WARNING!!

In this thread I'm questioning things people have proven to be true not only by math and science but also by practice. It will not seem logical. If you want to close the tab and go about your day conniption-free, please do. I won't blame you.
If you're willing to continue on, thank you in advance for your patience. Also, buckle up

So I'm wondering about f/stops and brightness and different focal lengths and a whole load of other stuff.
NOTICE: All my assertions about brightness and stuff is based a metering system seeking to average the light and strive for an average of zone V. I haven't even thought about how it would change with, say, zone Vi'ing people's faces.


From what I understand about the f/stop, mostly from http://www.uscoles.com/fstop.htm , the f/number is a ratio between the diameter of the aperture and the focal length of the lens.
Longer lenses need bigger apertures diameters to pass the same amount of light, so we use f/stops to make everything simpler. f/8 on a 90mm is f/8 on a 300mm, etc.

Ok. Makes sense to me so far.
But. Two different lenses will give the same exposure for the same subject only if the camera is moved so the subject fills the frame
Right?

Let's say we have a wall 4x5 feet with an even luminous density of 30 cdl/sq. ft., suspended in an infinite blackness. Total output of the wall is (4x5)x30 = 600 foot-candles. Both a 300mm f/8 and a 90mm f/8 will give the same exposure: 1/30, asa64.
But they need to be at different distances so the wall occupies the entire frame.(we'll neglect light loss from distance)
Right? If the camera was fixed at a point where the wall filled the frame of the 300mm lens, the 90mm lens from the same distance would show tons of black around it, and would need a longer exposure to attain an average zone V, right? Because the bright wall would take up a smaller percentage of the frame? I'm already starting to confuse myself haha.

However, there are some subjects you can't really get closer or farther away from. Let's say the sky. A 300mm lens and a 90mm lens are picking up very different fields of view. So for a theoretically even luminous density sky, there is less light entering the front of a 300mm than a 90mm, by virtue of it taking light from a narrower field. So to give the same exposure for less total light, the 300mm needs to be more 'light-efficient', right? kinda like the slow, wide river or fast, narrow river distinction.
https://petapixel.com/2014/01/29/pic...y-photography/
^^ in the above link, about halfway down, they mention this thing called clear aperture, which is their word for aperture diameter.

"The 24mm f/2 lens collects light from a comparably wider field of view than the 100mm f/2. Since they’re both f/2, they both capture light at the same “speed”. So for equal shutter speeds, they should provide the same illuminance at the sensor." <-- this is because they're talking about astrophotography, so both lenses are viewing (essentially) a wall of even luminous density
here "speed" is intensity X area

"So in terms of exposure value, the 24mm lens will produce equivalent brightness images for any given ISO and shutter speed because it’s pulling light from more of the scene than the narrower 100mm lens, hence the identical f/number rating. The long lens collects more light at a time from a smaller area of the scene while the short lens collects less light at a time from a larger area of the scene."

So this would seem to say that a longer lens is brighter than a short lens for any given subject angle of view, even if they are the same brightness for their own lens angle of view.

Am I talking crazy? The more I think the more I'm confused. I realize that people have been using these principles for longer than forever, so that's what I'll use when I'm out in the field.
However, I'm curious and want to know more. If anyone has anything that would help sort me out it would be greatly appreciated.

Thanks!

Benno