The full inquiry stated that the dark spots were usually seen in the sky part in photos and sometime they were almost indistinct, other times very noticeable. The camera used was a DSLR and the lens kept clean.
Here are samples of such spots. These five photos (these are enlarged sections) show the same sky and tree at right. They were taken at different aperture settings, as indicated.
You can see that for large apertures (small f/number) the spots are larger and lighter. In the photos at even larger apertures the spots were not noticeable.
This explains the mystery of sometimes getting spots and not at other times – the aperture will, of course, be different for different photos. So sometimes, with a small aperture (large (f/number), the spots are there, and they are not visible in photos taken at large apertures.
These spots are made by very tiny dust flecks on the rear of the lens. Indeed, this dust was almost invisible to the naked eye. When the lens is stopped down the light bundle forming the image at any one spot is very small and even the tiniest dust particle can cast an appreciable shadow onto the sensor.
It is very easy to miss cleaning the rear of a lens. It is not exposed to the elements except when changing lenses, so you might not think about it. Normally we keep the rear caps on, so there are but a few seconds when dust can get on the rear of the lens. Unfortunately, that is time enough. So be sure to not only keep the front lens surface clean, but the rear of of lens as well.
(Oh, yes, I intentionally got dust on the rear element of a lens to take these photos – the things one has to do to investigate readers’ mysteries!)
© 2011 Ludwig Keck