So, how many times have you been given the following helpful piece of advice or criticism?
"Always keep your horizon level"
It may not sound like such a bad rule on the surface, and for the image to the right, it certainly seems to make sense. :-) :-)
Sadly, I find that it is one of the most overused comments from new photographers and by those who like to throw around rules without actually knowing what it is that they are talking about.
Lets take a look at why this statement rarely applies in the real world and why you need to question the other person's reasoning when you hear it being said.
This statement can only apply in one situation; when you can see the horizon over a large body of water that extends past your ability to see any further due to the curvature of the Earth. In this situation it is accurate and in most cases should be followed. (naturally, as with any rule of photography, rules are made to be broken -- when you know how and why).
In all other situations this "rule" can not possibly apply. In any situation where land is at the horizon, it may be affected by changes in elevation and by perspective.
The absolute dumbest comment I hear is "Always keep your shoreline level". Again, on the surface it almost sounds intelligent and to some it seems to mean the same thing. But think about it in places where the shoreline is not absolutely parallel to your point of view. i.e. where one point of the shore is closer to your point of view than the other.
Lets look at the following image. The shoreline is obviously not level.
And at first glance, our "expert" photographer buddy recommends that we need to straighten the horizon. OK, lets take a look at what happens when we do....
Is it better? Well, out of context I suppose I can see why one might think so. The waves do appear a slight bit off no, but maybe only because I know the context of the original scene and why this could no longer be correct. BUT WAIT... even out of context, looking closer now, one has to ask themselves; Why would someone build a lighthouse on an angle? Is it about to fall off the cliff? It is now clearly tilting towards the cliffs.
Perhaps with a little more context we can see what is going on here.
The shoreline to the right is cut off but you can still see more clearly that the shoreline is extending away from us from the right, out towards the left out into the ocean beyond the lighthouse.
If you look at the actual horizon you can see that the original image is in fact level even when the shoreline was not.
It has instantly become clear that our "expert" photography buddy didn't really pay attention to detail, and likes to brag about his vast technical knowledge -- knowledge that is clearly lacking.
Look at the following side by side. Out of context, the shoreline looks like it may need some straightening adjustment. Again, straightening the shoreline clearly introduces problems with the monument at the center of the image.
This is a bit of an extreme example, but it is honestly not too far off from a recent image for which I was quoted this rule of photography by an "expert" photographer. If the person had actually looked at the subject and understood the "rule", it would have been clear that the image was in fact level, regardless of the appearance of the so called horizon. Instead the person was more interested in impressing a group with his knowledge of "the rules", and ended up looking foolish.
So, my simple tip for today to help stop people from looking like fools is to stop re-iterating rules for the mere sake of expressing your knowledge of them.
Keep in mind that truly level horizons are rarely possible, nor desired!
We need to stop looking at the horizon and start looking at vertical lines. If there are any manmade object in the scene, this is more reliable method of determining what is "straight". There are few situations where manmade objects are intentionally created or left tilted. The "Leaning Tower of Pisa" comes to mind. Not much else.
Just be aware of wide angle lenses creating curved or keystone effects which will affect vertical lines near the sides of your image. It is best to look for correctable vertical lines near the center of your image when possible.
When there is no context for what is level of straight, then it may be perfectly acceptable to rotate the image create a sense of a level horizon... whatever that may mean for that specific image.