Better question: Full Frame vs APS-C ?
I've read things saying that after accounting for things like focal length and Crop factor that FF doesn't offer higher quality than APS-C. Is there any advantage to FF then?
I've read things saying that after accounting for things like focal length and Crop factor that FF doesn't offer higher quality than APS-C. Is there any advantage to FF then?
In general better glass is available on full frame since camera manufacturers produce pro full frame lenses, but infrequently do so with APS-C lenses. There are exceptions, such as Fuji, but in general the best glass is for full frame sensors.
Because full frame includes the high end, you can get features on full frame cameras that are not available on APS-C cameras.
APS-C's advantage is in lighter, cheaper glass for a given field of view. This is important when hiking! Another option is the m43 format, which has half the sensor area of APS-C, but in turn allows for even smaller and lighter glass and cameras.
It's all trade-offs. Full frame sensors will be less noisy and allow for wider fields of view with better quality glass. But you pay for that monetarily and in size/weight. Is the tradeoff worth it to you? Perhaps not. Or perhaps there is a killer feature that is only available on FF cameras from the manufacturer of your choice that you want, so you're willing to pay the monetary and size cost to get it. If you're really after narrow depth of field, low noise, or extremes of pixels, medium format is there and willing to suck as much money up as you've got available.
The point of full frame would be more flexibility at lower focal lengths, and the high correlation to being put inside of more fully featured and sturdier camera bodies with faster I/O. It doesn't have to be that way, it just is since the wafer cost is already higher and already segmenting customers by price, they just give a greater package to the remaining people that would pay more.
And yes, lenses increase in cost faster than the sensor area they illuminate, assuming they retain the same wide aperture/large incidence angles at the sensor end.
Thankfully, as a photographer you don't have to think about those optical restrictions. It's enough to know that there is a hard limit at f/0.5 that can't be reached with normal lenses (microscope objectives get much closer, though). Practically, everything below f/1.0 is disproportionally expensive.
A 50mm/f1.0 medium format lens would be far better than what APS-C could possibly do, just because you'd hit the f/0.5 wall before you'd reach APS-C.
The downside of larger sensors is that they are more expensive to make (less fit on a wafer, and the chance of catching a bad pixel is higher), and the optics are inherently heavier and larger, as well as often (but not always, if you can get away with a simpler optical design due to needing a less-open aperture) more expensive.
Thankfully, this only matters in two situations: using lenses that reach maximum resolution below f/2.0 with high-density sensors, or operating in darkness/with very short exposures.
Both that very-high-resolution scenario, as well as the darkness scenario benefit from larger sensors. The former from better frame-relative resolving power when diffraction limited, the latter from offering wider crop-factor-corrected apertures and thus capturing more light.
MOST FF sensors have bigger pixels (larger pixel pitch) than APS-C cameras. Noise tends to increase as pixel size decreases. If you crop a FF image down to APS-C size, it'll end up with lower resolution. So the narrower angle of view that an APS-C gets you gives an apparent zoom that can't be achieved with such a FF sensor. Whereas a FF sensor can take wide-angle shots better and has lower sensor noise.
There are exceptions to this. Notably the Sony a7R III, a7R IV, a1, and the Cannon R5 are FF cameras with small enough pixel pitches that they're actually higher resolution after cropping to APS-C than most APS-C cameras. So in these cases, there's no difference between shooting with the FF camera in APS-C mode and an actual APS-C camera.
In general, FF is better for wide angle (e.g. 14mm).
To a lesser extent - if you want a really narrow DOF, it's easier with FF sensors.
Otherwise - no real benefit.