Perspective is the sense of depth or space between objects in a photo, along with their dimension (2D or 3D), in relation to the viewpoint of the camera lens.
Perspective can vary depending on two things:
1) Your camera lens
Different lenses have different focal lengths which will all have different angles of view. For example wide lenses will offer a wider perspective than an image taken on a telephoto or macro lens zoomed in close to a subject.
Depending on where you are positioned in relation to your subject, you can get lots of different perspectives using the same focal length as shown in the photo below.
2) Your position
Changing your position can mean moving closer to or further away from your subject on a horizontal plane, but it also means moving up and down the vertical plane, shooting from above or getting down low.
As a photographer you need to learn to use the differences in size and distance to improve your composition, no matter what you are shooting. There are several ways you can add depth and three-dimensionality to an image:
The human eye views objects as being smaller the further away they are, relative to their size. When we look at a line that is travelling away from us, it appears to get narrower until it reaches a single point at the horizon (the vanishing point). This tells our eyes that the narrower the line gets the further away it is. This is called linear perspective.
By composing a photo with any continuous line (a road, fence or path, for example) leading from us to the horizon, rather than running across the scene, the viewer will get a sense of distance, like in the photo below.
Interposition is when we perceive an object to be further away if it is partially overlapped, by another object in front.
By placing an interesting object in the foreground of photos that overlaps the main subject, you can create a perception of depth, like in the image of the Falkland Islands below.
The same can be done by putting your subject in the foreground with another interesting object behind. Just remember to ensure the combination of subjects enhances your composition, rather than making it seem busy.
Shadows and Form
When we see a shadow behind an object or across part of an object, we visually perceive that object to be three dimensional.
To take advantage of this, place the sun, or your light source, off to your side, so that it casts a partial shadow across your subject, giving it form, like in the photo below.
Image taken by Wojciech Toman
This is a fairly obvious one. When you have two or more identical objects in the frame, those that appear smaller are seen as being further away, while those that appear larger are seen as closer.
By including multiple similar objects at varying distances we can demonstrate a sense of depth in our image.
Position yourself close to one of the objects, and compose the shot with its replicas located further away from you.
Image taken by Nicholas A. Tonelli
On the ground, when an object appears higher up in the frame, we perceive it as being further away. But in the sky, when an object appears higher up in the frame, we perceive it as being closer.
So how can we use this to our advantage? To help demonstrate distance on the ground, find a slightly raised platform (like steps) to stand on. Then compose your shot so that the horizon is very high in the frame, with little or no sky visible. The elements at the top of the frame will appear significantly further away, like in the image below.
When photographing a moving subject in the sky, (such as a bird or a kite), wait for it to get lower to the ground if you want it to appear further away, or catch it higher in the sky if you want it to appear closer.
Image taken by Stuart Hamilton
Forced perspective is where you make objects in the distance seem larger, smaller, closer or further away than they actually are. This is done by strategically placing a subject in the foreground, such as when people appear to be holding up a building that is actually much larger in size, or holding something far away in their hand, like in the photo below.