Seems that EdZ's write up concerns back focus and what ends up with moving the primary to reach focus--and that's ultimately causing aperture reduction.
But that still seems different from having an eyepiece located at the focal plane operating at full aperture, but the field stop exceeds the size of the baffle tube.
I don't know if my guess is right or David's regarding full illumination, however.
While it might be true that all of the light that can reach the eyepiece is there, if that is still getting vignetted by the aperture of the eyepiece, then I suspect we are losing light.
I thought vignetting would only reduce FoV, not light/aperture.
Quoting EdZ here regarding soft and hard vignetting...
"We just had this discuission perhaps a month or so ago. At the time I showed proof that the baffle tube DOES NOT limit the field of view. Yet i still see this claim made. Well it just aint so. What troubles me though is that people, even though they've received accurate information, keep repeating the same incorrect information over and over. It can be tiresome to keep correcting this misinformation.
The baffel tube, if smaller than the field stop of the eyepiece, will vignette the "amount of light" delivered to that outer portion of the field of view. For the baffel to actually cut off the field of view, that baffel would have to be so close to the field stop as to actually take the place of the field stop. Unless you are sticking your eyepiece right into the visual back, that will never be the case.
It has also been shown the light drop off caused by that vignette of the outer edges of the field of view is so gradual that in many cases, it cannot be seen at all. Isn't it ironinc that here in the SCT forum people raise false alarms about the outer fov vignette, and yet for years in the binocular foum we have been trying to explain to people the extent of the outer fov vignette, but they just can't see it.
The vignette will appear worse generally when it is also accompanied by hard vignette cut off. That is, if something is actually cutting off the aperture, then whatever is in the way is reacting as a stop and the field stop may be no longer acting as intended, therefore the outer fov might appear as significantly darkened."