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.
Modern cameras are a great time-saver, and they take a lot of the guesswork out of getting an excellent exposure for each shot you take. However, when a computer chip makes the decisions, it can't always read the photographer's mind.
One major area is the automatic "metering" which today's cameras bring to bear. If you take a picture, the metering circuits analyze the available light to decide the best shutter speed and/or aperture to make the image include light areas and dark areas in the most likely desired proportions. Every picture may be different.
Once you start stacking multiple pictures, however, you'll soon realize that some images are brighter and some images are darker. A few clouds passing overhead may change the scene drastically, even though two shots were taken only a minute apart at the same location.
To develop good composite projects like these, you will have to make the images fairly similar in lightness, or some objects will stand out more and feel somehow "wrong" to the viewer.
There are two solutions: make the camera think less when taking the pictures, or adjust the images after you get back to your computer.
The more advanced the camera, the more control you have over your exposure settings. Turn off automatic metering, and specify an aperture and shutter speed explicitly. Take all of your photos to be combined using exactly the same settings. One downside to this solution is that you must become more familiar with the science of photography instead of relying on the camera's own decisions, in order to get the best exposure for a series of shots. Another issue is that you have to have a camera which allows you to control these aspects.
The other alternative is to adjust the images of a series to match each other more closely, using the GIMP. Choose a key image that most resembles the brightness, contrast and saturation that you feel will work the best, and then edit each of the other layers in turn to match that overall exposure. When compositing, you don't need to have an exact match, but the better the exposure match, the easier the detail work will be when you're editing.
Shadows can be a very hard problem to tackle when dealing with composite images. By mid-childhood, the human eye has been trained with millions of examples of real shadows of all descriptions. However, the human eye often has a very hard time explaining why a shadow looks exactly the way it does, and how to reproduce a similar shadow convincingly.
Recreating shadows, or editing shadows that exist in photographs, can be a major complication to doing composition work like these projects.
Unfortunately, there's no magic solution here. There are a series of tools in photo editors like the GIMP, which I'll list here. There is also a lot of hand-editing with these tools, and unique situational judgements which you're going to have to do on your own.
Tip 1: always work on a Duplicate Layer and keep the original layer unmodified behind it. This effectively makes a "back up" copy of the original layer. You can delete the new layer entirely if you don't like the results, or you can make the new copy less opaque to soften the effect of the change you're making.
Tip 2: use the Dodge/Burn tools to edit most shadows. (These are named after similar techniques used in a darkroom.) When you dodge, you reduce the darkness of the image without affecting the color. When you burn, you darken the image, again without affecting the color. If you dodge or burn too much, you can either Undo your changes or you might be tempted to use the opposite burn or dodge tool. For the best color quality, you should prefer the Undo key over reworking an area of the image repeatedly.
Tip 3: use nature's example as often as possible. If you're trying to fake a colored shadow you can't burn or dodge, fake it to look just like similar shadows already in the scene. Use the Eyedropper tool to copy colors from nature, instead of trying to engineer the right color with the color picking dialog boxes.
Tip 4: the edges of shadows get softer as they get farther from the casting object. Stand on a sidewalk, and compare the shadow of your foot, the shadow of your hand, and the shadow of a high tree's leaves. The reason for this phenomenon is that the sun is not a pinpoint in space, but a visibly wide disc of light. This effect is even more pronounced when the light source is nearby, or with wide light sources like flourescent bulbs, because the light source strikes the object from a wider range of angles.
Tip 5: if you draw your shadows or other custom shapes on new transparent layers, rather than on a layer with imagery already on it, then you can use many filters to adjust the shadow's shape and intensity itself. Experiment with the effects of Gaussian Blur or Motion Blur, or adding various types of Noise to your new layers.