The Horizon Line
For your consideration, may I present, the lowly horizon line.
The horizon line is the very definition of "lowly" given how "down to earth" it is. The horizon line separates the earth below from the sky above. Although it appears to be a line, the horizon is of course actually curved. The earth itself is curved, so the image of it stretched out in front of us must be curved, even if the curvature is so subtle that the human eye doesn't detect it. Go high enough up into the atmosphere, and that curvature would be easily evident.
Frame the horizon line in a camera viewfinder and you may or may not see it as a straight line. Optical anomalies caused by the extreme field of view in some wide angle and fisheye lenses will show the horizon as curved no matter what its real shape. Used as a creative tool, such lenses can be a considerable source of fun and amusement. Anyone who's ever looked at themselves in a carnival funhouse mirror understands how much fun it can be to play with perception. A lens with a more "average" mid-range focal length though should be able to render things closer to the way we see them.
Yet there are inherent differences.
The horizon line can be a source of significant frustration for many new photographers. The darned thing just refuses to lie flat. Time and time again, they find out only too late that the horizon in an otherwise good image is crooked. The human eye doesn't require your head to be perfectly level to see reality as level. For the most part, no matter what angle your head may be tilted at, you will still see and understand the horizon as being flat, level, and normal. Yet another of the tricks your brain performs on your behalf, generally without you even noticing. And so, people tend to use a camera as if it, too, could see the world as level regardless of its true orientation.
Alas, such is not the case. And even when we know with certainty what we will end up with when we don't check that the horizon is level before we shoot, we frequently end up with crooked horizons nonetheless. It's a hard lesson to learn since we never had to consider such matters before picking up a camera. If you use a tripod that is too short, the difficulty becomes even more pronounced. Stooping down and tilting your head to see through a viewfinder below eye level will make you an even poorer judge of what is level. A spirit "bubble" level is about the only way to gauge inclination accurately.
But even once you've mastered the art of getting the horizon to stay level, it can still cause frustration for photographers. The problem is of a more existential nature. What do you do with the horizon? Where do you place it? Looking at the world with your naked eye, the horizon is naturally at ground level where it belongs. But a camera frame crops out all frames of reference outside itself. What you see is what you get, nothing more. The horizon line exists only in relation to everything else within the frame, and perhaps most notably, to the frame itself. Mountain lake reflections at dawn do lend themselves to placing the horizon line down the middle, lending equal weight to real mountain above and the reflection below. But such placement for most other subjects will generally look awkward. Most often, placing the horizon above or below center will yield images that look more natural. Precisely where you choose to place it will depend on which half of the frame you want to have more visual weight. Intentionally positioning the horizon very near the top or bottom of the frame can be another way to have fun with the differences between human and camera perception. There's a whole world beyond that that we are aware of that our cameras are oblivious to. We see the world as more than just what is within our immediate eyesight.
The horizon line also tends to serve as a form of "no man's land" for most objects. Things on one side of the horizon line tend to stay on that side. Things on the other side tend to stay on that side. There isn't much fighting for turf between the two camps. Nor fighting for sky, if you'll pardon the horrible pun. Trees and mountains appear to rise up from the line, but most everything else seems as if it knows and respects its place in the order of things. Rarely do you see pigs fly, for instance.
Another way to play with perception is to photograph objects "on the wrong side" of the horizon. Birds land often enough that we accept them standing on the ground, but clouds don't belong anywhere other than in the sky. Recording them as reflections in water can yield some surprising and rewarding results. Getting down low to shoot up at flowers and mushrooms can also be fun. This sort of thing allows the photographer to show things in a way they aren't normally seen, yielding images with increased impact potential.
This horizon "no man's land" is the premise by graduated neutral density filters work. Such filters, gray (or "neutral density") on one end and clear on the other attempt to cut down brightness from the sky while allowing full exposure for the ground below. Trees and other objects that dare to cross the horizon line create the tell when looking for the use of graduated filters. Today, HDR techniques, though far more tedious than the use of graduated filters, provide a means of dealing with extreme brightness differentials that don't rely on things staying on their own side of the horizon. As such, they open considerable creative potentials for working with points of view that break the horizon line.
It's easy to view the lowly horizon line as irrelevant in photography, or perhaps down right inconvenient when its presence lets on that the camera in question was crooked at the time the shutter was pressed. But just as with everything else, it can serve as a useful creative element, one best not overlooked.