Post processing practically involves everything that happens to a picture after your camera transfers data from the sensor to the memory card.
While photographers somewhat belong to different schools of thought regarding post processing and apply them in varying degrees (if at all), there are basic techniques that are undeniably useful for everyone.
George Bailey enumerates these basics nicely in his blog. I have also seen a number of articles that concurs with Bailey, stating that these are perhaps the most important things you need to know how to do. Indeed, these are the extent of my needs for maybe 95% of all the pictures that I post-process:
Cropping and Angle Adjustment
There is no arguing that the ideal thing to do is to do everything you can to properly compose and shape your frame. However, like anything else in the real world, there will always be times when you are faced with less than ideal shooting conditions that will prevent you from composing your shot exactly the way you want. That is why, in times of doubt, I step back or zoom out to give certain allowances in case my aim is off. This will give me some leeway for cropping and rotating an image to get an ideal or at least a best-effort composition out of the original image.
Brightness and Contrast
Again, the ideal practice is to get your exposure the way you want as you shoot, but nobody’s perfect. Fortunately, exposures can still be corrected by adjusting lightness and darkness off-camera. There are many ways to go about this. Some will recommend to go straight to using more sophisticated approaches. But there really isn’t anything wrong to start learning with the basic brightness and contrast controls. In most cases, it will probably be enough.
There are all sorts of ways to tweak a picture’s colors. White balance is usually handled in-camera, but can also be adjusted off-camera if supported by your editing software. The second parameter we should keep in mind is saturation, or intensity of colors. By tweaking saturation a picture can go from having no intensity at all, which effectively transforms a picture to black and white, to having extremely loud colors. Specific colors or ranges of colors can also be tweaked to balance a picture more acutely.
There are really no rules to follow here, as this is very much dependent on the photographer’s sense of aesthetics and objectives. This can also take a lot of figuring out and practice before you can find what works best for you.
One of the first things people notice in a picture is how focused or out of focus it is. Sharpening can, and has been a savior for many photographers. Even with an adequately focused image, sharpening can still work as an enhancement tool. Unfortunately, it can also be overused, especially by beginners.
Sharpness is simply the amount of contrast between adjacent pixels. Applications like Photoshop provides several ways to sharpen a picture. Usually, the basic sharpening filter/effect is adequate. But some people would prefer using other methods which employ more advanced methods.
Manipulation of depth of field in-camera allows you to dictate which areas are to be in focus and which are out of focus. But for one reason or another, the areas supposed to be out of focus end up not being blurry enough for the photographer. Blur can be further accentuated through software. It is also commonly used to mask blemishes on the subject.
However, doing this tastefully requires a subtle touch and a lot of practice.
It is good practice to resize an image that is appropriate to wherever it is to be packaged in. This is especially useful when uploading to the Web. It helps in rendering in the fastest possible time, as well as ensure that what you upload is exactly what you will see in Web pages. In the above examples, the original unedited image was over 3000 pixels wide and 4000 pixels tall. Cropping significantly reduced the size, but was still pretty large 2653×3537. To accommodate small screens and slow Internet connections, I had to further reduce the image to 400×533 to easily fit just about any modern screen and keep download times short.
I am still undecided as to whether this is a good or bad thing, but popular services such as Facebook does its own resizing and optimization to make it more convenient for those who won’t take the time to go through their pictures and download everything straight from the camera. It’s good if you are a casual mass shooter, but bad if you’re the type who wants to carefully go over each shot to decide which ones you want to share.