1. Image stabilization was left on when you used a tripod. The IS (Canon) or VR (Nikon) feature is designed to be used when handholding the camera. When your gear is mounted on a tripod, though, it should be turned off. There are some lenses that are said to be unaffected by this issue, and they produce sharp pictures whether the stabilization feature is left on or turned off.
In my experience, though, I have never had sharp pictures when the IS function is turned on and I'm using a tripod. This is also true when I'm shooting on safari and my support is a beanbag. I lost some great shots of lion cubs nestled in a gnarled tree because I assumed the beanbag would be similar to handholding the camera. I was wrong. My images were unsharp until I turned the IS off.
3. Your shutter speed was too slow and you are hand-holding the camera. This is one of the prime culprits that result in unsharp pictures. If the shutter speed is slower than 1/60th of a second, chances are that your images will not be tack sharp. This guideline is useful for lenses in the 50mm range and wider. For telephoto lenses, the general rule is that the shutter speed should be the reciprocal of the focal length of the lens. In other words, if the telephoto is a 300mm, then the shutter speed should be 1/300th of a second or faster. If the lens is a 500mm plus a 1.4x teleconverter equaling 700mm of focal length, the shutter speed should be at least 1/700th of a second to insure sharp pictures. This is what I used to photograph the blue grosbeak at my feeder.
When you are shooting in a low-light situation, the strategy you should use is to raise the ISO until the shutter speed becomes fast enough to give you tack sharp images. This is assuming you are already using a large lens aperture. If not, then open the lens up all the way before you start raising the ISO.
5. Autofocus can fail in low light environments. In order for the autofocus mechanism to function correctly, it needs contrast - the difference in light areas of the composition versus dark areas, or the difference between colors. When shooting at night or in a dim interior such as a restaurant or cathedral, for example, I recommend turning the autofocus off and focus the old fashioned way ... manually. This will guarantee that your pictures will be sharp.
6. When there are several planes of focus, the autofocus mechanism can be fooled. This results in unsharp pictures. A leopard in tall grass, for example, presents a challenging proposition for the autofocus feature. It can't know what the subject is, and most likely it will focus on one of the blades of grass and leave the animal out of focus. Therefore, the only solution is to focus manually. That takes all the guesswork out of the equation.
8. Doing macro photography in the wind guarantees blurred pictures. If you insist on doing macro photography when it's windy, even if you use a tripod you might as well buy a time-share in a facility with padded cells! It will truly drive you crazy. Even the slightest of breezes makes macro work virtually impossible. If you are shooting rock patterns or bark, for example, you won't have any problems (unless the wind is strong enough to buffet the camera). However, if you are trying to photograph flowers, leaves, grasses, butterflies, spider webs, seed pods, and other subjects that are at the wind's mercy, then I would encourage you to wait until the wind has died down.
Next, use either the self-timer built into the camera or a wireless trigger to take the picture. I use the 2-second option and it works fine. This prevents the camera from being jarred when your finger depresses the shutter button.
Finally, make sure your tripod is tight. All the sections should be firmly tightened, and the ball head must be fastened tightly onto the tripod itself.
10. Many lenses don't focus correctly at infinity. When you manually turn a lens all the way to the infinity mark, it is reasonable to expect that this means the lens will be focused on subjects at great distances. This is often not true. Sometimes you have to pull the focus ring back slightly. The autofocus mechanism should accommodate this discrepancy, but if focusing manually, it's important to be aware of this. Focus by your eye instead.
Article by Jim Zuckerman. To learn more about photography, explore the many online photography and Photoshop classes offered here at BetterPhoto.com.