First of all I want to say that this article does not apply to the use of every camera, but only and exclusively to the usage of digital SLR cameras.
There’s no sense to talk about this for analog cameras, which by its very nature have no sensor to be cleaned. The problem does not arise for any compact cameras and other cameras with fixed lens (not interchangeable) too. Instead the problem may arise for all those digital cameras in which it is possible to change lens.
What happens: when a lens is changed by another for shooting a different subject, some small grains of dust can get inside the camera. At this point there are more possibilities. If the grains are deposited in some hidden meander which are not in the same trajectory of the light towards the sensor and the eyepiece, you will never notice them. If instead the powder is drawn from the sensor or one of the elements that make up the trajectory toward the eyepiece, then it will be possible to see them.
If the powder is deposited on the mirror or more easily on the viewfinder, the advice you will get from any experienced photographer is to keep it where it is. The danger is that it can move, but as long as it is there, it will only cause a little discomfort while you look inside the eyepiece. Photographs will not be affected, because once the mirror is raised, the light does not encounter this obstacle impressing the sensor. If you try to remove it at any cost, is a clear risk of worsening the situation and break the viewfinder which is very delicate and should never be touched.
The real problem arises when the dust settles on the sensor. In this case, you will not see while you are shooting (unless you use the live view mode and have the very large dust grains), but you will realize you got a problem in post-production while watching your images.
There are several ways to check the status of your sensor. Note that the dots will be visible on very low contrast subjects and will be almost invisible on high contrasting and colorful areas. Furthermore, their visibility is inversely proportional to aperture used. Some people will tell you to photograph the wall at f/16, some others to take picture of a computer screen at f/18. I believe that the only serious and professional way to check the status of the sensor is to photograph a blueand bright sky with correct exposure at f/22 with a focal length in a range between 20 and 120 mm.
If doing this test you will notice some dots in your image, you should adhere to the following:
If this attempt fails and the dirt is still there, you can try these solutions:
If the dust grains are a lot or they are quite large or you’re not going to shoot at higher apertures, you need a manual sensor cleaning. You can do it yourself, or contact an authorized service center: