Among the factors that contribute to mistakes is the tilting where an image is either "falling" to the left or "falling" to the right. It is very easy to make this mistake and there are several theories that explain it but I would say that we are generally concentrated on the subject and we forget the background. The direction of the tilt varies but it seems to tilt more to the left because we grip the camera with the left hand and control it with the right hand, the latter being lighter and delicate.
The end result of a tilted picture is that it transmits a sensation that something is "wrong" and unpleasant.
Let's open a parenthesis here: sometimes, pictures are taken diagonally with the purpose of seeing things from a different perspective. I am not talking about these "tilted" ones because they are in fact diagonal.
Therefore, a picture is considered tilted when the inclination of the horizon line in the picture (imaginary or present) lies between the actual horizon line and those diagonal pictures (that is, the pictures intentionally taken tilted). The objective here, then, it is to correct the tilted pictures or the "wrong" ones.
Let's consider the example below.
|Fig 1 - A fisherman in Etretat, France.|
Nevertheless, he seems to be struggling not only with the fishing rod but also with the fact that he is "falling" to the left. Also, the viewer has the impression that the piece of white rock in the background will fall on his head any minute.
Therefore, this picture sends a message of struggle and pain although taken in a beautiful place simply because it is tilted. Given that our natural reaction is to seek "protection" against pain, we tend to ignore it and go for the next picture.
To start the fix, you have to find an obvious or logical line in the scene that you know is leveled up like the horizon line, the surface of water against the horizon, any kind of frames like doors, windows, edges of buildings, etc. In particular with buildings, you must be careful because they might distorted or be shown in perspective in relationship to the borders of the picture due to the angle that the picture was taken. Use them only as the last resort.
|Fig. 2 - On the way to make it straight.|
In the photo above, the obvious reference is the water line and the next step is to straighten it up to the it becomes something like the Figure 2 where there was around two degrees rotation to the right. The black area displays this.
After the rotation, you just need to crop in the dotted lines to produce the final image as shown in Figure 3.
If this photo would have been taken in such a way that water line would not be visible, another alternative to find its right alignment would be to imagine a vertical line that goes through in the middle of the fisherman's head all way down to the middle of his legs. Although not perfect, in this case it seems that he is quite straight and this approach will work quite well.
This procedure is quite simple but there is a better and even simpler alternative to it: to take the right picture in first place for two main reasons:
1) There will be no need to spend time fixing the picture;
2) Depending the amount to be fixed (namely, the rotation applied), significant portions of the picture will be cropped out which might render it unusable.
|Fig. 3 - Now, the fisherman is spending all his energy to do his real job: fish!|
The rules to take the right pictures are also very simple and only require a little practice. Instead of pointing and shooting, pointing and shooting, just point to your scene, observe how the scene is being displayed in your display or viewfinder, notice the horizon or feel it whenever it is not there and then - very gently - shoot. You are going to notice much better results coming out from your camera.
A trick to assist you in doing the proper alignment is to set a grid that some cameras have available. This grid usually divides the screen in 3 by 3 rectangles and provide a good reference.
Where do you go from here? Basically, you need some tilted pictures and a software to correct them. Regarding the pictures, I am sure that you must have plenty of them just like everybody has (including myself).
As per the software, the piece of choice par excellence is Photoshop, quite sophisticated and expensive too.
You can download for free GIMP which tries to mimic Photoshop.
Another excellent program to start with and use free of charge is IrfanView with lots of other picture operations.
A good alternative is Picasa, also for free, that performs other operations needed for photographers .
Finally, if you have Windows and you installed Microsoft Office, you must have available the Microsoft Office Picture Manager which is a very simple tool to start with.