(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.

Setting up the Shot

Use a Good Tripod

The techniques in this tutorial each depend on having multiple photographs from the same vantage point. While it's possible to take two pictures from almost the same place using your hands, I recommend the use of a good tripod.

There are many forms of tripods on the market, and almost all cameras can be used with almost any tripod. The type and size of the screw mount for tripods has been a standard for many years. Which tripod you choose depends on your needs.

Typically, the heavier the tripod, the more stability it will give you for long exposures or for matching multiple exposures. Using miniature "table-top" tripods are almost like holding the camera yourself, unless they can be strapped or suctioned or magnetically attached securely to a firm surface. Long thin legs with lots of adjustments may be convenient for packing and storing, but each joint on a tripod leg may potentially weaken the stability and allow jitters to affect your photography.

With that said, my examples here were all taken with a fairly inexpensive, lightweight tripod. Digital editing gives you, the photographer, a lot more leeway for slightly imperfect shots. Select a tripod that matches your budget and your goals for what applications you'll have for it.

Don't Touch the Camera

Tripods are designed to hold the camera for you. This allows you to position the camera where you like, and then take the picture without touching the camera.

There's just one catch. How do you actually take the picture, if you don't want to touch the camera?

The simplest way, without extra equipment, is to use the camera's own self-timer feature. All but the cheapest cameras have them. Usually, you select a self-timer mode, and then squeeze the trigger release. Some time later, often ten seconds, the camera will then snap a photograph without your hand on the camera. Read your camera's manual to learn how to use the feature on your model.

More advanced cameras will have other options for taking a photograph without touching the camera itself.

Some cameras have a small threaded port where a cable release may be attached. A cable release is a long snakelike affair, with a screwmount at one end, and a squeezable trigger at the other. This allows you to take the picture at the moment you choose, but with a minimal upset to the camera's position.

Recent cameras have infrared or radio remote-control options, just like a television or stereo component. Hold the remote control discreetly and command the camera from afar. Some films or digital camera sensors are sensitive to infrared light, so you may need to take care not to get a sparkle from the controller into the image itself.

While this technique is suitable for film or digital cameras, the only sort of camera which would pose a slight problem would be for film cameras that require manual intervention to advance the film, such as those old all-manual cameras with a film advance lever or knob. These are not impossible to use, but extra care will be required to take a series of photographs without jarring the tripod too far out of position between shots.

For those using film cameras, this tutorial assumes you can and will develop and scan the film or paper into a digital file format before we begin.

Align the Images

Once you have multiple photographs from the same vantage point, you'll want them merged into one file to be able to composite them. The GIMP has a window for controlling several images at once as layers. These layers are composited automatically and can be edited individually. If you don't have your Layers, Channels & Paths dialog open, find it in the GIMP's main menu.

Here's a quick example for aligning two images as separate layers in the same file. I took two photographs of a traffic signal, one with the green lamp lit, and one with the red lamp lit. You may want to try the same with your own photographs. Open both images. In the Layers dialog, you should see a thumbnail for one of the images. Drag the layer thumbnail to the other image. (It doesn't really matter which image goes onto the other image.) The first screenshot below shows the red-light photo being dropped onto the green-light photo. The result is a single file with two layers.

dropping a new layer

reducing layer opacity

aligning layers

Now that the images are both in the same file, our goal is to align them visually. Even with an excellent tripod, a day with no breeze, and a perfect shutter release, there may be some small misalignment which would pose a problem to some projects.

In the Layers dialog, you can select any layer you like, and edit that layer without changing the others. Imagine them like sheets of translucent paper stacked on top of each other. The top layer listed in the dialog box is at the top of the stack, and shows up on the image window. Other layers are hidden behind this top layer. If you click on the eyeball icons next to each layer, you can show or hide the layers individually. For example, if you hide the red-light photograph layer, you'll see the green-light photograph beneath it. For now, select the top layer and make sure the eyeball icon is on, so that we see it in the image window.

It's difficult to see how these layers align when you can only see one of them at a time. If we had X-Ray vision to see through the top layer slightly, we could see how the two images matched up. Reduce the opacity of the top layer. As you do so, the top layer will fade away accordingly. At 100% opacity, the layer is opaque and you can see nothing through it. At 0% opacity, the layer is completely hidden. The middle screenshot above reduces the opacity to about halfway transparent. This way, we can see both the top layer and the bottom layer at the same time. The example photographs don't align very well, as you may see.

The next step is to align those layers so the images mesh up. In the third screenshot above, we have chosen the Move tool and used it to move the top layer to line up with the image in the bottom layer. You can drag the image around in the image window, or you can use your arrow keys for fine control. Zoom in on the image if you like, using the image's View menu choices, to get the best alignment. The street-sign looks as if it were one photograph, and the traffic signal's cables have merged also. Note the signal looks like both the green and the red signals are halfway lit. This is due to both images being visible at the same time.

If the top layer were turned or twisted, this would make for a more challenging alignment problem. The Rotation, Shearing Scaling & Perspective tool can help with such challenges, but is beyond the scope of this tutorial. If taken with a tripod, multiple images should always get a reasonable fit with just a little moving of each layer.

Save Layered Images

When you want to save your progress, use the GIMP's own XCF format instead of a flat image format like JPG or TIFF or GIF. The GIMP's files will remember each layer, their opacity settings, their alignment and all the other GIMP features you've been using for editing.

When you're done with your masterpiece, you can always export a final image back to a flat image to share on websites or print to the printer.

Next Section: Creating Translucent Objects

Contact Ed Halley by email at ed@halley.cc.
Text and artwork are Copyright © 1996-2005 Ed Halley
Copying in whole or in part, with author attribution, is expressly allowed.