[IMPOSSIBLE] Red, Green and Blue at different ISOs.

[Carlos Morillo]

Please keep in mind that I have no clue what can and can't be done but I noticed you have dual ISO which as I understand it from what I've read you have 2 lines from the array read at one ISO and the next 2 lines at a different ISO and so it goes back and forth. I was wondering if it would be possible to sample blue pixels at one ISO, the Green Pixels at another ISO, and the red at a different ISO. I am not sure how that would be de-bayered for color even if it were possible to have 3 ISOs, however for Black and White photography it would be amazing to have 3 ISOs. As an example if you had blues at ISO 100,  greens at ISO 800, and reds at ISO 1600. If a color image could be pulled from it even better but even if it ware only available for Black & White images that would be great.

Thanks for all your work,

Carlos Morillo

[ayshih]

As far as I know, what you have asked for (three ISOs) is not possible.  With the settings we have available (more details below), half of the green pixels will be read out at the same ISO as the red pixels, and the other half of the green pixels will be read out at the same ISO as the blue pixels.

There are two amplification stages that combine to achieve a certain ISO:

• The first stage allows us to read out pairs of horizontal lines at different gains (CMOS gain).  This gain setting is what is modified for dual ISO.  Since the horizontal lines are paired, the colors cannot be decoupled here.
• The second stage allows us to read out vertical lines at different gains (ADTG/DFE gain).  Since red and blue pixels are always on different columns, it's possible to set these gains so that red and blue pixels have different ISOs.  As a practical matter though, since this amplification stage comes after the first one, you'd have to minimize the amplification from the first stage to avoid saturation, and thus you may see no real benefit from multiple ISOs.  Put another way, if the blues are at ISO 100 and the reds are at ISO 1600, the reds may be much closer to the noise performance of ISO 100 rather than ISO 1600, and so you might as well have simply taken a regular photo at ISO 100.

[Carlos Morillo]

Thanks for your response. It's interesting to see how this works.

[Audionut]

The actual dual ISO ability is implemented in Canon hardware.  a1ex found a way to control that as needed/wanted.

I agree, being able to control the post exposure gain at the pixel level would be excellent from an SNR standpoint, but unfortunately not with current hardware.