FF vs. APS-C
Many shooters with full frame cameras in their bag choose to shoot wildlife and other subjects that require longer focal lengths with APS-C cameras. Especially with wildlife, there's no such thing as "enough" reach.
I'm one of those shooters. I have several FF bodies that I use for most of the photos I take. But when I am shooting field sports outdoors I use an APS-C Canon 7D Mark II as my main camera. The primary reason is to get more "reach" out of my EF 70-200mm f/2.8 L IS II lens. The 150-600mm lenses don't work for me because most of the field sports I shoot are under artificial lighting at night. Even with f/2.8, I still need to use around ISO 3200 to keep the exposure time to around 1/800-1/1000 seconds. To get the same reach at the same aperture I'd need a 300mm f/2.8 lens on a FF camera with the same number of megapixels.
EOS 7D Mark II + EF 70-200mm f/2.8 L IS II = $3,500 at current market prices.
EOS 5D Mark IV + Sigma 120-300mm f/2.8 = $6,000 at current market prices. The Sigma 120-300mm is a pretty good lens, but not quite as good optically as the current Canon or Nikon 70-200mm f/2.8 lenses at 200mm.
The cheapest 300mm f/2.8 lens in Canon's current lineup is the EF 300mm f/2.8 L IS II at $6,100 for the lens. Nikon's cheapest 300/2.8 is currently $5,500, Sony's is $4,900 (and isn't quite as good optically as the Canon and Nikon lenses).
Top tier APS-C bodies like the Nikon D500 or Canon 7D Mark II do handle a little faster than many similarly priced FF counterparts. There's not as much, if any, difference in handling speed between more expensive FF bodies and these specialized APS-C sports/action models. Handling speed refers to things such as frame rate, the number of frames in a maximum continuous sustained burst, AF system speed, and multiple direct controls for things such as aperture, exposure time, ISO, focus point selection, etc.
There are many wildlife shooters that combine the 150-600mm lenses with APS-C cameras in brighter lighting conditions. Their primary reason is the extra reach it gives them. An additional reason is that sports/action/wildlife tend to be "high mileage" types of shooting. It's much easier to stomach replacing a fast handling APS-C camera like the D500 or 7D Mark II that each cost around $1,500 when it's been worn out in only a few years than it is to absorb the cost of replacing a $3,500 FF body that's been worn out by the same amount of use.
The key parameter regarding "reach" to keep in mind when comparing sensor performance between FF and APS-C cameras is to look at pixel density. That's basically how many pixels are in an unit area of the sensor.
If a FF sensor and an APS-C sensor from the same generation of technology have the same density, then one can get fairly equal results by cropping the images from the FF body to cover the same angle of view as the APS-C body. For instance, the 20MP Canon EOS 7D Mark II has pixels the same size as the 50MP Canon EOS 5Ds and 5Ds R. The pixel density of the newer 45MP EOS R5 is only slightly less dense.
On the other hand, if one uses a 20MP FF body, by the time one crops to the same size as an APS-C sensor that only leaves about 8-9MP, depending on whether it's a 1.5X crop or a 1.6X crop. But 20MP APS-C sensors require less data processing per frame than 50MP sensors, so when using the fastest UDMA-7 CF memory cards A camera like the 7D Mark II can shoot at 10 fps for about 30 raw frames or for as many JPEG frames as the memory card or battery will last. The 50MP 5Ds, on the other hand, is limited to 5 fps for only about 14 raw images or 500 JPEGs before the frame rate bogs down when using the fastest CF memory cards. Nikon models are similar.
Nikon D5600 does have some limitations with low-light photos.
Every full frame camera also has limitations with low light photos.
The marketing departments of camera manufacturers make it sound like their newest models have solved every photographic problem ever known to man. If you want to know what the weaknesses of that particular model are, just wait until they announce the replacement in a few years. They'll then tell you how the even newer model has completely solved those problems that the last new model had.
The difference between FF and APS-C cameras that have sensors with the same generation of technology is about one stop when the images from neither camera are cropped. If one crops an image, the total area that is used to collect light for the resulting photo is reduced and it's basically the same thing as using a camera with a sensor the size of that cropped area.
If that extra stop gives:
- An exposure time half as long
- An ISO setting half as high
- Allows a lens with a max aperture one stop slower
- One needs that extra stop to get the shots one wants all day long
then it's probably worth the extra expense for the FF body and longer focal length lenses for most shooters.
If that extra stop only gives one an extra ten minutes to shoot at dawn and dusk, then the added expense might not be worth the added expense for many wildlife shooters.
D5600 DSLR vs. α7S III MILC
The differences between your current D5600 DSLR and a FF α7S III MILC go far beyond sensor size and pixel density, though there are some stark differences there as well.
- One has an optical viewfinder, the other has an electronic viewfinder. Both have advantages and disadvantages, but they're certainly different.
- Battery life. Long shooting sessions tend to use up a lot more battery power using MILCs than when using DSLRs. Keeping the sensor and viewfinder running all of the time consumes power that DSLRs don't use when one is looking through the viewfinder at wildlife for long periods of time.
- MILCs tend to be smaller and lighter than DSLRs. But the Nikon D5600 is on the small side for a DSLR and the Sony α7S III is larger than many other MILCs. The D5600 is actually slightly smaller and 3/4 the weight of the α7S III.
- At only 12.2 MP, the FF Sony α7S III is more of a video oriented camera that can take stills than a stills oriented camera that can shoot video. You won't have much leeway at all to crop images from such a camera. Cropped to APS-C angles of view, images would be 5.4MP. The Sony α7R IV, at 61MP, would probably be more useful for wildlife photography.
- To get the same pixel density as your APS-C D5600, one would need a 55MP FF camera.
- The Sony α7S III is significantly faster handling than the D5600. The Nikon D500 is as well.
Here's a full comparison between the Sony α7S III, Nikon D500, and Nikon D5600 at B&H.