Dynamic range and RAW file "bit depth"

Started Jan 25, 2014 | Questions thread
Mark Scott Abeln
Mark Scott Abeln Forum Pro • Posts: 13,216
Signal to noise — illustrations

Someone mentioned that 1 bit-depth images can be adequate, and here is an example:

(click ‘Original size’ to see full resolution)

I think this is illustrative of one of the controversies in digital photography — are more, smaller sensels better, or are fewer but larger capacity sensels more desirable? I think the trade-off depends on how much post-processing is to be done to an image.

It would be difficult to do any kind of basic editing on the 1-bit-depth image, but it is certainly adequate for display.

Here is another image:

This uses only a handful of colors. It looks fine if you don’t look too close, but would also be difficult to edit.

I’ve seen some complaints lately regarding the Nikon D800 camera, that its files are somewhat difficult to edit well, particularly when it comes to skin tones, due to noise.

A camera with larger and less noisy pixels would be easier to edit well — albeit with less resolution.

And here is an illustration of signal-to-noise ratio:

(click ‘Original size’ to see full resolution)

There is a considerable amount of signal that can be visually picked out of the noise, and using a SNR of 1:1 is pretty conservative when estimating a usable dynamic range. But it is difficult to edit such a noisy image well, without doing lots of averaging of pixels. So do we cut off the most noisy part of an image, to allow good editing, or do we keep it for the detail? I see that as a difficult trade-off in some respects.

I suspect this might be a good area for more research, which might lead to a way to broaden the dynamic range of an image without having too much objectionable noise. One unit of noise in a particular part of an image might not be as bad as a unit of noise in another part of the image — as others have mentioned, highlights have a larger absolute amount of noise compared to shadows. But the visual effects of noise will vary depending on the color of the object being photographed.

The places where the typical eye is more sensitive to shifts in hues will likely be more important than less sensitive regions — the range of human skin hues is actually quite narrow, and noise there can look quite objectionable. Also, the range of hues around blue are quite sensitive to noise, but sadly this is noise-prone in Bayer sensors. Doing some sort of variable cut-off of dark pixels depending on color might be quite useful, as well as targeted noise reduction.

But this again is a trade-off between keeping good visual detail versus making an image easily editable.

 Mark Scott Abeln's gear list:Mark Scott Abeln's gear list
Nikon D200 Nikon D7000 Nikon D750 Nikon AF-S DX Nikkor 35mm F1.8G Nikon AF Nikkor 50mm f/1.8D +2 more
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