"Consider the following.
Imagine my 10x8 is set up with lens fitted, focused and the required exposure set. It’s film holder is loaded it with Ilford FP4 and the shutter tripped. Next I transfer the lens to my 5x4 mounted in the same position, focus on the same spot, and make an identical exposure, again on FP4. Now consider my 35mm panoramic film back fitted to the 5x4 without changing any of the camera or lens settings. It’s loaded of course with FP4, and an exposure made. Following processing of the film, what difference in quality may I expect to see when examining the negatives?
None!"
Absolute hogwash. You MUST be playing devil's advocate here, or I am just flabbergasted!
This is ignorant of field of view and composition, which are the real issues when photographing. You don't make a photograph to display image quality. You make one to display composition and/or content. Of course if you crop an 8x10 neg down to the point where you are only using a section of neg as large as a 4x5 neg, image quality will be the same as if you had popped a 4x5 reduction back onto the same camera setup that you had shot with 8x10. Any idiot can figure that one out on their own.
*This has almost ZERO real-world application, so is useless as far as comparisons are concerned.*
You're not going to be sitting there thinking, "Gee, I have my shot composed on 8x10. Maybe I should leave the camera set up exactly like it is, but use a 4x5 reduction back instead..." That's throwing away 3/4 of the shot you just set up and changing your shot entirely. That is like deciding whether to print something in 12 point font size and cut a square out of the middle of the page to read it, or printing it at 6 point size so it all fits on the same-sized square. Doing it the first way makes the page "impossible" to read as intended by the author.
The only possible application of your info is when your lens is simply too short for what you want, and you know you will need to crop way way down and waste 3/4 of your negative anyhow when you print. In that case, use the 4x5 reduction back rather than wasting a piece of 8x10 film. Similarly, if you know you will need to extensively crop your full frame digital shot to get the field of view you want on your print, use an APS-C camera instead for better image quality...but you won't ever ask yourself, when your full frame camera is sitting there on a tripod, shot beautifully composed, "I wonder if I should swap the camera out for my APS-C body instead...."
Instead, do this little test using the same horizontal field of view with each of the formats, then enlarge them. Put a 300 on your 8x10 and take the shot. Put a 150 on the camera, put on the 4x5 reduction back, compose just like the first shot, and take the shot. The 8x10 shot has to be enlarged 1/2 as much to make the same sized print. Your 35mm bit makes no sense, as you are using a view camera anyhow (wasn't that the original gripe...having to use a big camera?), and using the same width of film you are using on a piece of 4x5 film, so if enlarged the same amount horizontally, "image quality" will *obviously* be identical save for differences in processing, and other variables such as base thickness, different emulsion batches, etc.
Your way, you would shoot the shot on 8x10 with a 300mm. Then pop a 4x5 reduction back on and shoot it with a 300mm, changing nothing. I suppose that if you brought your 35mm camera with a 300mm lens, pointed it the same way you could take a shot with it as well.
With your way, you simply DO NOT get the same shot. It is a ridiculous and useless technical exercise that only illustrates that magnification is independent of format. Big whoopdeedo. You will never, ever have this consideration come up in real life shooting.
How are you these days?
