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HDR Photography: Exposure

Because high dynamic range photography involves getting around the limitations of dynamic range in modern digital photography, it stands to reason that exposure is a critical concept to understand. When you’re setting up your camera to shoot bracketed photos, you need to be comfortable thinking and acting quickly.

Exposure is how much light enters the camera for the length of time the shutter is open. An exposure is shorthand for a photograph, or frame (which is a throwback to a frame of film).

You control exposure by changing how the camera operates. There are three primary ways to do this, as shown in the table. Each variable has the same effect — raising or lowing the exposure. They all do it differently, as well as having a different major side effect.

Controlling Exposure
Variable How It Works Side Effect
Aperture Changes the size of the opening in the lens that lets light in the camera Shortens or deepens the depth of field
Shutter Speed Changes the length of time the shutter (the thing that keeps light out) is open Stops or blurs motion
ISO Changes the digital camera sensor’s sensitivity to light Raises or lowers noise
Here are some of the more important concepts related to exposure and how to control it for HDR:
  • Aperture: Aperture is the size of the opening in the lens, measured in f-numbers. The f-number (also called an f-stop) is inversely proportional to the size of the opening (the aperture).

    In other words, a larger aperture has a smaller f-number and a smaller aperture has a larger f-number. The size of the lens and the actual size of the opening are irrelevant in comparing exposure. You can see this relationship play out in this figure, where each succeeding f-stop is half as large as the previous (but the numbers get larger).

    image0.jpg

    A larger aperture creates a shallow depth of field, which is the area you perceive to be in acceptable focus. A smaller aperture increases the depth of field. This is illustrated in these photographs, where the wide aperture of f/1.4 has a much smaller depth of field than the second shot, taken at f/16.

    image1.jpg

    For HDR, it is best to keep aperture constant between brackets so the depth of field does not change.

  • Shutter speed: Shutter speed is the length of time the shutter stays open and allows light to enter the camera and strike the sensor. You control this value to let less or more light in, which in turn darkens or brightens the photo.

    For HDR, fast shutter speeds are less important than traditional hand-held photography because you will often be taking photos of inanimate objects with the camera mounted on a tripod. For hand-held auto bracketed HDR, a faster shutter speed is more important.

  • ISO: For digital cameras, ISO equates to sensor gain (the sensitivity to light). The higher the ISO, the greater the camera’s ability to capture scenes with less light. The counterweight to this is always noise, as shown in this figure. Notice in particular the noise is much more apparent in areas of even tone — the green concourse floor and white cinderblock wall.

    image2.jpg

    For HDR photography, the lowest possible ISO is the best setting. Raise it only if you cannot get the shot any other way.

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