is overexposed.
In any case, it is important to remember that it is not a question of judging the quality of the picture but only its exposure, one could say “its reality” in relation to the scene in front of you. You can voluntarily decide to overexpose or underexpose a photo to emphasize an atmosphere or a sensation of the moment.
Low key and high key techniques are proof of this (photo techniques which voluntarily consists in over/under exposing a scene to its extreme to bring out a subject or an atmosphere). These are simply the things to remember about photography exposure.
The determining parameters: the exposure triangle
I hope you will have understood that exposing a picture correctly therefore depends more on a choice and on your desire to transcribe a certain reality, than a truth in itself. The photographer will thus have the pleasure of being able to manage the exposure of his picture himself by adjusting three main parameters, those which constitute the famous exposure triangle.
- ISO sensitivity: this is simply the sensitivity of your camera’s sensor to light. At the same shutter speed and aperture, the more you increase the sensitivity of your sensor (you increase the ISO), the brighter your picture will be,
- The aperture of the diaphragm: this is another very important parameter to consider for the exposure of your photo. It is this last one which will essentially allow you to blur part of a scene or on the contrary to have the whole thing totally sharp (notion of depth of field). Technically, this is the diameter of the diagram of your lens at the time of shooting (expressed in ” f/”, example f/2.8 or f/11). For now, remember that a large aperture = small f/,
On the left, the background of the picture is blurred (f/2.8 = large aperture) while on the right, the background is in focus (f/11 = small aperture).
- Last element of the triangle, the shutter speed: this is the time of the shutter opening when you take your picture (ex: 1/200).
The base itself is not complicated. Where things are more difficult to grasp is that these three parameters of exposure in photography are inseparable from each other, interconnected. It is therefore impossible to change one of them without affecting at least one of the other two.
Understanding exposure in photography: the interaction between the 3 parameters
To understand correctly the concept of the exposure in photography, we will see why and how these 3 parameters are connected. In the 3 cases below, twice as much light is sent to the sensor:
- When opening the aperture (for example from f/4 to f/2.8),
- When we increase the ISO sensitivity (ex: ISO 100 to ISO 200)-> It is after the capture of the light that there is an amplification of the signal allowing the sensor to receive 2 times more light. It’s a little technical detail, I wanted to summarize!
- When we increase the exposure time (ex: 1/100 to 1/50) -> We can see that the notion “to increase the exposure time” can be misleading, by seeing a number that decreases.
It is important to remember that each of these parameters will influence the exposure, but not only. For your information:
- Modifying the aperture will have an impact on the depth of field of the image (the focus area, as opposed to the blurred zone also called the bokeh),
- If the exposure time is changed (shutter speed), the motion captured may be more or less blurry (a tripod must be used to obtain proper sharpness),
- Finally, the more you increase the ISO sensitivity, the more grainy your photo will be (the photo will be more “coarse” we could simplify).
I know there are a lot of technical terms, but they will all be explained later in more specific articles. In other words, there are more or less automatisms to keep in mind concerning the choice of parameters depending on the shooting conditions (e.g.: ISO at its lowest during the day in the sun).
The goal to taking beautiful pictures will be to find a balance between these 3 parameters depending on what you want to do or shoot.
For example, the 3 following combinations will give the same exposure, but not the same photo result.
f/11 – 1/500 – ISO 400
f/8 – 1/1000 – ISO 400
f/8 – 1/500 – ISO 200
Keep in mind that there are standard shutter speeds, aperture and ISO (understand that every time you change one notch, the exposure will be doubled or halved). Here are the values:
For ISO: 50 – 100 – 200 – 400 – 800 – 1 600 – 3 200 – 6 400 – 12 800 – 25 600 – 51 200 – 102 400
For shutter speed: 30s – 15s – 8s – 4s – 2s – 1s – 1/2s – 1/4s – 1/8s – 1/15s – 1/30s – 1/60s – 1/125s – 1/250s – 1/500s – 1/1000s – 1/2000s – 1/4000s – 1/8000s
For aperture (f/): 1 – 1,4 – 2 – 2,8 – 4 – 5,6 – 8 – 11 – 16 – 22 – 32 – 45 – 64 – 90 – 128
I’ll give you a more detailed point below regarding the EI (exposure index) values. In any case, if you increase a parameter (ISO, aperture or exposure time), you will have more light coming in, and therefore, a brighter picture in the end. This is also true the other way around. Keep in mind that every time you double a parameter, the exposure doubles too (example: go from f/5.6 to f/4 or from 1/1000 to 1/500 or from ISO 100 to ISO 200).
Here are 5 concrete cases explained in detail below
The purpose of the exercise is to find the same exposure on the picture by modifying one or more parameters of the exposure triangle. The starting parameters chosen are on the left of the table while the modified values are on the right.
- The blue value is the one voluntarily modified for the exercise,
- The value(s) in red are the values changed to compensate for the voluntarily changed value (blue),
- The values in black remain the unchanged values in the parameter compensation.
Explanations: For this first case, the aperture is modified. We therefore opened our aperture (diaphragm) by two stops and went from f/8 -> f/5.6 -> f/4. Remember (see the grayed-out table above), each time we change a parameter by one notch, we double/divide the exposure by 2. Here we have opened our diaphragm (aperture) by 2 stops (large aperture = small f/ number), so we have 4 times more light at f/4 than at f/8 (2 times doubled). For this first case, we only decide to change the shutter speed parameter to compensate this excess of light. The ISOs remain the same.
Compensation : We have to compensate 2 stops to keep the same exposure on the picture. We therefore decide to increase the shutter speed, i.e. to take the picture faster (there will be less light reaching the sensor). We had a starting shutter speed of 1/60. If we decide to compensate by one stop, we take the picture twice as fast, i.e. 1/125 (see grayed-out table). But we have to compensate 2 stops, we shift the shutter speed another stop to reach 1/250. We have thus shifted the shutter speed by 2 stops to compensate for the 2 aperture stops changed for the exercise.
The photo will keep the same exposure between f/8 – 1/60 – ISO 400 AND f/4 – 1/250 – ISO 400.
Impacts :Because the aperture is larger (f/ smaller), the depth of field (area of focus in the image) will be reduced. Concretely, you will have part of your image that will be more blurred, often your background. This is ideal for example to highlight a subject and blur an unsightly background. Your shutter speed is now faster, which allows you to limit having a blurry picture when you take it. This speed could be ideal for capturing a nearby fast scene (for example, your cat running in front of you…).
Explanations: For case N°2, it is the shutter speed that is modified. The latter has been voluntarily changed from 1/60 to 1/500. By referring to the greyed-out table above, we have shifted the shutter speed by 3 stops (1/60 -> 1/125 -> 1/250 -> 1/500). So we take the picture faster than initially (small calculation 60 x 2 = 120 then 120 x 2 = 240 then 240 x 2 = 480, i.e. (3×2) about 1/500). This new setting allows 6 times less light to enter (3 stops). We decide to compensate by changing the aperture.
Compensation: As you can see, to compensate for the loss of light through the 3 shutter speed stops, we must let more light into the sensor. We will therefore open our diaphragm (aperture) by 3 stops. Initially, it is f/8. We change it to f/8 -> f/5.6 -> f/4 -> f/2.8. By choosing an aperture of f/2.8, we have thus well compensated for the loss of light in the exercise.
The photo will keep the same exposure between f/8 – 1/60 – ISO 400 AND f/2.8 – 1/500 – ISO 400
Impacts: As in case 1, the aperture is now large (f/2.8). This parameter will be ideal for blurring your background for example. If you want a large depth of field (focus area) on your image, you will have to go back to a smaller aperture (e.g. f/11), which will directly impact your picture, as you will have to lower the shutter speed several stops (note: it is possible to have a sharp image at f/2.8, all will depend on the distance from your subject, we will talk about this later). The risk of motion blur will then appear.
Explanations: For case N°3, it was decided for the exercise to modify the ISOs (ISO 100 to ISO 400). We have therefore changed 2 stops, i.e. double the light (4 times). We therefore have 4 times more light reaching the sensor than initially. To compensate for this excess of light, we choose to change the shutter speed.
Compensation: We must therefore adjust our shutter speed by 2 stops as well. Remember that we had an initial shutter speed of 1/250. By shifting one stop (1/500), we decrease the light by two (the picture is taken faster, so less light reaches the sensor). If we shift one more stop, we get to a shutter speed of 1/1000. So we have compensated for the excess of light by taking the picture 4 times faster (1/250 to 1/1000).
The photo will keep the same exposure between f/4 – 1/250 – ISO 100 AND f/4 – 1/1000 – ISO 400
Impacts: The important thing to note in this case is that by choosing to increase the ISO to 400, it allows us to take the picture four times faster. This will therefore be very useful in low light conditions where a minimum acceptable shutter speed will be necessary to prevent the picture from being blurry. If we decided to go to ISO 800 (shift it another stop), the shutter speed would be 1/2000 for the same exposure. Keep in mind that raising the ISO allows you to obtain (while keeping the same exposure) a faster shutter speed. Note that it will be necessary to find a balance because the more you increase your ISOs, the more you lose in image quality. Another possibility to compensate for this excess of light, we could have changed the aperture by 2 stops (i.e. from f/4 to f/8).
You will see below in the article the advantage of increasing your ISO with a long focal length in low light conditions (story with my orangutans).
Explanations: Case n°4 is similar to case n°1. We have chosen to modify the aperture by closing our diaphragm one stop (from f/5.6 to f/8). So we halved the light of the image. To compensate for this loss of light, we decided to change the ISOs.
Compensation: We therefore decided to give more ISO sensitivity, by shifting them one stop (from ISO 100 to 200). This doubles the amount of light reaching the sensor. The loss of light is in this case compensated.
The picture will keep the same exposure between f/5.6 – 1/500 – ISO 100 AND f/8 – 1/500 – ISO 200
Impacts: Going from ISO 100 to ISO 200 has little impact on a picture. On the other hand, choosing to close by one stop our aperture has an impact on the depth of field. Concretely, the whole picture will be sharper everywhere at f/8 than at f/5.6.
Explanations: This last case is the most complex of the 5 cases. We choose for the exercise to modify the aperture (f/11 to f/5.6). We thus multiplied by 4 (2 aperture stops) the quantity of light in the picture. This time, to compensate for this excess of light, we decide to modify the 2 others parameters and not just one (as in the other cases above). Remember, we have two stops to compensate for.
Compensation: We first decide to double the shutter speed (from 1/1000 to 1/2000). The picture was taken faster (1 stop), the amount of light is cut in half. But this is not enough, knowing that we have 2 stops to make up. We then change the ISOs by halving them (from ISO 400 to IS0 200), which again divides the amount of light in the scene by 2. We therefore halved the amount of light twice to compensate for the 2 stops of the exercise.
The photo will keep the same exposure between f/11 – 1/1000 – ISO 400 AND f/5.6 – 1/2000 – ISO 200
Impacts: Case n°5 proves that you can compensate an excess or a loss of light by two parameters of the exposure triangle.
Remember the following:
- If you need to compensate for an excess of light by one stop, simply change one of the other two parameters that affect the exposure of your photo,
- You can compensate 2 stops with the other 2 exposure parameters (e.g. 1 stop each),
- Every compensation you make will have an impact on the result of your photo and not on the exposure itself (if your compensation is correct). It could be a greater depth of field (e.g. if you compensated by closing your diaphragm at f/13), or an increase in graininess in your photo (if you compensated by increasing the ISO), or a risk of motion blur (if you decided to decrease the shutter speed of the photo for example.
The goal will be to find a balance between the 3 parameters according to what you want to emphasize on the photo or your light conditions for example.
Don’t worry, you won’t always have to do these exercises to compensate for this or that parameter. Once all 3 parameters influencing the exposure are well understood, you will be able to use, if you want, the semi-automatic modes, which will do part of the work for you. I personally use the Av Mode a lot, where I only need to set the desired aperture and ISOs (and even then, I could set them to Auto). I’ll tell you about it below.
How to correctly expose with my camera then?
You’re going to tell me, it’s all well and good, but it’s very technical. Let me explain it to you in concrete terms. You have 4 possibilities:
- Either you use the automatic mode and you let it manage by itself (but you didn’t buy a DSLR or a mirrorless camera for that, did you?),
- Either you use the aperture priority mode (“Av” at Canon / “A” at Nikon) and in this case, you only fix the aperture (the “f/”) and the ISOs, the camera alone manages the adequate shutter speed,
- Alternatively, you can use Shutter Speed Priority mode (“Tv” at Canon and “S” at Nikon) and set the shutter speed and ISOs, the camera will then select the appropriate aperture,
- Either you use the manual mode and in this case, it is up to you to choose the values of the 3 parameters (you will get there once the concept of exposure is understood).
To see if a picture is exposed “correctly” to the scene in front of you, you can use the histogram of your photo. It is visible on your camera for each photo. This allows you to see at a glance whether your photo is normally exposed, underexposed or overexposed. Here is an example of 3 histograms to show you.
Histogram representation of the 3 pictures shown at the beginning of the article (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 I explain more precisely about the histogram in photography, learn how to read it in detail and how it will help you improve your pictures and your exposures.
To summarize, remember that the histogram shows you the tone curve of your picture. If your histogram is stretched to the left (as in the first picture), then your photo has a lot of dark tones, or even black (if the histogram is completely to the left). A “properly exposed” photo will often have a histogram with both light tones (to the right) and dark tones (to the left), see the histogram in the center of the image above. Conversely, the histogram on the right in the image above is pulled to the right, showing that the picture has a very bright if not completely white part.
EV for better understanding
To end with the exposure in photography, here is a last technical precision. I mentioned the term in the introduction which specifies that EV is a unit for measuring the exposure value. I explained in the above paragraphs that there are standard values (for which the exposure is doubled or halved). On DSLR cameras, the EV is usually increased by 1/3 (or 1/2) increments each time (when you turn the dial to adjust a parameter). Three 1/3 (or two 1/2) notches are therefore required to double a parameter.
Here’s an example of an intermediate EV value for 1/3 notches (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 the 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) with 1/3 stops: you set the aperture to f/16, but you want a brighter picture (keeping the other settings fixed), so you decide to open the diaphragm. Use the dial to shift the aperture by one third (1/3) to f/14. The picture is now brighter. If you move it a step further, you are at f/13, and then at f/11 at the last notch. By shifting from f/16 to f/11 you have doubled the amount of light reaching your sensor (loss of a stop).
If you are in aperture priority mode (Av – Canon): it’s the same principle except that the camera compensates for this excess of light by automatically reducing the shutter speed.
Ex : f/16 – 1/1000 – IS0 100 then after your change, the camera compensates at f/11 – 1/2000 – ISO 100. (the camera has reduced the exposure time by 2 -> understand it takes 2 times faster).
The result in terms of exposure is the same except that your depth of field has been reduced (your picture is overall less sharp).
Photography exposure settings with a concrete example!
I’m showing you a real case I had a few years ago while taking pictures of an Orangutan in the jungle in Sumatra, Indonesia.
To put it in context, the place was very dark, completely in shadow. An increase in ISO was necessary in this case. I wanted to isolate my subjects (Orangutan) and blur the background. To do this, you have to open the diaphragm as much as possible (it all depends on the maximum aperture of your lens). Mine only opened at f/4 maximum. I therefore find myself limited by ISO 1600 (from my camera body) and an f/4 aperture. I was in Av mode (aperture priority) at the time, and the camera was displaying shutter speeds that were too slow (less than 1/200th) to take my picture. Concretely, what does this imply in reality?
The result was that a lot of my pictures were blurry, the shutter speed not being fast enough to take a sharp picture in these conditions. So I had reached the limits of my equipment here (both my camera body and my lens).
The reason I’m telling you this is to try to make you understand why there is a price difference between a Canon 24-70mm f/4 (900$) and a Canon 24-70mm f/2.8 (1900$). The price is doubled, that’s the reality. But if we look back at my Orangutan case, if I had a lens with an aperture at f/2.8, I could have actually taken the picture twice as fast (remember the table above, the shutter speed is doubled between f/4 and f/2.8). Now also imagine that if I had a camera body that went up to more than ISO 1600 (for example 3200), I would have been able to double the shutter speed on my picture (therefore sharper). I’ve now switched to a Full frame camera and it’s much better.
If you understood that, that’s good enough. It explains to you concretely why some lenses or bodies are very expensive, but especially the relation between the 3 parameters of the exposure in photography. The choice of your camera is therefore also important. Did I lose you with my Orangutans? ? 🙂
If you have understood my explanations, there are endless possibilities by modifying these three parameters to play on the exposure in photography. Depending on what you want to take, how you want to shoot, at what shutter speed, what type of photo, etc., you will have to choose the right parameters to enhance your subject.
You must now come to understand all the difficulty in choosing the best compromise between these 3 parameters. For the moment, the concept of exposure may remain a bit vague, but I will invite you (as soon as they are written) to read the detailed articles on the parameters: aperture, ISO sensitivity, and shutter speed. To summarize, you can increase the exposure of a picture in the following way:
Increasing the aperture of your diaphragm (one f/ smaller number)
Increasing ISO sensitivity (higher ISO number)
Decrease shutter speed (1/… smaller)
And vice versa to lower the exposure of your photo.
Attention, I remind you that:
The more you open your diaphragm, the shorter the depth of field (reduced focus area),
The more you increase your ISO sensitivity, the more noise/grain you will have on your picture (loss of quality),
The more you lower the shutter speed, the more blurry your picture is likely to be.
That’s it, I’m done with exposure in photography, and I hope you were able to understand the general idea and the three concepts that influence it. I’ll explain to you later the importance of the RAW format and post-processing in relation to exposure. You won’t become a professional photographer with a few readings (neither am I), but you will soon know how to shoot, capture, freeze a particular moment according to your photographic equipment and its limits. This will give you beautiful pictures!
Was I clear? Too technical? Do you need any precisions? Do you have any questions? Feel free to leave a comment and we’ll discuss it together. To continue your learning, don’t hesitate to come and read the article on aperture in photography, or ISOs.