The Nikon D850
camera is fully capable of action photography.
However, a lot of action photography means powerful telephoto
lenses... and that's where a full frame (FX) camera like the D850 comes up short. In order to have the same "reach" with an FX camera you'll need a whole lot more powerful lens. Where you might use a reasonable size/weight/cost 300mm telephoto on a DX camera, you're basically going to need a much bigger, heavier and more expensive 500mm lens for use on the FX camera (actually it's 450mm, but no one makes a lens that focal length so you have to choose between 400mm and 500mm). You probably won't want to shoot for very long hand holding a 500mm lens, so will probably also want to invest in a fairly sturdy tripod rig. That will make you less mobile.
Yes, an alternative with the D850 is to switch the camera to "DX crop mode". But, when you do that you'll essentially be reducing the camera from it's 46MP image potential all the way down to around 20MP. There are far less expensive DX cameras that give you higher resolution than that!
A ton of resolution like 46MP with an FX camera is generally not necessary, ideal or even desirable for action photography... But it is great for landscape photography, portrait photography
if you will be making very large prints from your images. You'll really need to make prints larger than 13x19 before you'll see much difference between what the FX camera produces and images from a 20MP or 24MP DX camera (or now 32.5MP, just introduced by Canon).
An ultra high resolution camera also is very demanding of the lenses used upon it. It's not very forgiving and will bring out any short-comings of the glass used upon it. Big image files from a 46MP camera also require bigger memory cards, fill up hard drives faster and call for a computer with more processing power and RAM to work with them.
In fact, the lenses you choose to use will make more difference than the camera you use them upon. Good glass is more important than all the "gee whiz" features of the camera. FX glass is widely available, but of necessity is larger and heavier. It has to use larger lens elements to cover the larger image sensor. DX lenses can be smaller and lighter... and more affordable. Plus, DX cameras can make full use of both DX lenses and FX lenses. FX cameras essentially need FX lenses (again, yes, DX lenses can be used, but will "hobble" the FX camera for all practical purposes).
I use Canon gear... both crop sensor APS-C (DX) models and full frame (FX). I shoot a lot of action photography, including many equestrian events, and the vast majority with the croppers. Full frame just isn't all that necessary. I've had clients make as large as 16x20" prints from images I made with my crop sensor cameras... no problem! They came out great (even with older, lower resolution models I used in the past).
Below were shot with various Canon APS-C DSLRs with 8MP (30D), 15MP (50D), 18MP (7D), 20MP (7D MkII)...
I haven't tried one yet, but based on early reports the new Canon 90D with its 32.5MP sensor should be a great APS-C camera for many things! That's higher resolution... in an APS-C camera... than in all but one of Canon's current full frame cameras (50MP 5Ds and 5Ds-R). Should be quite good for landscape photography... at least for all but the very biggest enlargements. Plus it can shoot at up to 10 frames per second, matching the 7D Mark IIs that I currently use. It also has a high performance 45-point AF system that's usable with lens/teleconverter combos with effective f/8.
The 90D also suggests we'll soon be seeing an 83MP full frame camera from Canon. They recently patented a sensor with that resolution. Plus 83MP just happens to be exactly what 32.5MP APS-C scales up to in full frame. That's only rumor so far, but I'd put money on a bet that this very high resolution full frame camera will be a mirrorless camera in the EOS R-series... able to take advantage of the new line of RF lenses Canon is beginning to produce. I've heard some rumors of Nikon having a 60MP camera in the works, too. That's likely because Nikon buys some of their sensors from Sony, who just intro'd a 61MP full frame camera. We'll see!
Now that cameras are computers, they are always getting "bigger", with new features added, while costs keep dropping. (Old digital cameras depreciate rapidly, as new models are introduced.)
Really serious landscape photographers.... and some wedding/portrait photographers... go even bigger than full frame. Medium format digital cameras have been available for some time with 50MP and 80MP sensors. Now we are seeing 100MP and even 150MP!
For a lot of things, mirrorless cameras can offers some advantages. They use AF sensors embedded right in the sensor, which means they can have many more than a DSLR (the EOS R has over 5000!) and can focus almost anywhere in the image area. There's also no need to "calibrate" mirrorless AF systems, as may be necessary with DSLRs for optimal focus accuracy. They also use electronic viewfinders that can give you a preview of the image your current exposure settings will produce, as well as assist with manual focusing. However, for action photography, most still prefer an optical viewfinder such as a DSLR uses. Electronic viewfinders draw a lot of power, so 2X to 3X the number of spare batteries may be needed. There also are rather limited selection of lenses for mirrorless... Sony, Fujifilm and Olympus/Panasonic have the most extensive system of lenses for their cameras, but still less than half what's available for Canon or Nikon DSLRs.
Sony makes both full frame and APS-C mirrorless. Canon and Nikon are both relatively new (one year) to full frame mirrorless, so have pretty limited lenses so far, but many in development. Canon has a separate system of APS-C format mirrorless, but it's more amateur geared and has only around 8 lenses available after five years.
Panasonic has just introduced a full frame mirrorless, but has very few lenses for it so far. It uses a mount that several other manufacturers will be using, including Leica. But those will be relatively pricey.
Olympus'/Panasonic's other mirrorless are all Micro 4/3 format. They share the same lens mount, so between them are able to offer a fairly good selection of lenses. The upper limit of Micro 4/3 sensors currently is around 20MP... Most are still closer to 16MP.... But who knows what we might see in the future.
In terms of area... millimeters squared... APS-C sensors are approx. 50% larger than Micro 4/3 sensors (330 to 360mm sq. versus 225mm sq.)
And, in turn, a so-called "full frame" sensor area is over 100% larger than APS-C sensor area (856 to 864mm sq. versus 330 to 360mm sq.)
Still, medium format digital sensor area is 70% to more than 100% larger than full frame! (1350 to 2178mm sq. versus 856 to 864mm sq.)
The Nikon D850 i camera /i is fully capable of a... (
