Often times, a smudge from a dirty finger, a water drop, or a particle of some sort can get on the camera lens and produce what looks like orbs in photos. The easiest way to spot a smudge on the lens is if a similar-looking orb appears in the same spot in a series of photographs taken with the same camera.
Orbs that appear in the same section of a photograph are almost always a lens smudge. For example, if the orb appears three inches from the top and two inches from the left in more than one photo, a smudge should be suspected. The three photos below show different smudges on a camera lens surface...
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Created with a spot of white liquid on the lens. The opacity of the liquid caused the orb to appear dark shadowed
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This orb was caused by a water droplet on the lens surface. It is very see-through, clear.
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Grease smudge on the camera lens. Notice how it is grayed-out, a blurry effect.