I've recently been playing around with two interesting cameras. One is a Canon 5Dmk3 and the other is the D800, a predecessor to one of my recent favorite Nikons, the D810. My romp with the cameras comes shortly after having divested all of my Sony mirrorless cameras and lenses. I have now shot professionally (and as an enthusiastic hobbyist) with full frame cameras from both Sony lines (DSLT and mirrorless) as well as many of the recent (and older) Canon and Nikon cameras including 1DSxx cameras and D8xx cameras, I think I've come to some conclusions.
Both the Canon and Nikon cameras are mature products which, when used to create raw format files, deliver very good results and very detailed images. But why, with all the competition out in the world do these two brands still dominate the marketplace and what might change?
If we look at the niche in which the two brands have the most dominance it's in the higher end, full frame markets. It's no mystery that a full frame sensor can deliver extremely good files with less noise than other formats but the real idea most people have is that by using the bigger sensor it's easier to create good files while using smaller formats might require more shot discipline, better technique and more attentive processing in order to get the same quality results.
That being said my first two brand examples (above and below) are both from APS-C cameras and are from older products that have not been on the market for a while. It's obvious to me that both are capable of capturing very good color and, for the web at least, very satisfying levels of detail. I am always surprised when I revisit the photo just below and remember that it was taken with a 2003 vintage 4 megapixel camera, the Nikon D2H.
I think that if everything else was equal and people wanted cameras to use to most easily create day-to-day images of their family, friends and events in their lives that most people would be better served by a mirrorless camera like the G85. It combines an affordable purchase price, a good kit lens with lots of range, wonderful image stabilization (which I think is much more of a boon to casual amateurs than to working pros) a more than ample file size for both social media and actual prints, all combined with the highly useful, constant feedback of its live view system. What you see is mostly what you get... But instead people tend to end up with Rebel Kits or Nikon 3x00 kits because, like an incumbent president or congressman, these brands have much more name recognition, and because of their long tenure in the market and their overall market share they have bigger budgets with which to advertise. Most of the other brands and their current models just get lost in the noise.
But I also tend to think that looking at the bottom or middle of the market really isn't interesting to people like us, who have a keener interest in photograph. The place to look is in the upper middle and top of the market. It's interesting to understand what drives people in this sector to select the big two instead of other options.
The best answer is that most people who are in this market are more likely to have been in photography longer and to have made some selections that created a pathway to future purchases long before mirrorless options where even available. Chances are you started out with a Nikon FM film camera or a Canon AE-1 and upgraded from time to time as new models came to market. When digital came into vogue you selected from one of the big two because they came to market with what looked like more mature and usable products. Plus you already had some lenses that worked.
Nikon and Canon users traded systems back and forth, depending on their photographic specialities, until both systems had in place full frame cameras with more than 20 megapixels, then the system abandonment based on ever changing standard specifications slowed down. People locked into their lens silos more faithfully. There were a number of bleak years for Nikon in which Canon had full frame bodies with 16 megapixels counts while Nikon's flagship offerings topped out at 12 megapixels in a cropped frame body--- but that's all history now.
I think reason for overarching market segment loyalty is that for the longest time people have been taught and marketed to that full frame represents a gold standard for formats and one that pros and serious hobbyists aspire to in their tools. For the longest time they were really the only game in town. Yes, Sony came out with their a850 and a950 but they were already considered obsolete by the time they hit the markets and, by comparison, there was nowhere near the number of branded or third party lenses available for that mount. It's only since the second generation of Sony's A7 series cameras that Nikon and Canon have had any competition to speak of in the full frame space.
Obviously the big disruptor since 2013 has been the ever expanding line of Sony A7 series, full frame cameras along with an increasingly well filled out lens offerings. They are new, novel and fun and Sony's success seems to have taken Nikon and Canon by surprise. For me the advantage of the Sony line has little to do with the size of the camera bodies or the inclusion of various features; it has everything to do with the inclusion of an EVF as an eye level viewing mechanism. I was lured into the Sony system because of the potential of combining high image quality with the distinct usability of the constant live view of the electronic finders.
I am still a big fan of the EVF but I'm not so sure anymore that there is an image quality advantage to the Sony line when it comes to the character of the files; raw or Jpeg. Sony seems to make some color and tonality choices that are less advantageous for portrait photographers than the color and tonality from cameras in similar price and performance ranges from Canon and Nikon.
One of the benefits of having shot with a variety of cameras when wearing my "professional photographer" hat is that I have a rich store of "test" shots and "real world" shots I can go back and examine when I get in my head that one camera or system has better or worse color or tonality than a competing system. I can look a ten or twenty thousand examples, shot in a range of job types, from low light, available light to studio flash and just about everything in between.
During my recent down time I started looking in earnest at lots of old files across systems. One observation is that nearly every camera I've owned is able get into the "excellent" ballpark without much effort. Some are better than others, some are easier to use than others.
If I look back through all the modern cameras I've used (from 2008 onward) I'd have to say that there are two that really stand out as portrait cameras. One is the Canon 5Dmk2 and the other is the Nikon D610. Both had their foibles but with the right lens on the front both made files that made peoples' skin look better than other cameras I've shot. Both were capable of high sharpness tempered by good handling of highlights. Both were solid and reliable.
In an age with lots of good choices I can see why wedding, baby and portrait photographers are drawn to traditional cameras. I think it has a lot less to do with usability and mirror versus no mirror and a lot more to do with how the two big players have optimized their files for rendering humans more beautifully. I don't have extensive experience with Fuji cameras so I can't really compare them.
If I were to counsel someone today whose goal was to make the best possible portraits, budget not an issue. I'd direct them to either the Canon 5Dmk4 or the Nikon D850.
You can do good work with any good camera and across formats. It might just be easier to make a classic portrait with one of these two cameras. Mostly by dint of the sheer amount of color science research and development that's gone into them over decades.
Which one is the best? The one whose lens system you already own.....