The sharpness is one of basic factors of a good picture. Factors, which affect it are generally well known – exposure time equal to the inverse amount of the focal length, continuous release control, stable position of the photographer including firm camera holding...
There are also two other factors, which are not so discussed but can also have an indispensable effect for the picture sharpness:
1.Lens with a high lens speed – high-quality lenses are quite expansive. Photographic technology made a big progress and that is why you can sometime hear: „Why should I buy an expensive lens with a high lens speed if I can reasonably adjust ISO without a big noise increase, i.e. quality decrease? If we do not speculate about other aspects ( depth of field for example ), it could be right.
BUT: light going through the lens is divided to the part reflected to the viewer ( you can see the scene ) and to the part going to the AF sensor ( adjusts the zoom to make the picture sharp ). Of course, if there is more light ( lenses with high lens speed ), the AF sensor can make its job much better and you recognize it especially during bad light conditions.
2.AF points – according to the type of camera it can have from 3 to 51 ( generally ) of them. Mainly in middle class cameras, probably in an effort of producers to save money, all points do not have the same sensitivity. Some of them are more sensitive for horizontal, some of them for vertical edges ( and their shape corresponds with it – look to the picture ).
Only the sensor in the middle ( Nikon D90 ) is the cross-type sensor and is sensitive for both horizontal and vertical edges and makes the best job. In the praxis, it may be advantageous, especially in case of poor lighting conditions, to use so-called point-recompose-shot. To focus the object with the middle AF point, recompose the picture and finally shot. The AF system is a very useful technology, but it is good to know its limits.
Have a nice light!