More on 5dmkII black spots

Started Dec 5, 2008 | Discussions thread
didd Regular Member • Posts: 261
Re: Why are they on the RIGHT SIDE of high lights

My interpretation =

It's due to the saturation of the amplifiers used in the CDS (correlated double sampling). It's in the analog part before conversion, but not AGC pb since there is no AGC in this part.

The purpose of the cds is to cancel the fixed pattern noise at "per pixel" level. The sampling is done inside every pixel, that includes one black level + one video level. The difference of black level from one pixel to another one is the cause of the fixed pattern noise. This difference (FPN) is the same for black level and video level.

The purpose of the CDS is to substract the black level (B+FPN) to the video level (V+FPN) so that the result is =
video after CDS = (V + FPN) - (B + FPN) = V - B . No more variable FPN.

Now, imagine a very Bright point for which V is saturaded in one amplifier. The saturation of the amplifiers needs some time to recover (few pixel clocks), during that time the B level is wrong (> at the normal value)

Consequence = if you have a very fast transition White -> Black, the result of the calculation V - B can be false for few pixels just after the transition, corresponding to the time for the amplifiers to recover from the saturation. The result can be that for those pixels V - B is

It's visible only on the right side, because it's temporal and analog my friends

Hope this helps (sorry for my english)

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