The Bastards Book of Photography

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

Exposure Through Spot Selection

You can practice setting exposure with your touchscreen camera phone

  • Exposure value:
  • Shutter speed: 1/7937
  • F number: 2.4
  • Iso: 64
  • Focal length: 4.3 mm
  • Flash used: Off, Did not fire
View on Flickr Taken with Apple iPhone 4S / on Jan 31, 2012 at 01:47 PM
I shot this while on my lunch break. The sun is directly overhead and the tight row of skyscrapers along Broadway near Wall Street creates a focused chasm of light. Here, I've selected the sky as the exposure spot, which is why there is detail in the bright sky and everything else is just shadow

Camera phones may be limited in their controls, but they (most of them, anyway) let you select a spot in a scene to expose for. This is a great interactive way to learn exposure – and it will bring more dramatic lighting to your phone photos.

How do I adjust exposure on my (touchscreen) camera phone?

Note: This works on smartphones made in the last couple of years, including the iPhone 3GS on up and I assume most of the Android phones. Some digital-cameras also have touch functionality on their LCD screens.

1. Point your camera at a well-lit daylight scene.

Without doing anything, your camera should calculate an exposure for the scene. Dark areas will be black and details in the lightest areas will be “blown out” as white.

Helmsley Plaza on Broadway. At the camera’s default exposure, you can still make out some blue in the sky.
2. Touch one of the lightest areas, such as a cloud.

The overall scene should darken. Your camera is now exposing for the highlights, letting you see detail in the lightest areas while blacking out the details in the relatively darker areas. This will underexpose the photo.

I’ve chosen to expose for the clouds, allowing for actual detail of the clouds and more blue in the sky.
3. Touch one of the darkest areas, such as a shadow.

The overall scene should lighten. Your camera is now exposing for the shadows, letting you see detail in the darkest areas at the expense of blowing out the detail in all of the relatively lighter areas. This includes the sky, which may appear to be plain white instead of blue. This is overexposing the photo.

I’ve chosen to expose for the shadow of the cube. You can make out the details on the people’s clothing. However, the sky is blown out as completely white.

One thing to keep in mind: The touch-selection will also change the focal point of the image. In other words, if you touch on an object near you, your camera will expose and focus on that, and all objects far from that focal point will be out of focus.

So try to pick a spot that not only brings a desired exposure, but is near to where you what sharp details. The Camera+ app for the iPhone lets you separate focus and exposure selection.

How is this useful?

It lets you select the parts of a scene that you care about. On an evenly lighted day, the camera’s auto-exposure will pretty much match what you want to see. But in particularly dark and light situations, you may want to highlight an area different than what the camera does.

How do I do spot selection on my dedicated camera?

Cameras without touchscreens don’t have this handy functionality. By default, they usually sample from a variety of points in the scene. Most DSLR-type cameras will have a spot-metering mode which samples from the center of the image.

This requires you to point the camera’s center spot toward the desired spot in the scene. And than manually recompose for what you actually want to shoot for. This is a kind of a tricky technique in itself…

But the key takeaway is: in spot-metering mode, the camera determines exposure according to the light balance at the center of the image.

The diagram above shows two exposure evaluation modes of the Canon 5D. On the left is the default mode, which evaluates the light in the entire scene. On the right is the spot-metering mode, which evaluates the light at the center of the scene, in an area roughly 3.5% of the viewing area.

More advanced cameras allow you to manually select a spot, in the same way that you can choose a specific focal spot. Refer to your instruction manual for more details.

If all you have is a camera phone, you can jump ahead to the lessons on overexposure and underexposure for an in-depth exploration of artistic and practical technique.

If you have a dedicated camera, we’ll learn about the Program mode, which lets us dial up/down the exposure of a scene regardless of where the exposure spot is.


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