(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.

Controlling Lighting

The Problem

The family is all over for a holiday celebration, and they plead with you for some quick portraits. "The lights in the Christmas tree are so pretty, can you get them as the background?" You consider your options. There's just no way to get the pretty ambient lighting and the flash lighting to match up, unless you start improvising all manner of studio flags, reflectors, gobos, barndoors and beauty dishes.

The Solution

Instead, let's combine multiple shots, one with each of the lighting elements we want. This way, we can skip all the complicated studio light control equipment, and do all the lighting controls at home.

ambient lighting
flash lighting
As in previous examples, we take these two shots from a tripod, so that the background elements will align perfectly. One shot has a dim room with the twinkling tree lights. We ask the models to settle into position for the second shot with flash lighting. We aim or reflect the flash unit up to the white ceiling, so it will bounce the light all around the room and provide soft shadowing for the portrait.

We can inspect the lighting in each shot. The Christmas tree lights came out well, but the rest of that shot is pretty uninteresting. The models are in the perfect position, but the lighting is a little limited and pretty much equal everywhere. We need more separation from their background, and as expected, you can't see the lights on the tree at all.

Screening the Highlights

Now we want to isolate the Christmas tree lights, and boost their importance in the final scene.

First, duplicate the ambient lighting layer, and set the upper layer to "Multiply" mode. This dims the dim areas, while keeping the brightest areas bright. Merge these layers again, and use a black airbrush to erase everything outside the tree lights.

Now we have a nice black layer with a few colored lights and reflections twinkling the way we like them. Put this over the flash lighting image, and set it to the "Screen" mode. This strengthens the tree's lights behind the models, but as you can see here, the lights are also peeking through our models, also. Just keep painting black on the tree lights layer wherever you spot a problem.

just the lights
lights show through
black out some lights

Separating the Background

The room surrounding our models is pretty brightly lit, thanks to the strong ceiling-bounced flash which we had available. This extra spill lighting is canceling the punch our portrait could have, so we should work to reduce it.

We will now work on the flashed scene in much the opposite way that we worked the ambient scene.

Duplicate the layer with the models. Set the top of those two layers to the "Multiply" mode. Let's ignore the faces for a moment and look at the background areas of the room. The darkening effect may be too strong, so we can adjust the opacity of the layer down a bit, to say 50%. It's a matter of taste. Notice how the lights on the tree stay bright while the room grows dim; that work is starting to pay off.

Add a layer mask to that layer. The GIMP will ask if you want it all white, or all black, or a number of other choices. There are several ways to develop a layer mask, but we'll start with an all-white (opaque) layer mask. In the Layers window, a white icon will appear next to the image icon. Clicking on the image icon will let you paint in the image, and clicking on the layer mask icon will let you paint in the layer mask; try it and see how the GIMP indicates which icon is active. (You can Alt+click the mask to see just the mask in the editing window, but for now, we don't need that.)

Click on the layer mask's icon in the Layer window, so we are painting to the layer mask. Select a medium-sized brush and the color black, and start painting wherever the kids are, in the window. If you paint too far, just paint again using the color white. Whatever areas of the mask you paint in white, the layer is allowed to contribute; whatever areas are masked black, the layer will not contribute. By painting black on the models, we ensure that this layer is only affecting the background areas of our scene.

When we're finished, your layer stack should look something like the one in the screenshot below. Don't worry about the layer's names, but notice how they work together to control the lighting in the scene.

mask out models
final layer stack

Next Section: Blending Exposures

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.