The Canon EOS and Photography Forums

when i look through the viewfinder on my 40d there are marks..little brown spots that i can see and are coming up in my photos. i'm not sure if these are on my sensor or not. I'm certain they are not from my lens. Can they be on the view finder? If they are on my sensor what is the best way to clean them off?

You can't see the sensor though the viewfinder. In general anything you see through the viewfinder is on the viewfinder screen. As such it will not appear in your images. If there is dust in the lens, you won't see it in the viewfinder and it won't appear as spots on the viewfinder screen. Similarly dust on the mirror won't really show up in the viewfinder, but again it won't appear on the image anyway.

Any spots which appear in the same place on all your images are likely to be on the surface of the filter which covers the actual sensor. The only way to remove them is to physically clean that filter (sometimes called cleaning the sensor) or to have someone else (like Canon) do it for you.

It's quite possible to see spots in the viewfinder (dust on the screen) and spots on the images (dust on the sensor cover), but the two are not related and the spots will, in fact, be in different places on the image.

To check for dust on the sensor cover, point your lens at a uniform target (e.g. the blue sky or a white wall), defocus the lens, stop down to f22 and take a few shots. If you see dark spots in exactly the same place in each image, then those dark spots are dust on the sensor cover. If you open up to f8, the spots should blur a little.

For information on sensor cleaning see http://www.bobatkins.com/photography/digital/sensor_cleaning_ii.html.

Many people clean their own sensors using commercial products like those made by Visible Dust (http://www.amazon.com/exec/obidos/ASIN/B001BZOUSQ/bobatkinsphotogr). While they aren't cheap, they're cheaper (and more convenient) than sending in your camera to be cleaned every few months! They are also probably somewhat safer and more reliable than "home brew" methods of sensor cleaning.