Photographs have no depth. They're two-dimensional facsimiles of a three-dimensional world.
But photographers, like magicians, deal in illusion. If we understand what creates the illusion of depth in a photograph and how to enhance that illusion, we can make our images more believable.
Perhaps the most common way to suggest depth in a photograph is to have objects of recognizable size in both the foreground and background. Here's an example:
The tufts of grass in the foreground are taller than the mountains in the background. A viewer might conclude that we are looking at steroid-enhanced grass and shriveled mountains. A viewer is more likely to conclude, however, that both grass and mountains are of relatively ordinary size, and that the mountains are much farther away than the grass. When a viewer draws that inference, the illusion of depth arises.
Once we grasp this idea, we understand one reason why including foreground interest in a landscape photograph can make for a better photograph. Images that have no foreground interest can appear flat and lacking in depth. Where I live, attractive backgrounds are abundant. Most of my effort goes into finding something interesting and appealing to include in the foreground.
Another common way of suggesting depth is to include converging parallel lines on an image. Consider this photograph, which was taken in a stand of aspens in northern Arizona last fall. (By the way, the effects of compression have produced color banding in the sky in the following photograph that isn't present in the uncompressed version.)
This could be a forest of weirdly slanting trees. Our minds are more likely to conclude, however, that the aspens are really upright and parallel, and that their apparent convergence is due to the fact that the treetops are far away. Our minds draw a similar conclusion when any lines we decide are parallel--including railroad tracks, fence rails and the sides of a highway--converge as they approach the background. In reality, of course, the lines in such photographs are not parallel at all. The perception that they are parallel in the viewer's mind fosters the illusion of depth.
The mind will also infer depth when objects in the background exhibit less contrast and less color saturation than objects in the foreground. That's because our planet's atmosphere scatters light. The effect increases with distance. Whether we've ever thought about it or not, our minds will read the softness and diminished saturation of distant objects as indicators of depth. This effect is increased when the atmosphere contains haze, fog or dust, up to the point where any of those obscure the background completely.There are a couple of reasons why the viewer's mind will conclude that the background mountains are far away.
One reason is the small size of the mountains in relation to the foreground. The other is their softness and lack of contrast when compared to the foreground, due to the effects of the atmosphere. We can increase the effect of these atmospheric clues to depth—or mimic them—during processing. All it takes is to subtly reduce the local contrast, the sharpening and the saturation in the background.
Another way to suggest depth is through the use of shadows. The depth of objects in a photograph will be reflected in the kinds of shadows they cast. When we see the kind of shadow that a three-dimensional object casts, we conclude that we are looking at a scene with three dimensions. The prominent shadows in the following photograph are a major reason why the photograph appears to have depth.
This photograph was taken a couple of weeks ago in Petrified Forest National Park, shortly before sunset. The strong side lighting produced long, prominent shadows. The availability of such lighting near sunrise and sunset is one reason why many of us prefer to photograph landscapes at those times. Had I taken the same photograph at midday or on a very cloudy day, the relative absence of visible shadows would have made the result look much flatter.
I had to wait for this shot for precisely that reason. When I first decided that I liked this composition, the sun was obscured by a cloud. I looked at the sky and concluded that the sun would probably sink below the clouds before it set. That's what happened about ten minutes later, and that's when I took the photograph.
It's no coincidence that all of these photographs were taken with wide-angle lenses. The difference in size between foreground objects and background objects is exaggerated by such lenses. So is the effect of converging parallels.
All of these clues to depth are recognized by scientists who have studied depth perception. Humans have two eyes. Our two eyes triangulate the distance to an object through a process known as stereopsis. A camera can't do that. It's like a one-eyed human. Since one of my eyes has never worked well, I know from long experience that one-eyed people can still perceive depth, although not as well as two-eyed people do. We do so by recognizing depth clues that do not rely on binocular vision. Those clues include the relative size of known objects, the convergence of parallel lines, atmospheric effects and shadows—precisely the same clues a one-eyed camera relies on.
Because depth in photographs is an illusion, it's easy for it to be lacking if we don't make a conscious effort to enhance the illusion. That task is much easier if we have a working understanding of what the primary depth clues are.
You can see more of my work at my Smugmug.