(H) Ed Halley's

GIMP Tutorials

Using Multiple Exposures in Photography

The following is a brief tutorial on how you can take and edit multiple distinct digital photographs taken from a single vantage point for creative photo-manipulated effects. This includes separate scanned film images, or separate images from a digital camera.

Blending Exposures

The human eye is very good at adapting to complicated lighting situations. The iris can open up to see details in shadowy areas, and it can close down to avoid the pain of bright light.

A single image from a camera, however, is limited to one exposure setting at a time, and the film or sensor may not be able to retain details in both light and dark extremes.

If you take multiple exposures, however, this can be blended later for excellent detail over a very wide range of exposure. Many cameras offer an automatic "bracketing" feature to help with this task, though any manually-adustable camera can be used as well.

Take at least two shots; one shot which is exposed brightly enough to capture the details in dark areas, and one shot which is exposed darkly enough to keep the details in bright regions. If you're using a tripod, bracketing for shutter duration is the better course, but bracketing by aperture is usually acceptable. These exposures may be two or more full stops apart.

There are several ways of making a "mask" which will combine the two images nicely; we'll only try one such method here. This method works in most types of exposure-blending situations.

In your photo editor, copy and align the two exposures, with the darker-looking image above the lighter-looking one. Create a layer mask for the darker image. Select the entire lighter image, and copy it into the mask of the darker image. It should appear as a grayscale copy.

The mask allows shadowy details to appear, while suppressing the overexposed bright areas. However, a closer look may show that there are sharpness problems near contrasty edges. Select the newly made mask and apply a small blur, such as a three-pixel gaussian blur, to smooth out the transitions from light to dark exposures. If the blur is too large, halos will appear along contrasting edges.

Once you've got the right blending, you will likely notice that the images have lost some of their colorful lustre. This is because color saturation is related to the overall range of the exposure, and blending across the expanded exposure range has not expanded the color range as well. Flatten the image, and boost your overall color saturation to taste.

Next Section: Selective Portraiture

Contact Ed Halley by email at ed@halley.cc.
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