Exposure Explained – How to Capture the Light

In the end, all your camera does is catching the light. As the photographer you decide just how much light you want to capture. Back in the day this was a real skill. Just manual controls, and only your eyes to judge the scene. Nowadays you can just lean back and let your camera do it for you. Nevertheless, understanding the concept of exposure will help you to become a better photographer.

Midnight Seeker by Jonhy Blaze on 500px.com

By purposely underexposing this shot, the photographer created an unique atmosphere

Why exposure matters

Sure your camera knows exactly how to expose an image according to its own rules, but it cannot read your mind. Lighting is one of the most important aspects in photography, and it can make or break your image. Your camera’s metering can not make that decision for you.

As you can see from the images throughout this article, breaking the rules is sometimes necessary to end up with great images. Nevertheless, in order to break the rules on purpose, it makes sense to first see what the rules actually are.

The exposure triangle

Traditionally exposure is explained as a triangle with three components. Shutter speed, aperture and ISO speed. Each of these influences the amount of light captured in your photo. But beware, each of these three comes with its own trade-off.

Shutter speed

The influence of shutter speed on exposure is intuitive enough. By dividing your shutter speed by two, you are halving the amount of light captured. The trade-off is that you need a fast enough shutter speed to prevent a blurry photo, due to either a fast-moving subject, or movements of your hands. To prevent the latter, a generally accepted rule is to keep your shutter speed above the reciprocal of your (35mm equivalent) focal length.

Aperture

You can see the aperture of your lens as a circular opening through which the light is captured. It is easy enough to understand that a twice as large opening, will capture twice as much light. And indeed, this is true. However, the aperture number used in your camera is equivalent with the reciprocal of the diameter of this opening. Now we have to do a little math, but hopefully you remember that the surface of a circle is A=π·r^2. This means that in order to double your lens opening, you need divide your radius (and hence your aperture number) by a factor √2, which is roughly 1.4. So, an aperture of f/2.8 captures half as much light as f/2, which captures half as much light as f/1.4 and so on. Of course the other parameter affected by aperture is the depth of field. The larger your aperture (the smaller the number), the shallower your depth of field will be.

ISO

The ISO speed is something carried over from the film era. In this time, the ISO was the sensitivity of your film. By doubling the sensitivity (for example moving from ISO100 to ISO200), the amount of light captured also becomes twice as much. This same concept applies to the sensitivity of your camera’s sensor. Of course, this extra light does not come for free. In the days of film, a higher ISO film had a less fine grain. Now in the digital age, higher ISO values also introduce more noise in your photographs. However, this is becoming less of a disadvantage with newer camera’s. Where earlier camera’s were barely usable at ISO800, the cream of the crop can easily go to ISO6400 and beyond, and still produce acceptable images.

A slightly overexposed imageOverexposure is more challenging, but in this example it is applied tastefully.

Stops – Where it al comes together

Now we get to the interesting part, where we combine the shutter speed, aperture and ISO into one value. In the end, we can calculate a number which calculated the amount of light in your photograph. We can for instance say: this image is exposed by 5 stops.

The general rule is that 1 additional stop of exposure captures twice as much light. And as you might have guessed now, the light in your photograph is halved when decreasing the number of stops by one.

The baseline

We prefer to define the number of stops as an absolute scale. In this way you can get a feel for different weather conditions, what the exposure should be. In order to do that, we need a baseline. The baseline at 0 stops is defined as the light level that requires a 1 second exposure at f/1 with ISO100. That is easy enough to understand, but in fact it is already very dark by then.

An example

Nevertheless, this baseline gives us a starting point. Now as an example, consider a photo, taken with a shutter speed of 1/8s, an aperture of f/2.8, and at ISO400. The shutter speed has halved three times over the baseline (from 1s to 1/2s to 1/4s to 1/8s). The aperture has been multiplied three times with the previously explained factor of 1.4 (from f/1 to f/1.4 to f/2 to f/2.8). And finally the ISO has been doubled twice (from ISO100 to ISO200 to ISO400). This means that in total we have 3+3+2=8 stops of extra light in our image compared to the baseline of 0 stops. Therefore we can say: this image has been exposed by 8 stops.

Working the triangle

If you understood the concepts so far, you can see how the exposure is made up by the three different components. In fact, the equation can be given by:

total number of stops = stops due to shutter speed + stops due to aperture + stops due to ISO.

What you can immediately see from this equation is that you can play with the balance of the of these three parameters. In order to obtain your desired exposure, there are multiple routes to go. For instance if you need to have a shutter speed that is twice as fast due to a moving subject, you can maintain exactly the same exposure by doubling your ISO, since then the total number of stops will not change. Another example is where you would need to have a large aperture, to blur the background behind your subject. In this case you can crank up the shutter speed, and still end up with the same total number of stops. This is what gives the photographer the flexibility to achieve different kinds of images at the same light level.

The sunny 16 rule

So now you know what stops are, and how they are influenced by the three parts of the triangle. But it is all still a bit theoretical and abstract. In the end we might want to be able to look around and say: ‘well I guess the weather today is around 10 stops’. Or: ‘wow, it is really dark outside, I will be lucky if I can see 3 stops’.

PARAALBUM-38This image is exposed according to the sunny 16 rule. 1/125s, F16, ISO100.

Back in the days of film, the photographers relied on a very simple rule to do something like that, the sunny 16 rule. This rule states that:

On a sunny day at aperture f/16, your shutter speed should be 1/ISO.

Simple enough right? Of course it is not always sunny in the real world. The table below serves as an extension to the sunny 16 rule, for different weather conditions. Just replace the aperture number in the rule above, and keep all the rest the same.

Aperture Lighting conditions Shadow detail
f/22 Snow/sand Dark with sharp edges
f/16 Sunny Distinct
f/11 Slight overcast Soft around edges
f/8 Overcast Barely visible
f/5.6 Heavy overcast No shadows
f/4 Open shade/sunset No shadows

Oh, by the way. If you calculate f/16, 1/100s and ISO100 back to our baseline, you will find out that this more ore less matches an exposure of 15 stops.

Using exposure compensation

In the end you can still let your camera make the exposure decisions. To be honest, most of them do a really good job while doing it. There is however one little trick to override what your camera thinks is best. This is called exposure compensation. Most camera’s feature this somewhere in a menu, or by an actual physical dial.

Imagine that it is sunny outside, and your camera picks a combination where the exposure is 15 stops. Imagine that the sky is really beautiful, and that you want to emphasize that by making the foreground darker, bringing out the details in the sky. In fact, you want to underexpose your image with one or two stops. This is exactly the setting you do by using exposure compensation. You tell your camera: ‘your settings are quite all right, but in the end, I am the boss, and I want two stops less light for this shot’. And you should, because, you are the photographer. You are the only one that can see the light with the end result in mind.

Gig by Rui Caria on 500px.com

Another example where underexposure is used by the photographer

Understanding exposure

If you want to learn more on the topic of exposure, there is one book you should definitely check out. The book is ‘Understanding exposure’, by Bryan Peterson. Understanding Exposure is ranking of one of the top ten books on photography sold on Amazon. It has sold over one million copies. Read more about the latest edition in our blog post here, or check it out on Amazon.

Understanding Exposure, fourth edition. By Bryan PetersonThe fourth edition of Understanding Exposure by Brian Peterson.

If you still have any questions on the topic, please drop them in the comments below, and we will try to help you out!