Did you ever ponder that dust in the air actually gets attracted to your DSLR sensor because it has a bit of static charge in it? So even though you have the mirror in down position when changing lenses, you end up attracting dust from the air. Your sensor is can thus be termed as a dust magnet.
While most professionals take care to clean up the lens, the lens cap and the filters that they use, dust still gets through into the body and then on to the sensor. While we change the lens, the mirror is positioned down covering the sensor. However the inside of the camera is still exposed to dust; as a result of which dust get can still get through.
The easiest way to detect whether your camera sensor has dust on it is to point it at something white (or neutral), set it to its narrowest aperture and then take a picture. Now magnify the picture that you took and look for black specs on it. If you are not sure, take another image and then compare. If you see the black specs appearing at the identical locations, it means you have dust on the sensors.
Now that we know for sure that we have dust on the sensor we come to a point where we have to roll up our sleeves and do a bit of cleaning. There are two ways to clean a DSLR sensor, first one is easy, send it to a professional cleaning company and let them clean it without damaging the sensor. The second is having a go at it yourself. We obviously want to clean it ourselves so here we go.
Every DSLR (at least all the good ones that we know of) have an option to lock the mirror in up position. This is known as, well, mirror lockup. This is convenient when shooting images when you are hand holding the camera and don’t want any camera shake because of moving parts inside. But it is also great for cleaning the sensor, as it basically keeps the mirror out of the way.
Ok so once the mirror is out of the way, bring out a hand blower and squirt some air on to the sensor. Sounds less techie? This works if you don’t want to spill something on the sensor that you are unsure about. But even though it sounds kind of simple it actually works. However there is a small caveat. Do not use a strong blower such as a motorized one which can be great to work with but could actually damage the delicate sensor and other parts. Having the camera held pointing downwards is advisable. When you are happy, put the lens back on and then take another shot pointing at the white wall. Can you see the black spots still? No? Congratulations you successfully cleaned the sensor. Yes? Move on to the next step.
Specially formulated lens cleaning fluids are available which can be used to clean the lenses. These can be used for water based stains and also to clean dust on the sensor. You will need a swab that can be purchased separately. Always ensure that the swabs are tailor made for touching the delicate sensor and only purchase those which are designed for DSLRs. Just tip the swab with a drop or two of the lens cleaning agent and then swipe the sensor once or twice gently. Put the lens back on and test the sensor. This should clean any dust on your DSLR sensor