There are several levels of air turbulence: atmospheric turbulence, local turbulence and internal turbulence.

In addition to air movements inside the tube (they can be seen on a defocused star), bad thermal equilibrium can lead to distortions of the mirrors. The upper face of a mirror cools down more quickly than its other face, then its shape changes and spherical aberration appears. It is very common to see, in a hot telescope, a strong spherical aberration that disappears slowly during the night. It is evident that a very good mirror in bad thermal equilibrium becomes a mediocre mirror, and to benefit from all the lambdas measured in the laboratory (where temperature is controlled !), thermal equilibrium must be irreproachable. Moreover, this deformation of mirrors changes their focal lenght and leads to defocusing (see The focusing).

Because of the air turbulence, the image of a star given by a telescope is usually not the perfect pattern that books show us: it is agitated or blurred. In most of the cases, both effects come together in variable amounts, depending of the seeing and the diameter of the telescope. Thanks to short exposure times, CCD can fight against agitation of the image by 'freezing' it. However, contrary to adaptive optics, CCD can do nothing when the image is blurred.

For more information about turbulence, read High Resolution Astrophotography (see bibliography).