No matter how much care one takes, dust accumulates on sensors of DSLRs

Dust appears as spots, as shown at right (crop)

Spots are usually not seen except in clear areas such as sky

The main incidence of dust entry probably occurs when lenses are changed

Periodic cleaning is required, usually a matter of months

Manufacturers provide instructions for do-it-yourself

Extreme caution is required to avoid damaging sensor

Dust on CCD