D810 vs D800E dark-field histograms

People have asked for comparisons between the dynamic range of the D810 and its immediate predecessors. I don’t have a D800 at my disposal, but I do have access to a D800E, and I thought I’d do some testing.

But first, a little discussion of dynamic range.  The dynamic range of an imaging system is the ratio between the brightest pixels the camera can register and the dimmest pixels that have some defined level of quality. For the definition of acceptable quality in the dimmest pixels, I like to use a signal to noise ratio (SNR) of 10. However, that’s not standard throughout the industry. In fact, the closest thing to a standard seems to be something called engineering dynamic range (EDR), which defines the dimmest pixels to be considered as having an SNR of either 0 or 1. Zero is a convenient number, since picking that means that the EDR is simply the ratio of full scale over the dark-field noise.

Since full scale is the same — 2^14-1 or 16383 — for the two cameras, comparing their EDRs simply means comparing their dark field noise. That should be pretty easy, right?

Wrong.

The D810 and the D800E — and, presumably, the D800 — play by different rules. Let me show you.

Here’s a dark field histogram (as for all the histograms in this post, made with single shot mode at 1/8000 second) from the D810 at ISO 100, which is the lowest real ISO setting the two cameras share:

 

Note the nice tight distribution around 601. That’s about the black level for the camera, and will be subtracted out by the raw processing software that you use on your computer.

Here’s a dark field histogram from the D800E at the same ISO, also with the bucket size set to 1:

D800E ISO 100

Now just about all the pixels are at zero.

What’s going on? The D810 makes the — more sensible, in my opinion — assumption that deciding what the black level is should be the responsibility of the raw converter program, while the D800E does it in camera, and subtracts that value from all the pixels before writing the raw file.

That means that it’s not simple to figure out what the D800E dark-field noise was before the camera threw away all that data (to be fair, it’s data to somebody like me researching the dark-field noise, but the camera figures it’s noise). There are ways to calculate what the dark-field noise probably was, but they involve, for every ISO setting of interest, making a series of ever-darker exposures and fitting a curve to the raw values.

Now let’s look at the highest setting the two cameras share, ISO 25K.

First, the D810:

Missing a few values, aren’t we? You ain’t seen nothin’ yet. The D800E at the same setting:

Whoa! Only one in every 16 buckets has any content, and that is really sparse. At ISO 25K, the D800E is a 10 bit camera, and one that does some serious black clipping.

Now that we’ve seen the extremes, we’ll march through the whole one-stop ISO series that the two cameras have in common. You saw ISO 100, so we’ll start with ISO 200:

Quite a difference. A well-behaved Gaussian distribution in the D810’s case, and a truncated shadow of that in the D800E. It’s not just clipping at the middle of the distribution, it’s clipped well to the right of where the mean must be.

ISO 400:

 

 

The D810 histogram looks like it came from a textbook. The same severe clipping shows in the D800E histogram.

ISO 800:

More of same. The gaps in both the cameras’ red and blue channels are because of Nikon’s digital white balance prescaling.

ISO 1600:

 

 

Interesting. We’re seeing gaps even in the D810’s green channel. There are gaps developing in the D800E green channel, too.

ISO 3200:

Both cameras are now 13 bitters. The unreasonable black-biased distribution continues in the D800E.

ISO 6400:

The D800E is a 12 bit camera at ISO 6400. I’d call the D810 12 and a half, since we’re not seeing the level of combing that you’d expect with straight 12 bit precision. This is probably the result of digital gain that’s not a power of two. We are finally seeing some clipping in the D810 histogram. I see no good reason for this. There’s a lot of room to the left of the clipping point.

ISO 12K:

There’s some funny combing in the D810 histogram, but nothing like what’s going on with the D800E. The older camera is now digitizing at the 11 bit level.

The different approaches to raw processing will make for some difficulties in graphical read noise comparisons. More next time.