For users of digital cameras with interchangeable lens, dust can be a real problem. This is especially true for photographers who shoot at smaller apertures (large f/stop numbers). Why? Because the dusts are not sitting directly on the micro lens of the sensor but on the anti-aliasing (AA) filter. The typical distance between the AA filter and sensor surface is much greater than the size of typical dusts. At large apertures, the light rays that pass through different part of the lens surface can go around the dust and focus on to the pixel. The dust may not be visible at all or appears to be a fuzzy dot in your photos. When shooting at small aperture, the light is restricted and therefore more directional (coming through a smaller hole). It causes the dusts shadow to show up clearly and sharply in the photo.

It is not a good surprise when you come back from shooting then found out there are dark spots on the exact locations of your otherwise great photos. You cannot completely avoid getting dusts on the sensor even if you seal your camera in a air-tight bag. The moving parts on your camera and lens can also generate some particles. So, what are the options?

Purchase a camera with a anti-dust sensor

Camera manufacturers have realized the problem and are now offering many choices especially in the entry level DSLR markets. Example models include Canon 40D, Sony Alpha A100, Pentax K10D, Olympus E510, etc. Word of caution is that the effectiveness of the system may vary greatly. Visit camera review sites and forums to do some research before making the purchasing decision.

Avoid the dusts

Avoid dusty environments if possible when shooting. Try not to change lens in a dusty or windy places. Cleaning your camera bags and work area can also help.

Live with the dusts

The dust can be a serious problem but you should be able to live with that by inspecting and cleaning the sensor frequently. The simplest (and probably the best) way to identify the dusts is to shoot a bright uniformly lit area (blue sky, white wall) using a small aperture (eg. f/22) then view the image at 100% on your computer screen.
If you don’t have access to blue sky or a white wall, here is a trick that worked very well for me: open the notepad program (or other program with white background) and maximize the window to fill the entire screen. Now take a picture of the screen at f/22 or smaller aperture using manual focus mode at the lowest ISO. The shutter speed may be very low but you do not need to use a tripod. You will not notice any blurring due to camera shake because the dusts are on the sensor. To make the dusts easier to see, open the image in your favorite photo editor and do an “Auto level”. If the software only has manual level adjustment, simply drag both the shadow and highlight sliders towards the center of the histogram until the dusts are clearly visible.