Now that the digital sensor has fully replaced film, there is something all photographers need to be concerned with - dust on the sensor. However, many do not recognize the difference between dust on the camera sensor versus dust on the mirror or view finder. Although the latter is annoying for some, it does not affect your image. This post will discuss how to recognize where the dust is and what to do about it.
When there is dust on your sensor, it will show up on your image as small dark dots. The best way to detect dust on your sensor it to take a photo of the clear blue sky by turning off your auto focus and putting your lens to infinity focus. Then review every part of the image by zooming in on the image and inspecting it for small dark spots. If you see these dots, then your sensor is dirty and needs to be cleaned. Although it is possible to clean the sensor on your own, it is not necessarily advisable. Please note that it is not possible to detect a dirty sensor by looking through the viewfinder of a DSLR camera. When you are looking through the viewfinder you are not using the sensor to see the image, but rather you are looking at an image that is reflected onto a mirror and through a glass prism.
If you do not see these dots on your actual image but see dots when looking through your viewfinder, then the culprit is on your mirror, in focusing screen or in the rear view finder. First start with the easiest part - the view finder; take a q-tip with just a little bit of water on it and clean the view finder thoroughly. If the problem is not solved, turn your camera off and remove the lens in a dust free environment. Inspect the mirror in the camera body beneath the the lens which was just removed. If you see any dust on the mirror then turn the camera upside down and blow the dust off. You may also use compressed air or a bulb blower. Be careful not to hit the camera too hard with compressed air. This should fundamentally clear the mirror and focusing screen above the mirror at the same time. If you still see dust on the mirror, then use a q-tip to remove it. If the mirror is clean but you still see dust in the view finder, then it is on the focusing screen. This screen is a bit trickier to clean because of its location. Check your manual to see if you can remove the screen. If so, then remove the screen, per the manufacturer's instructions and clean both sides of the mirror outside the camera. Please note that sometimes pushing dust on your mirror around with a blower may actually send it to the sensor!
The bottom line is that dust on the mirror and focusing screen does NOT affect image quality at all. Don't waste too much time cleaning this and don't risk damaging your camera for a couple specks of inconsequential dust. If you must, send the camera back to the manufacturer for a cleaning. They'll clean the sensor and everything else at the same time! The manufacturer is also the most trustworthy to clean the sensor.