Mon 17-Dec-12 01:36 AM | edited Mon 17-Dec-12 02:23 AM by mpage
Are you sure you don't have dust on the D3100? For most people they will never see dust in their photographs. The D3100 sensor is physically smaller than the D600, and it has considerably smaller megapixel images. You are less likely to see dust with the D3100 than with the D600 or D800 cameras.
Test your D3100 to see if has dust. Shoot an image at f16 or f22 of a clear blue sky or a smooth white wall. Examine it on the computer.
Frederic posted two images in the following thread that show the result of sensor dust. His photos are near the bottom of the post: