I repeat: a 200mm lens in DX format and a 300mm lens in 35mm format each take an image of about 40º angular field of view and record a portion of it with about 8º field of view, which measn that they use abot the same fractio of the image. Please explain how one format is cropping more than another.
Well, I'll answer this with a few other points from your previous posts.
That leaves only your example of the 400/2.8: but Canon and Nikon do both have 400/2.8 lenses that are perfectly usable with EF-S and DX bodies! Surely you know that at such long focal lengths, nothing is gained by designing for a smaller than 35mm format image circle?
Let me reapt anothe pont that yopu have ignored: longer telephoto focal lengths are almost always achieved essentialy but putting a teleconverter into a lens of shorter focal length, so that the resolution in lp/mm decreases in proportion to the increase in focal length, giving about the same resolution in lp per picture height. (Your low opinion of the effects of teleconverters on resolution has been noted already in these forums.)
My view has been for a long time that whatever the focal length, if a smaller image circle is the design criterion, then a higher resolution is possible. A 300mm lens designed for the 35mm format can produce significantly higher resolution (in terms of lp/mm) than a standard 300mm lens for an 8x10" field camera.
I don't understand precisely how this is possible, except in very layman's type jargon, ie. robbing the quality of the outer edges to provide greater resolution in the centre of the lens. According to basic principles, a 300mm lens throws an image circle 300mm in diameter. But modern lens design has changed that result considerably.
I've checked the Photodo results for the Canon 400/2.8 and 600/4 and it's clear the 400/2.8 has a higher MTF, is a better lens; but only marginally. At f8 we have a rating of 84 for the 400/2.8 and 82 for 600/4. This is the sort of difference between the Canon 50/1.8 and 50/1.4, but I have no doubt that a 600/4 on a 1Ds2 would deliver higher resolution than a 400/2.8 on a Nikon D2X.
Now, if you have some insights into lens design that can shatter some accepted fallacies, then I'm all ears. Please explain them clearly without hand waving, ambiguity and smokescreens