Greg,
There are many sites that can help compare cameras, and some that can lead you astray.
DxOMark performs a standardized noise level test on all cameras. For a standard low light level, they measure the ISO which gives noise at -30dB.
https://www.dxomark.com/cameras/laun...taType=rankLln
The top 5 full frame cameras in this ranking are :
Sony A7s
Sony A7Rii
Pentax K1
Nikon Df
Canon 1Ds
I used filters to exclude medium format cameras which can rate higher.
The Canon 6D is 26th on the list and has an ISO yielding -30dB noise that is about 2/3 of the top cameras. It is showing its age.
Note that they don't state the light level used. Given that they call this sports photography, I've always assumed is likely to be night time sports ground lighting levels not 2 minute exposures.
Photons to Pixels site has these comparison tools
DxOMark data using Photographic Dynamic Range (PDR) for different ISO's.
http://www.photonstophotos.net/Charts/DXOPDR.htm
http://www.photonstophotos.net
http://www.photonstophotos.net/gif/b1ff00ff.gif Photographic Dynamic Range Chart (2017-06-03)
http://www.photonstophotos.net/gif/b1ff00ff.gif Photographic Dynamic Range Shadow Improvement Chart (2017-06-03)
http://www.photonstophotos.net/gif/b1ff00ff.gif Read Noise in DNs Chart (2017-06-03)
http://www.photonstophotos.net/gif/b1ff00ff.gif Input-referred Read Noise Chart (2017-06-03)
http://www.photonstophotos.net/gif/b1ff00ff.gif Sensor Heatmaps (2017-06-03)
http://www.photonstophotos.net/gif/b1ff00ff.gif Photographic Dynamic Range versus Sensor Area Scatter Chart (2016-12-06)
http://www.photonstophotos.net/gif/b1ff00ff.gif Low Light ISO versus Sensor Area Scatter Chart (2016-12-06)
The Nikon 810, Sony A7s and Pentax K1 yield similar results - not surprising since they all use the same or similar Sony sensors. The Sony probably makes the best use of the sensor. Nikon has the best external camera control and software support and the biggest range of lenses to choose from. Pentax has poor external connectivity but the best internal functionality for stand alone operation.
Sony A7s - lower noise at very high ISO's for nightscape "snapshots." Lenses are very good but expensive and limited in variety.
Nikon 810 - best connection to camera control software. Widest range of OEM and 3rd party lenses (Nikon, Tamron, Tokina, Sigma, Samyang etc)
Pentax K1 No external computer driven support. Limited high quality lenses, very expensive. Possibly has the best(?) inbuilt standalone support.
I have a K1. I'm not especially advocating for the K1 and I have not had nearly enough time in the field with it to explore its full potential. Mine is not modded and I won't be modding it. I use my camera to teach photography workshops. I do live imaging for class demos with live view to screen and don't want to have to explain the strange colour shifts.
I'm yet to see anyone advertise mods for FF Pentax. My choice was in no small part driven by the many Pentax lenses, flashes and other accessories I already owned. You would have to determine if it suits your needs.
Below is a list of features I find useful for AP. Perhaps 810 & A7s users can provide lists of useful features for you.
Starstream - automatically stacks brightest parts of sequential images to make composite star trail images;
Astrotrace - uses GPS & image stabilizer to shift sensor to pseudo autoguide exposures up to a couple of minutes. Longer focal lengths can only be guided for shorter times.
Time lapse support - Inbuilt time lapse drivers for movie or series of stills
Exposure bracketing - 5 steps of up to 2eV gives up to 10 stops of autobracketing. I have found this feature, standard in all non-entry level Pentax bodies, very useful for capture at solar eclipses.
I have posted a centre cropped jpg of a dark taken at ISO 6400, 5 mins at 21C. The IIS engine has compressed it.
If you are interested in the K1, I can send you some other darks. A 16 bit PSD files are 200MB ea
, RAWS 50MB and would have to made available be via ftp so please don't ask me to do this unless you are really seriously interested in this camera.
Good luck with your search.
Joe