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<blockquote data-quote="Eyelight" data-source="post: 304865" data-attributes="member: 24753">This could be way off, but:- Not moving for a year would seem to indicate it is not a lose &quot;thing&quot; and unlikely to be hanging around the shutter plane.-&nbsp; Changing with ISO sensitivity would seem to indicate it blocks light,&nbsp; i.e. the higher absorption rates of the sensor as ISO is increased is&nbsp; not absorbing light through the &quot;thing&quot;.- Constant regardless of lens indicates not a lens or lens mount issue.-&nbsp; Changing with F-stop indicates it is not the sensor (I think) or a&nbsp; &quot;thing&quot; on the sensor. It's in the DOF spread behind the focal point of&nbsp; the lens, but this would not change if the &quot;thing&quot; were on the face of&nbsp; the sensor (I think). A smarter person than me could use images taken at&nbsp; different f-stops to calculate the position of the &quot;thing&quot;.That&nbsp; leaves the low pass filter, which, If I am not mistaken, is what you&nbsp; see when the mirror is up and the shutter is open.&nbsp; Also likely what the&nbsp; professional cleaning cleaned, but only the outside surface.All&nbsp; this seems to point to the &quot;thing&quot; being on the back of the low pass&nbsp; filter or possibly a micro-crack in the filter. Since &quot;things&quot; are&nbsp; unlikely to get behind the filter, micro_crack seems more likely,&nbsp; assuming a low pass filter can crack. Getting worse over time would also indicate crack.Have reached the end of my basic understanding of how things are put together. Good luck in resolving the &quot;thing&quot;.</blockquote>

[QUOTE="Eyelight, post: 304865, member: 24753"] This could be way off, but: - Not moving for a year would seem to indicate it is not a lose "thing" and unlikely to be hanging around the shutter plane. - Changing with ISO sensitivity would seem to indicate it blocks light, i.e. the higher absorption rates of the sensor as ISO is increased is not absorbing light through the "thing". - Constant regardless of lens indicates not a lens or lens mount issue. - Changing with F-stop indicates it is not the sensor (I think) or a "thing" on the sensor. It's in the DOF spread behind the focal point of the lens, but this would not change if the "thing" were on the face of the sensor (I think). A smarter person than me could use images taken at different f-stops to calculate the position of the "thing". That leaves the low pass filter, which, If I am not mistaken, is what you see when the mirror is up and the shutter is open. Also likely what the professional cleaning cleaned, but only the outside surface. All this seems to point to the "thing" being on the back of the low pass filter or possibly a micro-crack in the filter. Since "things" are unlikely to get behind the filter, micro_crack seems more likely, assuming a low pass filter can crack. Getting worse over time would also indicate crack. Have reached the end of my basic understanding of how things are put together. Good luck in resolving the "thing". [/QUOTE]

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