This is a continuation of a series of posts on the Nikon D850. You should be able to find all the posts about that camera in the Category List on the right sidebar, below the Articles widget. There’s a drop-down menu there that you can use to get to all the posts in this series. You can also click on the “D850” link in the “You are here” line at the top of this post. Because this relates to the a7RII, this is also a continuation of a series of posts on that camera. You should be able to find all the posts about that camera in the Category List on the right sidebar.
In this post, I showed you some strange lumps and bumps in the a7RIII photon transfer curve (PTC). In a particularly informative set of comments to that post, a reader suggested that the raw data scaling that the a7RIII apparently performs could be the culprit. It occurred to me that the D850 does such scaling on the red and blue raw channels. You can see it in this histogram:
This is called “white balance prescaling”, and is used on many Nikon cameras, including the D850.
Here are PTC’s for the two green channels, at ISO 64, using EFCS, 14-bit precision, and lossless compression.
The data points are different for the two green raw channels, but the curves are virtually the same and look just like you’d expect a PTC from a well-behaved camera to be.
Here are the red and blue raw channels:
Look familiar? Pretty much like the a7RIII curves.
By the way, the full well capacity (FWC) of the D850 falls out of this test, and turns out to be about 62000 electrons if you look at the green channels, and 51000 and 54000 electrons respectively if you look at the red and blue channels. That means that the FWCs that I computed for the a7RIII are probably low.