There's just a different field of view. If I use a 24-70 on my Canon and my 12-40 on the Olympus (crop factor 2x), and set the Oly at 30mm and the Canon at 60mm I am standing at the same exact spot with each camera to get the same framing. Right? There is no reach advantage. [...] My 5D Mark III beats my Olympus hands down, just like it killed my old 70D. So get all the pixels on target you like, M4/3 will never beat FF.
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Only advantage in my experience is weight.
First, I don't own either a m4/3 nor digital Full Frame camera, just an 80D. So I have no real experience on the mattter. But I have been giving this some thought recently since I want to purchase a new lens to get more reach than my 55-250 IS STM provides.
So I believe you've demonstrated the point really well. The only differences between sensor sizes are the area which captures light, and for equal focal lengths this changes the field of view. To see the reach advantage, you just have to construct your example a bit differently.
You're standing in the same spot with two cameras: One with FF and one with m4/3 sensor. Both have a lens with the same physical focal length attached, say a Canon 300mm 4.0 L IS.
The m4/3 captures only one fourth of the lenses image circle's area. So it gives a tighter framing and collects only one fourth the amount of photons projected by the lens. Some people express these facts by saying the 300mm 4.0 behaves like a 600mm 8.0 lens on the m4/3 cameras. Think of that way of putting it as you wish.
The FF camera obviously captures more of the image circle, giving an image that is twice as large in both dimensions and contains 4 times the photons. However, it frames way less tightly.
But you could simply crop into the image. To match the framing of the m4/3 camera, you would have to half your image height and width, leaving you with the same FoV as the crop body and the same amount of collected light. So now your images should be equal in framing and noise, given the same output view size and similar sensor technologies. Your 5D image would have been reduced to 5,5 MP though, where the Olympus would have 16 so for large output sizes, the Olympus image might be better suited.
You argument from your original example was that you can just use a better lens on the FF body to avoid the cropping. Well, go do that. A Canon 600mm 4.0 will cost you more money than either camera did when you purchased it though. You could also just use a 2X teleconverter on the FF body and leave the crop as is for a mild price increase, but you would still lose 2 stops of light, bringing you back to a similar situation where both cameras frame equal and produce the same noise when viewed under identical output circumstances.
So, the reach advantage boils down to: Theoretically, at 300mm the higher pixel density of a crop body yields a greater potential for magnification using the same lens. Or, put differently, for a constant weight and price, you get more reach from a crop body.
If my understanding of this topic is wrong I would love to hear about it. I've used this argumentation to estimate how much value I'd get out of a Sigma 150-600mm C compared to a Canon 100-400mm IS II and so far I'm thinking I'd be losing some AF capabilities and build quality but getting equivalent optic quality and reach with 118% the weight for 45% the cost. Which seems a good deal for me. If I see this wrong, please enlighten me.