Got Dust?

by Steve on

camera sensor dust at f/22

What in the world is that awful blob in the middle of that beautiful sunrise photo?!

One of the most common topics found online and discussed in the picture-taking community is the presence of system dust.  Most commonly dust particles settle on the sensor, but they can also adhere to the lens and the viewing system as well.

The first problem that comes to mind for most photographers is whether the dust particles, no matter where they are located, will affect the image outcome.  For those particles found on the lens elements, the answer is no.  Dust in a lens is mainly just a source of stress for the owner because they can see it.  For lens dust to cause image problems the particulates would need to be about the size of a pea!  Otherwise, if you have so much lens dust that it appears hazy when you look through the lens, you may get a soft focus image but you will not see any actual spots on the image.  Bottom line is, don’t worry about dust in your lens harming your images.

Where do you see dust?

Does it actually appear on your image or is it simple visible through the viewing system?  If you see spots on your images, you have dust on your sensor.  Much has been written about cleaning sensors (not all good by the way); however, if you search within this site you will find excellent information on sensor cleaning from reputable camera repair technicians.  Follow these easy steps for an easy method to test for a dirty sensor.

  1. Set your camera/lens to manual focus mode.
  2. Set the exposure mode dial to AV or A (for aperture priority mode).
  3. Dial in a very small opening, like f16 or f22.
  4. Manually turn the focus ring to infinity and take some photos of a clear blue sky or similar background.

Notice the image above, it was taken at f22–note the visible dust spot and fiber particulate.  The larger the aperture (smaller f-stop value, larger lens opening) the easier light will bend around the particulate and not be visible.  With a smaller aperture (larger f-stop value, smaller lens opening), less light will bend around particulates on the filter.  If your image looks like the one above, your sensor needs cleaning.  If you do not know how to clean a sensor properly I recommend taking your camera to a professional technician.  If the particulate is not stuck to the filter you may be able to blow it off with a small hand blower.  Remember the spot location on the image is inverted so look for your dust spot in the exact opposite position on the filter surface.  Also note that a small spot on the mirror will not be visible as it is reflected and diffused by the focusing screen.

Focusing Screen Dust

Spots on the focusing screen are annoying; however, they will not show on your image.  When you take an image the mirror swings up and the shutter opens uncovering the sensor to form the image.  Everything in the viewing system is bypassed or moved out of the way to form the image.  If you suddenly see spots in your viewing system while on a shoot, keep shooting.  If you see spots when you are looking through the viewing system (not on live view), those are located on the focusing screen or inside the prism area.  The focusing screen is two sided: the bottom side is visible just above the mirror after removing your lens.  Use a small hand blower to gently blow dust off the bottom of the focusing screen.  If it isn’t moving it may be on the top side.  Cleaning the top side of the focusing screen will be featured in an upcoming post.

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