DSLR cameras are highly susceptible to image sensor dust. Most digital DSLRs sold today including the
Nikon D3100 include a built-in sensor-cleaning function. If you see little black dots on your photographs, especially when they are in the same portion of the photo time and again, you’ve got sensor dust. To check your camera sensor for dust, here are a few simple ideas to try. Set the ISO at 100. Set the aperture to F/22 or higher. Turn off the anti-shake VR or any other image stabilizing feature on the lens. Take a picture of a grey card, a light colored even surface, or blue coloured sky. Move the lens around during the exposure. Be careful not to over-expose the shot or you won't be able to see the dust. Take a few different shots just to be sure. View the images on your PC computer or laptop. Increasing the contrast of the image will then show any dust spots.
