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There is a lot of buzz about the dirty sensor problem on the D600, which is apparently caused by some lubricant splattering onto the sensor and accumulating over time.
My question is about the test itself that people are using to demonstrate this problem. As this video shows, the camera is set up to photograph a well lit white background. Then the lens is always stopped down to f 16 or f 22. I have seen the test described in other places the same way, and they always include the high f-stop.
I don't understand why stopping down is important for this test, since the test is to verify that the spots are on the sensor. If the spot were on the lens in front of the diaphragm, I might expect to see the sharpness of the spot change with the f-stop (assuming it was in the field of view). But a higher f-stop shouldn't have any effect on seeing a spot on the sensor.
I'd like comments on whether I'm right or wrong about this. If I'm wrong, then why does a higher f-stop improve the visibility of the spots on the sensor?