Hey! I'm a portrait photographer and I'm looking at the Canon 5DS/SR for both the high MP count the nice colors from Canon and the dual card slots. I have a few questions and I was wondering how the files are in raw and how felxable are they in post, also wondering how's the low light performance, I very rarely shoot above 1600 ISO and I don't mind noise so much as long as the image is usable.
The EOS 5DS is an interesting camera, but it does have some tradeoffs that are worth considering.
As you said, the high resolution and great colors are excellent benefits of the camera, but in order to really get the most out of that high resolution sensor you have two main hurdles to overcome, and one somewhat smaller one.
The first is that you would need (NEED) to use lenses with a very high optical resolution to match the sensor resolution. This means sticking with the newer L series lenses in most cases since they were designed after or in conjunction with the expectation of these massively high resolutions, remember that today's estimates tend to put 35mm film anywhere between four and 16 megapixels, meaning if a lens had a maximum equivalent resolution of 18 megapixels (this is not exactly how resolution works between sensor and lens but it is close enough for demonstration) it would show itself as perfectly sharp down to the granular level because it exceeds the film resolution, but that same excellent lens on a 30, 45, 50 megapixel sensor will start to show its age simply because it had no way to test its sharpness up to those resolutions which we previously had no ability to achieve. For portrait photography you would likely find the EF 85mm f/1.4L IS USM, EF 35mm f/1.4L II USM and EF 24-70mm f/2.8L II USM to be some of the great newer lenses that really let this camera's sensor show its full potential.
The second hurdle may be one you are familiar with if you have shot medium format film, especially a medium format SLR like the Hasselblad 500 series or the Pentax 6x7, and that is camera shake. Due to the camera's much higher resolution and our ability to go down to the pixel level with these files, every little movment will show up more easily. For most EOS 5DS shooters this means utilizing that old rule of using a shutter speed at least equivalent to one over your focal length (1/50th for a 50mm lens for example) to prevent undue camera shake and double it. If you are using a 200mm lens I might go so far as to say 1/500 is a minimum with IS activated but some photographers find themselves to be a lot more steady than myself.
Now the third potential issue is one you already raised and that is one of low light capability. The downside to having more pixels on the same image sensor is that each pixel itself is smaller. Imagine a full box where I have four muffins. Instead of using that box for four muffins I can instead have 16 cupcakes, but each cupcake is then going to be smaller than the muffin. When it comes to higher ISO levels and light gathering ability, all other factors aside, a larger pixel is going to be better able to capture light. Acceptable levels of noise are going to be dependent on the photographer, so I cannot say for sure that you would be happy with the ISO 1600 images that the EOS 5DS would produce, but I was more than happy with my results up to ISO 3200 and I would say 6400 will do in a pinch if you nail the exposure. Most portrait and landscape photographers do not find themselves needing to boost the ISO this high and that's exactly who this camera is for. I'll use this space to mention tonal latitude for a second because you did ask about the file's flexibility and ISO does affect that, the camera's latitude is about as flexible as others in its generation, but it will take a bit of a hit in the extremes. If you are in a really contrasty situation and need all the tones present then I would definitely bracket for safety's sake.
Now if this were four years ago, I would say if you can handle these three hurdles without much issue then the EOS 5DS is THE option, but given the advacnes in technology in the last few years I would highly recommend taking a look at the EOS R5 if you have the budget for it. The resolution is similar, 45mp versus 50mp, but in every other area it is a vast improvement.
First, the new RF lens system not only allows for use of the older EF lenses like those I mentioned before, but native RF lenses are able to have a simpler and sharper design. This has to do with a number of factors including added communication points in the new lens mount but primarily this has to do with the lenses not having to be designed to accommodate the space that the reflex mirror takes up in an SLR system. Removing that mirror and by extension that space from teh equation means less optical corrections and distortions are introduced into teh lens design, and the resulting lens allows for increased sharpness all around.
Second, the camera shake the EOS 5DS experiences is mainly due to three factors, the movement of the mirror, the movement of the shutter curtain and the movement of the user. While we can certainly improve ourselves and find new ways to steady our stance, the other two are all in the camera. The EOS R5 does away with the mirror altogether, and the shutter has been improved, even allowing for the use of a fully electronic shutter in some cases which introduces no movement into the camera whatsoever.
And the last factor would be high ISO performance. In this area there is no contest, the EOS R5 will give nice usable images at least two to three stops above what you would find usable in the EOS 5DS. The improvements in the tonal latitude of the images out of the EOS R5 are particularly impressive as well, I have found it to be comparable with the EOS 1DX if not better, and with 45 megapixels!
I've seen some good prices on the EOS 5DS and I wouldn't really recommend it as an "everything" camera, but if portraits, landscapes, studio work are your bread and butter it is a great performer. The EOS R5 is a step up in a lot of ways, but if you don't need the added flexibility and newer RF lenses you'll be very happy with the EOS 5DS.
Mark covered all of the bases very well with a great explanation.
I shoot mostly sports using 1DX series bodies but I have a 5DS and 5DS R for other work and they are still nice and versatile bodies.
Pay close attention to Mark's point about camera shake, that is the first thing I noticed with the 5DS R that motion blur was obvious at shutter speed settings that work fine with other cameras. Both camera shake and subject motion play a role and usually it isn't an issue but it will interact with the ISO speed issue so getting to a shutter speed sufficient to avoid blur may force you too high into the upper ISO range.
I am shooting a soccer match this afternoon and my main cameras will be a 1DX III with EF 400 f2.8 and a 1DX II with EF 70-200 f2.8 but I will have my 5DS with 24-70 f2.8 in the bag to grab some casual bench, group, and other sideline shots and it excels at that sort of work. I also use the 5DS and 5DS R frequently with my studio strobe setup where it produces beautiful images. It also works well with the EF 100 and EF 180 macro glass but keeping in mind that it does accentuate the issue of shutter speed requirements to avoid blur so you have to account for that in setup.