Are faint spots visible at f/22 caused by sensor dust or lens dust?
Asked 10/28/2015
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I tested a Canon EOS 5D Mark III with a 24-105mm f/4L at 105mm by photographing blue sky. In a small crop, faint dark circular spots appear at f/22, and they become much more obvious after increasing contrast and clarity. In an otherwise similar frame taken a second earlier at f/7.1, the spots are not visible, even with the same enhancement.
The spots are mostly toward the edges of the frame, with one near the center, and they appear similar in size but different in density. Are these marks from dust on the sensor, or could they be caused by dirt or defects in the lens?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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That is dust on the sensor (well the IR filter over your sensor), dust specks in the lens are not capable of causing spots on the image like this.
The reason you can see them at a small aperture and not at a large one is that the light rays are far more linearly aligned at f22, hence they cast a shadow - remember the surface of the filter where they sit is not ON the sensor, but close to it.
At a wider aperture the incident light basically goes AROUND the dust particles, and therefore cannot cause such a defined shadow.
Edited to add: Dust on the surface of FILM will always cause spots on an image, at any aperture.
Originally by user9999. Source · Licensed CC BY-SA 4.0
user9999
10y ago
0
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These are almost certainly sensor-dust spots, more precisely dust on the filter stack in front of the sensor.
Why: dust on the sensor becomes much more visible at small apertures like f/22 because the narrower light cone casts a more defined shadow. At wider apertures such as f/7.1, light reaches the sensor from more angles and effectively goes around the dust, so the spots become very faint or disappear.
Lens dust behaves differently:
- front-element dust usually does not show as distinct specks; it more often lowers contrast slightly
- rear-element dust would not typically produce these well-defined, aperture-dependent spots
- lens dust may also be seen through the viewfinder, while sensor dust only appears in the captured image
So the aperture dependence is the key clue: visible at f/22, not visible at f/7.1, strongly indicates sensor dust rather than a lens defect.
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