Why do sensor dust tests use a small aperture like f/16 or f/22?
Asked 2/15/2013
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When people test for sensor dust or oil spots on cameras such as the Nikon D600, they often photograph an evenly lit white surface at a small aperture like f/16 or f/22. If the contamination is on or near the sensor assembly, why does stopping down make the spots easier to see? Wouldn’t aperture only matter for dust on the lens, not on the sensor?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
13y ago
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When you use a high f stop, the light that hits the sensor is highly collimated — the light rays are mostly parallel. The dust isn't actually directly in contact with the sensor; there's a thin filter in front of the actual CMOS or CCD chip, so the spots you see are actually the shadow. When the aperture is small and the light rays mostly parallel, this throws the shadow cast by the dust dust into sharp relief. With a wider aperture, light strikes the dust from a variety of angles, making the spots softer, sometimes to the point where they're barely visible.
This is only incidentally related to depth of field or to depth of focus.
Originally by user1943. Source · Licensed CC BY-SA 4.0
user1943
13y ago
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A small aperture makes sensor dust/oil spots more visible because the contamination is usually not on the sensor itself, but on the filter stack in front of it (such as the low-pass/protective filter). What you see is effectively the shadow or blurred projection of that dust.
At f/16 or f/22, the light rays reaching the sensor are more narrowly constrained, so the dust casts a darker, sharper shadow and the spots stand out clearly. At wider apertures, light arrives from more angles, which softens and spreads the shadow so much that the marks may become faint or nearly invisible.
So yes: if debris were literally on the sensor plane, aperture would matter much less. But because it sits a small distance in front of the sensor, stopping down improves the visibility of the spots. That’s why dust tests are typically done against a bright, uniform background at a small aperture.
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