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Composition: Putting Things in Perspective
Perspective refers to the appearance of depth or spatial relationships between objects in an image.  The world we live in is three-dimensional whereas photographs are two-dimensional representations of that world. One of our jobs as photographer is to represent the world around us in two dimensions.
The more we are able to control the 3D to 2D translation in our images the more we will be able to create powerful compositions. Perspective is an easy concept, but not one that is well understood. For instance, consider this. True of false: wide angle lenses distort perspective whereas telephoto lenses compress perspective. These are commonly held concepts that are, in fact, both false. Perspective is controlled solely by the relative distances between various elements of our composition and is independent of focal length.
Size and Distance One Object
To see why this is false, let's start with a simple idea: the linear size of an object in an image is proportional to the focal length of our lens.  That is, if we stay in the same location and double our focal length, we double the size of our subject; if we halve our focal length, our subject shrinks by 50% in height. Switch from using a 50mm lens to a 100mm and our subject gets twice as big; switch back and it shrinks back. Sounds reasonable so far, right?
How about this then: the linear size of an object in an image is inversely proportional to our distance from it. In other words, if we keep our focal length the same but step back from our subject to double our distance from it, the size of our subject will shrink by fifty percent. Cut our distance to the subject by half and it will double in size (height and width, not area). This is true regardless of whether we are using a 24mm wide angle lens or a 200mm telephoto. No matter what, if we double or halve our distance, the image size is halved or doubled accordingly.
Perspective Two or More Objects
Still with me? Good. Now let's add a second identical object. If we place it twice as far from the camera as our first object, we know that it will appear half the size. If instead we place it halfway between the camera and our first subject, it will look to be twice as big as the first.  Again, this is true no matter what lens we use. If we switch from a 25mm lens to a 50mm lens, both objects double in size.
If we start with one object 10 feet away and a second identical one 20 feet away (twice as far) it will appear half the size of the first. If we now move twice as close to the first object so it is now only five feet away, it will now appear twice as big as it started out. But what happens to our second object? The distance to it will now have changed from 20 feet to only 15 feet. This new distance is only three-fourths of the original distance so the second object will appear 4/3 (the inverse of three-fourths) of its original size. Relative to the second object then, we made the first object appear bigger than it did, and we did so simply by moving closer, without changing lenses at all. Again, if we do change lenses, both objects change sizes proportionally to the change in focal length so there would be no added change in perspective. Go from a 50mm lens to a 25mm and both objects shrink by half and they maintain their same size relationship to each other.
Changing lenses then didn't change the perspective of our scene while changing our distance did. Why then do people think wide angle lenses exaggerate perspective? For only one reason: you can focus closer with a wide angle lens than you can a "normal" or telephoto lens. In fact, you almost have to get closer since otherwise your subject would appear rather small. Conversely, telephoto lenses do not compress perspective. It is only that they are most often used for photographing objects far away from us such that the relative distance between objects in the frame isn't as extreme as it would be with a wider angle lens.
Showing Perspective
There are a number of ways that we can create the appearance of perspective in our compositions.
Blocking  An object that partially obscures another in the frame is clearly closer to the viewer than the object being blocked. This is clearly true regardless of the size of the objects involved. The effect can be easily achieved regardless of lens or subject distance.
Relative Size Two objects that appear similar except for size are assumed to be at different distances in proportion to their size. The closer we get to the nearer object, the more extreme their relative distances will become, and thus the more extreme their apparent sizes. If we focus on a fence that is receding into the distance, the fence posts nearest us will appear larger than those further away.
Converging Parallel Lines Parallel lines that move away from us appear to converge at the horizon in the classic "vanishing point" phenomenon. Railroad tracks seem to merge together at extreme distance as do the sides of roads or rivers. This effect is simply another aspect of the relative size effect. Even though the railroad tracks are actually a fixed distance apart the railroad ties that separate the rails will appear smaller and smaller as they get further away from us.
Lack of Sharpness or Contrast Any fog, dust and other impurities will make objects further away appear somewhat hazy and lacking in contrast. This is always true, but water vapor evaporated off of vegetation and bodies of water by the afternoon sun will bond with any pollution present making the effect even more apparent. Indeed, in this day and age, the problem is so severe it can be hard to take a good picture across a valley or other expanse at that time of day.
More on composition next week ....
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