Photography

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If you set up your tripod and took a photo of a subject with a 600, 300, 150 and 75mm lens all from the same place, then cropped the 300, 150 and 75mm images to the same as the 600 then the main difference would be the resolution, as you have to crop away more and more of the image.

If, however, at each step you moved the tripod forward half the distance then the subject would stay the same size but because the camera is getting closer each time the perspective to the background would be changing and the FOV would appear totally different because of that.

The longer focal length means that you can fill the frame with the subject from further away without having to crop into it as much.

You'll hit a point where you sill start getting worse results - not necessarily because of the lens but because of the atmospheric conditions. Heat haze caused by he sun heating up the ground will give a mirage like image and start to blur objects further away - tyhis all depends on location and conditions.

I’d say the real answer depends on what conditions you’re shooting in. If you have a 400 or 600 2.8 they make a gigantic difference over a 200mm lens both in real reach, sharpness and importantly bokah. But if you’re shooting on a hot football field for instance you’re going to run into distortion from the atmosphere.

Honestly there is just very little comparison between a 200 and 400, let alone a normal focal length lens.

Subject compression is noticeable all the way past 600mm, just look at landscapes shot at that focal length and see how close everything gets together.

It's harder to tell with portraits because someone's nose to eye distance isn't that much to begin with. I love long telephoto portraits though, thinking about buying a 300/2.8 just for that reason...

it seemed like the FOV was just cropping in more rather than actually feeling closer to the subject.

Yes that's what changing the focal length of any lens does. It enlarges the image so that a smaller portion the image fills the sensor (the rest is effectively cropped out). The only difference is if you were to take a photo with a 300mm lens and then took one with a 150mm lens and cropped it to match the 300mm, and then took one with a 75mm and took that and cropped it, you'd still have the full resolution on your 300mm shot, your 150mm shot would be much smaller, and your 70mm shot would be tiny with very little detail.

You should see the same effect if you shoot something with your 24-70mm at 24mm vs 70mm.

More expensive lenses will be sharper. I'm certain if you compared your canon 70-300mm at 70mm vs your Sony 24-70 at 70mm, the Sony would probably be noticeably sharper, and I think that canon got worse at longer lengths.

And also you can hit diffraction, if you shoot at f/11 or smaller (f/16, f/22) that diffraction will further soften the lens. So shooting at f/5.6 to f/8 will be better, but if the lens is crap and soft at f/5.6, that doesn't help you as much.

Finally, there is atmospheric haze. If something is very far away, there is just a softness in the atmosphere from different factors from smog to mirage heat that can further soften your image.

"Is there really a significant difference between 300, 400 and 600m lenses? Of course I have no physical access to any such lenses at the moment."

yes

https://www.flickr.com/photos/186162491@N07/albums/72177720312764722/with/53080603595/

the shot of jupiter was only possible with a supertelephoto and a 1.4x extender.

Absolutely, they let you fill the subject in your frame.

Cropping images equals resolution loss and noise. You want to avoid that if you can.

Well clearly you’ve never shot birds, without long focal lengths most wild birds will be tiny in your frame.

Focal length always matters, depending on the job at hand