Taking pictures is as much science as it is art. A professional photographer has to not only be aware of the composition and colors of what he is photographing but he also needs to be aware of the light and the limits of his equipment.
Before he even snaps the shot he is weighing a number of factors, such as how what purpose the shot will have and how he will need to modify the image later on. It only takes a moment to snap a picture, but that moment is an eternity of decision and debate, with mental notes on each shot. Taking a shot is never as easy as it looks.
Breaking it down, there are a number of different phases to taking a picture and they apply to even the most improvised shot. From the moment he decides to take a picture to when he finishes manipulating the image, a photographer is worried about taking the best possible picture; only by putting some effort into the photo can he do the subject justice.
Because of that, the professional photographer will step into a different world in order to better focus on the subject, weighing a number of factors before even pressing the button. When that button is pressed all of the relevant choices have been made.
Lighting is one choice that is made on the spot.
While not all situations are perfect, a photographer can make the situation closer to ideal by messing with his f-stop and ISO settings. A good flash is great as well, but not all situations work well with a flash, especially candids and animal pictures.
Also, sometimes the photographer may be going for a bad lighting effect; a lens flare can help to sell a bright day, for example, or the darkness may be more interesting than a properly lit scene. Lighting is a decision that tends to be a lot more deliberate as it takes actual preparation, such as shifting settings or attaching an external flash.
When possible a professional photographer will bracket his shot, or take a picture with several different shots with variances in the lighting settings. This gives him options later on when he is taking a harder look at the picture. Other lighting effects will be taken advantage of, such as where the shadows lie and bright spots.
All of these become different elements to be played with and worked until the picture is as good as it can be. The goal is to avoid using the flash as much as possible and use natural light if possible; light and shadow are the primary tools of a photographer, after all.
Composition is an obvious choice.
Most professional photographers think in terms of the Rule of Thirds almost by default; centering the photograph is something only amateurs do. By positioning the subject away from the center of the picture the photograph concentrates on what is going on in the picture and creates a more dynamic portrait.
While some photographers do straight portraits, they are the exception rather than the rule; most photographers avoid portraits because they are just too boring. While other rules of composition apply, most of those are kept in mind and applied only as needed.
The colors at play are another consideration.
This goes beyond mere considerations of hue and contrast, as a photographer is looking for the best possible combination of colors. Just as a cook uses different textures in order to enhance the dish he is cooking, a photographer uses the play of colors in order to create the best possible picture.
The interplay between contrasts and tones can enhance a picture just as much as having chewy and crunchy elements in a good dish; by combining them a photographer can create a number of different emotions from the same subjects.
Another option under consideration is the question of sharpness and blurriness. Amateurs believe that the photo must be sharp throughout, that to demonstrate technical prowess is everything. Professional photographers recognize that blurriness can be used to focus on a subject, to make everything else disappear into the background.
One of the first things a photographer will do is to look at the autofocus settings in order to make sure that he can play with different kinds of focus. There are a number of different ways to use focus to concentrate focus on the subject, and a photographer will consider the subject more than the picture itself.
Lastly, a photographer will debate even the timing of taking the shot. While usually he will debate bracketing his shot, taking it just before and after as well as the preferred moment, sometimes there will only be time for one shot. In this case, the photographer is trying to make sure that he captures the shot and there is no way to predict the exact best moment; bracketing ensures that the moment will be captured. However, even then care needs to taken as the shutter is not always a precise instrument. The photographer needs to develop a perfect sense of timing in order to take the best possible shot.
A professional photographer also needs to keep in mind that an image can be manipulated. There is nothing new to this, as images have been manipulated since the first cameras. Software merely adds more tools to the toolbox. There are just some images that require some manipulation to accomplish, and nothing bad should be seen about taking pictures to be manipulated later. By the same token nothing bad should be seen about making subtle tweaks in order to make the best possible image; it is just part of the process.
Ultimately, the thought process of an amateur photographer is vastly different than that of a professional. The amateur is merely looking to take a good picture and just record a moment in time, while the professional photographer is looking to create something that will be appreciated, something that he can be proud of. That difference makes all of the difference in the world.