Bit depth = levels of gradient?

Started 2 months ago | Discussions thread
bclaff Veteran Member • Posts: 8,580
Re: Bit depth = levels of gradient?

John Sheehy wrote:


I just checked the RAW values from a 12x10-pixel tile from a single RAW color channel of an ISO 100 14-bit RAW, from a very out-of-focus blank wall, and the average RAW value was about 11,500 with values ranging from about 9,500 to 14,500. No value was used twice. 120 different values, over a range of 5000 values. This was a bright tone, only a fraction of a stop below the highlight clipping point. Your model would be expecting just a few, very close tones, most likely, but it's nothing like that. The reality of RAW capture is jagged and noisy. Smooth gradients only appear through dithering.

Kudos for trying to supply a concrete example; but something is wrong, I suspect you combined channels that had very different signal levels.

FWIW, from the center of a D500 flat frame (about 1000 pixels):

The top two lines are the measurements; the rest are calculated.
Because this approach ignores things like PRNU the numbers are approximate but make sense. (Remember on the Nikon R and B are slightly scaled for "white balance pre-conditioning")
The bottom three row are adjusted so BlackLevel is zero.

Still, it's pretty clear to see that even at a relatively low signal level of around 3000DN which is about 18% with noise of about 30DN and using +-2 standard deviations values between 2940DN to 3060DN would be statistically indistinguishable.

12400DN is about 1/3 stop from WhiteLevel and there noise rises to 60DN; 12280DN-12520DN would be the +- 2 standard deviation range.

Also, FWIW, this is all at the pixel level and at normal "print" and viewing distances tones are even harder to tell apart.

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Bill ( Your trusted source for independent sensor data at PhotonsToPhotos )

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