But for Lunar, tonal variation is part of the end goal. I can see both sides. What is the consensus of experts?
There's not a consensus, but what I can say about it is that for most people, you will never have the seeing conditions for 16 bit capture to be beneficial, even on the Moon. It should be noted that no camera is sufficient to capture the dynamic range of the Moon in a single exposure. The human eye has about 20 stops of dynamic range, which is why you can easily see both the earthshine side and illuminated side of the Moon at the same time. It should also be noted that although the 16 bit mode records a file in 16 bits, the amount of real data is typically only 10-14 bits depending on the camera. Most CMOS cameras only record in 10-12 bits if you select the 16 bit mode, but the file has to be saved in 16 bit format.
The reason that extra bits don't typically matter is that atmospheric turbulence causes slight fluctuations in the tonal value that registers on each pixel. This contributes to noise, and it doesn't take much noise to render the extra bit depth provided by 16 bit files to be meaningless. Emil Kraaikamp (the creator of Autostakkert) had an analysis on this several years ago. The summary is that unless you have almost zero noise in your images, which is only possible if you use low gain during truly extraordinary seeing (the likes of which most never see) then 16 bits is not going to do anything. If there is some noise in the system, then 8 bits is just as good if you stack about 1000 frames. This is also why going above 8 bits is never beneficial for planets, because the gain always has to be high in order to keep exposure low (which is not true for the Moon).
If you do some reading on the web and in books, you will see that many people have traditionally recorded the Moon in 16 bits, but it was common in the past to only stack 100 or so frames out of maybe 1000. Now with faster cameras and computers, it is more typical to record between 5000-10,000 frames and stack at least 1000, so this changes things (bit depth actually increases with stack depth....an 8 bit recording becomes more like 12 bits if there is some noise....which there usually is).
As always though, everyone has to experiment to come to their own conclusions. If you were going to work from a single raw image, or if for some reason you were limited to only a few dozen frames, you would want the 16 bit files for sure. But when you stack 1000 frames or so, the bit depth of the 8-bit files increases to basically match the same bit depth of a 16 bit recording.
There are significant disadvantages to recording in 16 bits with the IMX183 sensor. One is the frame rate, which will almost certainly drop to single digits. The other is the file size. Instead of 20GB per 1000 frames, the files are 40GB per 1000 frames. Even in truly excellent seeing (rare, even here in San Diego), I like to capture at least 2000-3000 frames per file. For a full frame shot, this is 80-120 GB file size in 16 bits, but only 40-60GB in 8 bits. When capturing several files in a session this size does matter from a practical standpoint. However, in more typical seeing conditions, I like to capture 5000 frames. 16 bit recordings turn this from 100GB into 200GB, and this fills up hard drives fast. When I first started using this camera, I recorded in 16 bits, but now I do 8 bits because of the file sizes. I haven't noticed any meaningful difference between my 8 bit and 16 bit processed recordings.
But these are just practical matters. If you had a relatively unlimited number of external SSDs (required to avoid snail pace file transfers) as well as an internal SSD hard drive of at least 2 TB, I would probably choose to record in 16 bits if I knew the seeing was extraordinary, as Emil's analysis does show that if the noise is very low, then 8 bit recordings can fail to capture all of the tonal data. So it becomes a bit of circular logic......8 bits will be just as good if you can stack enough frames and you have some noise........BUT you might not be able to stack enough frames if seeing isn't excellent........BUT if seeing isn't excellent you won't be getting a good result anyway and any bit depth over 8 is meaningless........AND if you take 16 bit files this limits your frame rate and increases your files sizes (which limits frames!!!!). Not so simple. Bottom line, 16 bit captures are probably better on the Moon if you have extraordinary seeing and have the hard drive space to spare and you computer can handle the frame rates.
Edited by Tom Glenn, 22 December 2018 - 01:21 AM.