When observing a scene with your eyes, your brain is processing many depth perception cues.
For example, binocular vision allows your brain to use triangulation to determine distances.
Let's say you're belaying down Lincoln's nose at Mt. Rushmore.
When you look at the visitor center below, the images of it, on the retina in each eye, are very similar.
Therefore, the visitor center is perceived as being far away.
However, when you look at the tip of Jefferson's nose, the images of it, on each retina, are very different.
So, Jefferson's nose is perceived as being close.
A viewer, looking at the photographs you took while climbing Mt. Rushmore, can't use his or her binocular vision to perceive depth.
You can enhance and reduce depth in a two-dimensional photograph with:
The convergence of parallel lines as they recede is a depth cue.
If there are objects of known size in a photograph, their distance can be determined.
Atmospheric haze lightens objects in the distance, producing a depth cue.
This is enhanced further, as the haze becomes more blue with distance (see below).
Reds, and other warm colors, advance from the surface of a photograph.
Blues, and other cool colors, recede behind the surface of a photograph.
Objects with the same hue, but with less saturation, recede.
If you place the horizon in the upper third of a photo, subjects will appear to be further away.
You can also use focal length to enhance or diminish depth.
A wide-angle focal length will enhance depth, using the relative size depth cue.
Subjects close to the lens appear larger, and distance subjects, smaller, compared to a telephoto focal length.
Backlighting can make a subject appear to be more separate from the background, and hence, closer.
A blurry background, and a focused subject, can make the subject appear closer, and the background, more distant.
If you photograph through something, the depth may be reduced.