No matter how careful you are when shooting, there will undoubtedly be times when you are left with a photo which is less than perfect.
This might be a small problem such as a slanted horizon on a landscape photo, or something seemingly disastrous like an underexposed wedding portrait.
Thankfully, you can often use your "digital darkroom" to tweak, adjust, and correct your photo, salvaging a seemingly ruined picture or turning a mediocre shot into a great one.
Not every image can be saved, and you certainly shouldn't rely on digital correction as a substitute for proper photographic technique, but as a tool for improving your photos it can often prove invaluable.
Straighten the Horizon
When shooting a scene you often have so much to think about - zoom, focus, shutter speed, composition and so on - that it is easy to forget to hold your camera level as you take the photo. The result is a slanted horizon, which can be very distracting.
Of course the best way to eliminate a slanted horizon is to remember to hold your camera level in the first place, but if you only discover it later it's easy to fix.
Load the image in your favourite graphics program and use the rotate tool to re-align the scene so the horizon is straight. Then simply crop the picture to make it square again.
Crop to Improve Composition
The human brain is excellent at analysing the world, figuring out what's important, and ignoring the rest. When photographing a scene this has the effect of making the subject appear more prominent in our mind than it actually does in the final photo. This often leads to a poor shot where the main point of interest is lost among all the background "clutter".
By cropping our photo we can remove this background clutter, and re-focus the viewer's attention on the object of interest.
When cropping, remember that you don't have to produce a picture that is the traditional 6x4 ratio - feel free to crop your image to whatever shape emphasises your subject best, such as a square, an elongated rectangle or even a circle.
Use Blur to Focus Attention
Modern digital cameras do an excellent job of keeping the vast majority of a scene in focus. But this isn't always what we want, because it can draw attention away from the point of interest to unimportant objects in a scene.
Use a digital blur to lessen the impact of these unimportant features, leaving your main subject in sharp focus. Most graphics programs offer a range of blur types, a Gaussian blur usually gives the most natural look but feel free to experiment.
Anyone who has taken a portrait photo has probably come across the curse of red-eye, a symptom of using a flash situated next to the lens, whereby the flash is reflected off the back of the eye, causing it to glow bright red.
Thankfully red-eye is very simple to fix - simply zoom in on the offending area and colour over the red pixels with black or dark grey. If that sounds too much like hard work, then most programs now offer a red-eye reduction tool which will automatically do the correction for you.
Remove Unwanted Objects With the Clone Tool
In an ideal world, all scenes would be perfectly picturesque and free from distractions. Unfortunately in the real world we are often faced with a situation where we have to include one or more ugly objects, such as lamp posts, road signs or rubbish bins, in an otherwise attractive photo.
Removing these objects used to be a task for a professional, but nowadays anybody with a decent graphics program and a willingness to learn can do it.
Most graphics programs now offer a "clone tool", or more advanced equivalents such as Photoshop's Patch Tool and Healing Brush. These can be used to copy one area of a photo and place it over another area. For example, if your landscape shot is ruined by an electricity pylon, simply clone an area of unspoiled land and place it on top of the pylon. It can take some practice to seamlessly blend a cloned section but it really can rescue a photo which would otherwise be destined for the recycle bin.
Warm Up or Cool Down Your Scene With Digital Filters
The human eye has the natural ability to adjust the way it sees colours, so that white always appears white and all the other colours look "right", no matter what the lighting conditions. Cameras attempt to replicate this but sadly cannot match 200,000 years of evolution.
Using your camera's built-in white balance settings can help, but often we can still be left with a photo whose colours don't appear anything like the way we perceived them at the time. A good example is a photo taken inside which comes out with an orange tint, or an outdoor photo with a blue tint (this effect is caused by something known as colour temperature).
We can correct for this tint by using digital filters to apply either a blue (cooling) or an orange (warming) filter to counteract the colour tint in our scene.
Adjust Brightness, Contrast, Levels, and Curves
Digital editing software programs offer four very useful tools for adjusting your photos' colours - brightness, contrast, levels and curves.
Brightness and contrast are fairly self explanatory - use brightness to lighten or darken an under- or over-exposed photo, and use contrast to adjust the difference in brightness between the lightest and darkest pixels.
The levels and curves tools essentially do the same thing, but they offer much more control over the output. They take a little more learning but you will find that it pays off in terms of the quality of your image improvements.
Remove Digital Noise
Noise is the digital equivalent of film grain. It especially affects photos taken with a long exposure time and those with a high ISO setting (digital equivalent of film speed). Night photos are one type of shot greatly affected by digital noise because they often use one or both of the above conditions.
There are several programs available to remove noise, and one of the best I have found is Neat Image, which is available as a Photoshop plugin or a standalone program. Simply load your image and let the program work its magic, and you'll be left with a smoother, more pleasing image.
Sharpen Your Photo
Sharpening is a technique often used in print media because it can make a photo appear crisper and better focused. The best tool for sharpening your image is called the Unsharp Mask, or USM (Note: the word "unsharp" refers to the technique it uses to sharpen your image, it will in fact make it more sharp).
When applying the Unsharp Mask, experiment with the settings until you get an effect which looks sharper but still natural - when in doubt, use less sharpening rather than more.
Add a Border to Your Image
A plain black or white border around an image can really help to enhance the photo's impact, and give it a more professional look. Avoid patterned or overly complicated borders at all costs; they just look tacky.