Maybe I've had this wrong the whole time, but as I understand it, if you shoot from an FX camera and switch to DX mode you're not really gaining any "reach". For instance shoot a pic with a 24.3MP FF D600 and then shoot the same pic in DX mode (effectively making it a 10.1MP DX sensor). Blow both those up to 100% and you should have the same pic essentially with the FX just having more image around the outer edges. Now if you shoot a 24.3MP FF D600 then switch to a 24.1MP D7100 and blow both of those up to 100% you should see that "reach".
I always understood DX mode in FX cameras being beneficial for performance reasons, not for gaining reach.
Maybe I've had this wrong the whole time, but as I understand it, if you shoot from an FX camera and switch to DX mode you're not really gaining any "reach". For instance shoot a pic with a 24.3MP FF D600 and then shoot the same pic in DX mode (effectively making it a 10.1MP DX sensor). Blow both those up to 100% and you should have the same pic essentially with the FX just having more image around the outer edges. Now if you shoot a 24.3MP FF D600 then switch to a 24.1MP D7100 and blow both of those up to 100% you should see that "reach".
I always understood DX mode in FX cameras being beneficial for performance reasons, not for gaining reach.
I've always found it to be a confusing topic, but my understanding is the extra "reach" comes from the higher pixel density that DX cameras usually have. When two cameras have the same MP but different size sensors, like the D750 and the D7200, the one with the higher pixel density (D7200) will allow you to crop much more and still have a detailed image. It's all about how many pixels you can put on target.
It gets a little more complicated with the D850 because it has enough px density to match the DX cameras. So in that case, it really does have the same "reach" as, say, a D500. If you crop the 850 down to DX size, it should look the same as if you took the shot on the D500. I say should, because there aren't really any tests out to show this yet. I did see one comparison, and I thought the D500 shot was ever so slightly better, but this was at ISO 25600.
It has nothing to do with the number of pixels. "Reach" means a DX sensor will record a smaller FOV than an FX sensors does simply because it's physically smaller.
Imagine taking a pair of scissors to an 8x10 print and cutting it down to 5x7.
I don't think that's what most people are referring to when they say reach. If it were simply recording a smaller FOV (which you're right, it is), there would be no advantage to using a DX camera because you could simply crop the FX frame down to DX and have the same image. However, the camera with the higher px density has more "reach."
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It is just cropping in the camera. You are a lot better off shooting in FX like the camera was designed for and cropping in post production. The only real time it is beneficial is when you need to shoot a lot more frames per second and quality does not matter..
Since when did shooting in DX mode increase your FPS any? I do know it increases your buffer capacity though. But FPS?
If you're firing away and know you're going to crop a lot in post anyway, shooting in DX mode (or Crop mode on a DX body) can garner you a higher burst.
"Reach" is about FOV, not pixel density. As in, "A 1,000mm has more 'reach' than a 500mm." You can 'reach' out further with the FOV of a 1,000mm than you can with a 500mm. Since a DX sensor is smaller than an FX, it's FOV is narrower given the same focal length. Hence, more 'reach'.
If pixel density were a factor in 'reach', then you'd have discussions about whether a D810 has more reach than a D750. Or whether a D7200 has more reach than a D7000.
If that's how you view it, then fine, but I personally think it's all about pixel density. Yes, a D810 does have more "reach" than a D750. If it weren't for the higher px densities you usually find on DX cameras, there'd be no point in shooting them with long lenses because you'd gain zero advantage. For me, reach = ability to crop, and the higher the px density, the more you can crop and still have a good image.
There are discussions about the extra "reach" of DX all over the internet,so I won't rehash it here. In essence, the D850 has about the same "reach" as the D500, because their pixel densities are about the same. DX mode on the 850 is simply cropping away over half the image, which can be done in post for the same end result.
Believe what you will, but try googling "crop sensor reach" and you'll find thousands of results to the contrary.