A couple of weeks ago, I looked at the dynamic range effects of Sony’s 12-bit shutter modes. Then someone asked about the visual effects of Nikon’s 12-bit raw mode. I did a visual test. For some, that’s enough. If you’re one of those, move along; nothing to see here.
Still around? I’m a numbers guy, too. Actually, I’m both, but the rest of this post is about numbers.
I did a photon transfer curve for the Nikon D810 in both 14-bit losslessly compressed raw mode and 12-bit losslessly compressed raw mode at ISOs of 64, 100, 200, and 400. I figured that by ISO 400, there’d be enough read noise dither that it wouldn’t make any difference whether the precision was 12 or 14 bits, but I wanted to have that data just to prove that to myself — and you all.
Here’s the ISO 64 data:
The horizontal axis is the mean value in stops below full scale. The vertical axis is the standard deviation of the noise in stops below full scale. I fit camera models to both sets of data. The solid lines are the modeled data, and the dots are the actual measured data points, each computed from 200×800 pixel samples of pairs of images. Only the first green channel (RGGB) is plotted.
Note the “ringing” in the 12 bit data. We’ve seen that effect before with the Sony alpha cameras. It occurs when there is not enough read noise to properly dither the analog to digital converter (ADC), or in this case the post ADC 14 to 12 bit quantizer in the camera. There’s not much difference between the 14 and the 12 bit case until the signal level drops to about 9 stops below full scale.
For the rest of this post, I’ll plot the signal to noise ratio instead of the standard deviation. It’s just another way of looking at the same thing, except now higher is better.
Here’s the ISO 64 and the ISO 400 data:
You can see that my assumption about the precision not making any difference at ISO 400 was right.
Pretty close down to ten stops below full scale. No ringing to speak of, so there’s enough noise to provide sufficient dither.
There are measurable differences, but I’ll bet you’d never see them in an actual photograph.
My bottom line:
- ISO 200 and up, 12 bits is fine.
- ISO 100, 12 bits is probably OK for almost everything.
- ISO 64: use 14 bits. It won’t cost you much, and it’ll give you peace of mind