Purple squiggly spots in photos: sensor dust, debris under the filter, or dead pixels?

Question

My photos show many small purple, oddly shaped spots. A shop cleaned the camera and said the contamination is under the first layer above the sensor, so it cannot be removed by normal sensor cleaning. Is that possible, or could these marks be stuck/dead pixels instead? The camera is only two months old.

user28109 · Answer

One way to tell if the sports are on the sensor itself, or on the AA layer above the sensor, is to shoot a white background at wide aperture (say F2.8) and one at narrow (say F22).

To understand the point, consider how a shadow is cast. If the light is a near point source (F22) then it's a sharp shadow. If the light is diffuse (F2.8) then there's no or little shadow, which is why people always say check for dust at high F stops.

So if you take one high and one low, if the specs are on the AA layer (well above the sensor) the F2.8 shot will show no or very diffuse spots. If they are instead on the sensor itself, they will be almost he same. If they are on some intermediate layer you'll get half-way shadows.

To actually tell, perversely enough, you might want to put a hair or spec of dust on the outside of the sensor (i.e. above the AA layer) and compare how it looks at F2.8 and F22, that will give you a good feel for where these are.

Purple squiggly spots in photos: sensor dust, debris under the filter, or dead pixels?

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