Photographer should be know this problem, dust is a common problem for SLR & DSLR camera, if you don’t know about SLR or DSLR stands for – you need to read this link. Not all photographers know that not all dust will be visible on picture. To explain it, let me categorize three kind of dusts (then you will know what kind of dust that will be visible on the picture) and you will know how to solve it:
*Dust on lens – have no visible effect on picture – why? because the image will be catch by image sensor, not the lens. Don’t be aggressive to clean the dust on lens, more lenses are damaged by cleaning (manual cleaning) – if you want to clean the lens better you go to canon service although it will cost you more.
*Dust on the focusing screen – will be very visible on SLR viewfinder but will not shown up on the picture at all. When you want to change the lens, you will see the focusing screen at the top of interior of the camera. To clean this problem simply using air blower to blow the dust away – careful to clean it otherwise it will enter the image sensor area. I think this isn’t a big problem because you only see in viewfinder but it won’t change the picture.
*Dust on the image sensor – not visible on SLR viewfinder but will shown up on the picture. To check this problem is very easy, just take photo of a white paper, then you see the picture in computer – if no dust on image sensor, the picture should be only white color. If you want to clean it – the simple way using air blower to blow the dust away – you need to go to camera setting to set manual cleaning – follow the instruction and don’t scratch the image sensor. Sometimes if the problem still happen maybe you need to take the camera to canon service.