This article is intended for the DSLR camera user, who has some notion of what DOF and apertures are, in relation to imagers. However, I welcome you to read the following regardless of your photographic experience or level, because it will eventually make sense to you. Be sure to check my other articles, as I will be covering apertures and Depth Of Field terminology for basic users in the future.
Aperture And DOF
When I'm asked about a camera's unsharp behavior from users, I immediately ask them about what type of camera they have--DSLR or a compact camera (DSLR-Type, Point & Shoot, etc.). Many DSLR camera owners are having difficulties understanding Depth Of Field (DOF), and how it relates to apertures, and imagers. One of the reasons for this and why there is so little experience with DOF, is because compact cameras have been used for the majority of a camera user's life, and are now discovering (albeit indirectly) the aperture ranges of a DSLR, have a much wider variance than their compact camera.
When shooting an image at f/2.8-f/4.0 (which is the general aperture range on compact cameras), most of the image is going to be in focus automatically (i.e, there is going to be a very large Depth Of Field). The reason, is because of the size of the imager. Basically put, the smaller the imager in relation to a 35mm camera, the larger your DOF will be. The average imager on a compact camera gives you approximately FIVE TIMES as much DOF as a 35mm camera. In other words, if you're used to shooting images with a Point & Shoot camera, and you shoot at an aperture of f/2.8, it would be like shooting at f/14 on a 35mm camera (5 * 2.8 = 14).
If you have purchased a Nikon D70, Rebel XT, or a Canon 20D, the DOF problem isn't so bad, but you will still be experiencing the pains of getting used to a different Depth Of Field and aperture settings if you've been using a compact camera most of your photographic life.
So, let's get to the first reason of why your images are unsharp. Since your DOF was significantly larger when you had a compact camera, and since you have a DSLR now, this could be the reason. Be sure to check your aperture settings on your DSLR and make mental notes of the apertures when you shoot. And probably the hardest thing you will need to do, is get used to the new DOF of your DSLR. I'm not going to tell you to FORGET the DOF of the compact camera you used in the past, because you might be still using it as your backup camera. So, you're going to have to learn two different sets of rules for shooting. It sounds hard, but it's not, once you get used to doing it. Practice, practice, practice.
Check the Focusing Of Your Camera
Unfortunately, due to the exponential increase of DSLRs being made, there are more defective cameras. The first step is to take your camera and lens and point it at an object that is not multi-patterned. Next, set your camera to it's largest aperture setting for the lens (the smaller the number is the largest). Find something with a solid color, a fair amount of light (but not too much), and shoot with your back to the sun on the object. Then, shoot at f/4.0. Then shoot at the next aperture setting. If you have a notepad, I highly recommend writing a few notes about your observations for each aperture setting. Record when the camera hesitates, what aperture setting, what ISO setting, and what shutter speed setting.
Check the Focusing Of Your Lens
And wouldn't you know, due to the exponential increase of lenses being made, there are more defective lenses than ever before too. Now, I'm not saying there are so many lenses which are problematic you should carry around a cross and garlic to protect yourself, but for those few people who do get a defective lens, it really doesn't carry much weight to say, "But hey my friend, most lenses are fine!" Initially, check your lens for any mechanical issues. Look through both sides of the lens against a relatively bright lamp (whiter the light the better) and see if you can spot any lens particles. Second, get the widest aperture of the lens you can, then shoot at an illuminated white material--could be a piece of white paper, white sheet, what ceiling. Just be sure the white object you shoot on is absolutely clean of any spots or whatever. Why? Because this could mislead you into thinking you have particles within your lens.
The Quality Of Your Lens
Suffice it to say, you really do get what you pay for. If you've gone out and purchased a $100 lens when most other major brand manufacturers sell an equivalent lens for 5 times as much...there's a reason for it. So, this could be an unfortunate problem of your images not being as sharp as they can be.
Electronic Contacts Of Your Lens And Camera Mount
SOME 3rd party lenses can perform just as well the major brands. However, this performance will often VARY FROM LENS TO LENS. Read that again. I've experienced this myself so I can say it without a doubt. I will not mention names but a particular 3rd party 15mm lens varied so much in quality I think a trip to a casino in Vegas would have resulted in better odds. Since you're reading this article, you have access to the net. Do some research BEFORE you buy a 3rd party lens. Do this even for the major brand lenses, but especially for manufacturers such as Sigma, Tamron, Tokina, and a plethora of other brands. Trust me, you don't want to get an inferior lens and find this out after a once in a lifetime vacation, a paid wedding, or even a special moment you want to remember. It's just not worth it.
The electronic contacts of 3rd party lenses are very tempermental. The reason is, the manufacturer has to balance its compatibility with Canon and Nikon cameras. Canon and Nikon have very different lens mounts and forms of electronic controls. And Canon and Nikon are not that helpful when it comes to divulging important schematics and electronic diagrams of how to best achieve maximum compatibility. No, what basically happens, is a 3rd party manufacturer buys a bunch of Nikon and Canon lenses, and then takes them apart. To the bone. Then hopefully has an idea of how the lenses interact with the camera.