Reply to thread

Message: <blockquote data-quote="aroy" data-source="post: 305837" data-attributes="member: 16090">It may be slight mark left while cleaning the sensor. The reason it shows up clearly at high ISO, may be that the mark is very light and shows up when the signal is boosted up a lot at high ISO.To check this, just keep the aperture (say F22) and exposure same, then shoot the sky at various ISO, starting from 1600 going all the way to the highest. The mark should keep getting more visible as you increase the ISO. If that is so, then use a loupe to inspect the sensor surface. If you have one with light it would help. Note that the mark is very light so it may only be visible under bright light and that too at an oblique angle.</blockquote>