The Bastards Book of Photography

An open-source guide to working with light by Dan Nguyen

Exposure: the Quantity of Light

An introduction to how your camera lets the right (or wrong) amount of light in.

  • Exposure value: 0
  • Shutter speed: 1/30
  • F number: 4.0
  • Iso: 1000
  • Focal length: 24.0 mm
  • Flash used: Off, Did not fire
View on Flickr Taken with Sony NEX-7 / E 24mm F1.8 ZA on May 4, 2012 at 08:33 PM
Rome's Colosseum

Exposure is the amount of light that your camera uses to produce a photo. The entirety of this book is pretty much devoted to how to control exposure by changing how your camera lets light in.

Sometimes you just have to make do with what light you have. Without knowing anything about manual controls, you can still move around and move your subject. Knowing manual controls gives you many more options and flexibility.

For our purposes, the complicated math formula behind this can be reduced to:

  1. More light == brighter photo
  2. Less light == darker photo

How does a camera automatically determine the proper exposure?

When turned on and pointed at a subject, your camera is taking in light. It then calculates how much light is needed to keep the subject (or the entire scene) from being too dark or too bright.

Try this.

  1. Point your digital camera at a dark place, such as a corner or under the table.
  2. Then point it quickly at a bright scene, such as out the window. The camera’s LCD should turn almost completely white. The following photo is taken from inside of Federal Hall on Wall Street, right after I had pointed it toward a dark corner:
  3. After a second or two, the LCD screen should darken a bit overall, revealing more detail in the scene:

This delayed reaction is caused by the lag in calculating the correct exposure. Your camera’s computer is trying to figure out the right amount of light to let in.

How much is the right amount of light?

The long answer: Figuring that out is the basis of this entire book, and the “right” amount varies per situation, subject, and photographer’s aesthetic sensibility.

The answer, for now: It’s what your computer determines is the right amount of light to properly show the subject.

In the graphic below are three iPhone photos of the same scene. The orange circles represent the areas that the phone camera is exposing for (you’ll learn how to do this in the spot-selection lesson):

In each frame, the camera attempts to make the area within the orange circle appear as a neutral midtone. Everything else in the photo has a brightness level relative to the exposed spot.

How does the camera know what the “subject” is?

By default, most cameras assume that it’s what the center is. In simple point and shoots, you can’t change this. In modern phone cameras, you have some flexibility in choosing the spot to expose, which I cover in the spot-selection lesson.

The default settings for most cameras, though, judge the light from multiple regions (and each camera may have a different mix). In the photo below, I’ve let my camera pick what it considers the proper exposure (an exposure value of 0, as I explain in a later chapter):

blue paint 1.3EV
According to my camera, this photo has an exposure value of 0 EV, which the camera judges as the right amount of light for this particular scene.

The camera’s sensor is getting a lot of light from the overcast sky and adjusts its settings so that details in the sky, such as the cloud in the top right, can be seen. Unfortunately, that ends up obscuring the details of the painter, who is dark in comparison to the sky.

Below, I’ve made a manual adjustment by telling the camera to increase its exposure +1.3 steps (or “stops”). This effectively increases the amount of light taken in by the camera by about 2.5 times as much as 0 EV.

But don’t worry about the numbers. It’s just important to see that the scene is significantly brighter than it was in the previous version. The sky is basically white, but you can now make out the details in the painter:

blue paint 1.3EV
The camera judges photo to have an exposure value of 1.3EV, which is about 2.5 times the amount of light of the same photo with a 0 EV

This isn’t a great photo op either way (and would look better with some post-processing work), but the bottom option with the increased exposure value is closer to how I want to capture the scene: the painter, not the sky, is the interesting part.

My eyes can see both clouds and details in the shadows, why can’t my camera?

This has to do with dynamic range: your eyes can see a greater range of light than the best camera. More expensive cameras have a higher dynamic range than cheaper ones. But the camera will always capture less detail than what you remember seeing.

What’s wrong with letting the camera properly expose the scene?

It’s not about right or wrong; it’s about what visual effect you intend to show.

In the last chapter, we learned how the sun behind the subject creates a silhouette.

When a camera is in auto-exposure mode, it will adjust the light so that your subject is not a silhouette.

As the subject gets brighter, so does the rest of the background. This is what is called, “blowing out the background.”

In the photo below, I deliberately reduced the exposure to use one-fourth the light compared to the camera’s default exposure. The model is almost completely a shadow. Had I let the camera do the exposure, the details in the model’s clothing could probably be seen, but the entire sky would likely be white, i.e. blown out.

  • Exposure value: -2.3
  • Shutter speed: 1/250
  • F number: 22.0
  • Iso: 100
  • Focal length: 24.0 mm
  • Flash used: Off, Did not fire
View on Flickr Taken with Sony NEX-7 / E 24mm F1.8 ZA on May 12, 2012 at 07:45 AM
Taken at the High Line, for Proof NY

In other words, the “well-exposed” photo is not always the best photo.

The camera generally does a great job when the light is right.

But under extreme conditions, such as bright sunlight on a snowfield. Or a dark room. Or for if a ray of light happens to shine directly into the region (generally, the center point) where the camera makes its judgment, the camera may do something you don’t want.

This is why we want to take control of exposure for ourselves.

  • Exposure value: 0
  • Shutter speed: 1/125
  • F number: 4.5
  • Iso: 400
  • Focal length: 48.0 mm
  • Flash used: Off, Did not fire
View on Flickr Taken with Canon EOS 5D Mark II / EF24-70mm f/2.8L USM on Jul 4, 2011 at 07:06 PM
I’ll be the first to admit that I don’t have a lot of great examples of well-exposed photos that readily come to mind. This photo has a decent smooth distribution of midtones with just relatively small amount of areas blown out (such as the young man’s white t-shirt). In a later revision, I’ll discuss histograms and how they relate to exposure. has a great primer.
  • Exposure value: 0
  • Shutter speed: 1/400
  • F number: 2.2
  • Iso: 80
  • Focal length: 6.0 mm
  • Flash used: Off, Did not fire
View on Flickr Taken with Canon PowerShot S90 / 6.0-22.5 mm on Aug 16, 2011 at 05:53 PM
Here’s where the camera’s default judgment of exposure worked out well. The subjects of the photo – the people and the dogs – have just about the right amount of detail and color. The camera obviously exposed for the subjects. Had it exposed for the Hudson River, which is reflecting back a ton of direct sunlight, these people and their dogs would be shadows. The tradeoff is that there is almost no detail in the background
  • Exposure value: -1/3
  • Shutter speed: 1/640
  • F number: 7.1
  • Iso: 80
  • Focal length: 6.0 mm
  • Flash used: Off, Did not fire
View on Flickr Taken with Canon PowerShot S90 / 6.0-22.5 mm on Aug 16, 2011 at 05:47 PM
In this photo, I’ve slightly underexposed the photo and I think the camera is exposing for a relatively dark area to begin with. This is why you can see New Jersey and a little detail in the Hudson River. The volleyball players in the foreground are mostly silhouettes.
  • Exposure value: 0
  • Shutter speed: 1/15
  • F number: 2.8
  • Iso: 1250
  • Focal length: 7.3 mm
  • Flash used: Off, Did not fire
View on Flickr Taken with Canon PowerShot S100 / 5.2-26.0 mm on Jan 21, 2012 at 05:51 PM
An evening taxi ride. I’m not sure if this exposure is the result of the camera’s judgment, or if this was as much light as it could let in given my settings. Either way, I got the intended effect: only the LED lighting – and the lights of the city and traffic – are emphasized.


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