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Downtown Lens: High Dynamic Range (HDR) Part 2 -- Multiple Exposures

By Dave Bullock
Published: Sunday, September 14, 2008, at 03:49PM

What your eye sees is not always what your camera captures. Your camera's dynamic range, the level of tones it can capture, is much narrower than what nature presents.

High Dynamic Range photography allows you to capture a view that more closely mimics real life. To do so, you need to take multiple exposures of the same scene.

In the last Downtown Lens I discussed the required materials for HDR work: a tripod and a camera with exposure controls. In this second part, I'll discuss capturing the multiple images required to make a tone-mapped HDR photo.

Once you have the proper gear, you need to find something to photograph.

For a good HDR image, you want a scene that has a wide range of tonal values. My favorite HDR subject is a city scene on a cloudy day. My favorite clouds are the ones in the sky after a rain. These days are also nearly smog free, so I'm looking forward to some rain soon.

Once you have your scene picked out, set up your tripod and attach your camera to it. Look through the view finder and frame the shot to your liking and then lock your tripod's head in place. Make sure it's locked steady as you don't want it to move between exposures.

Next, set your camera on either Manual or Aperture Priority mode. It is very important that you change your shutter speed between shots and not your aperture. If your aperture changes, it will have an effect on the shot's depth of field and cause some shots to have objects in focus that are blurry in others. That will confuse the software processing your image and cause it to not come out as well as it could.

If your camera has an auto-bracketing mode, use it to easily take three or more shots of the scene. Whether using the bracket function or controlling exposure manually, you want the photos to be at least 2 EV apart from each other. So take the first shot at 2 stops underexposed, the next shot properly exposed and the final shot at 2 stops overexposed.

You aren't actually limited to 3 shots, the more the merrier as far as HDR is concerned. If your scene has a very large range of tone you may need more than three shots to capture the scene, but typically three will do. If your camera can shoot in RAW mode you can even get away with taking just one photo, and although the result won't really be HDR, you will be able to pull out more info later in the process and fake it.

Once you've taken three or more photos without moving your camera, take a look at them on the LCD. The underexposed shot should have nice detail in the highlights (the clouds, in our example), with the rest of the scene being almost black. The properly exposed shot should have plenty of good detail in the mid-tones, but the sky should be almost white with only a small amount of detail in the highlights. Any dark portions of the image, such as the space under bridges or in shadows, should be black. The overexposed shot should have no detail at all in the sky, the mid-tones should be washed out, but there should be good detail in the dark spaces (seen here under the bridge).

Once you're happy with your shots, re-frame and take another series or two. Digital storage space is cheap, but who knows if you'll ever get the shot again. Better to have more shots than you'll use rather than missing the one you need. When you're done, pack up your gear and go home keeping in mind that the fun is about to begin. Shooting HDR is a lot like shooting film as you won't know what you really have until you develop it in your digital darkroom.

My next installment will cover that digital development, so keep an eye out for that next weekend. Have fun and happy shooting!

This post is the seventh part in a weekly series entitled Downtown Lens in which I will discuss a photograph and the technique that relates to it.


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