Vignetting on super tele primes?

read off some forums that when primes of 400mm or greater (with max aperture) is attached to FF bodies, there'll be some small amount of vignetting.

is there any truth to it or completely baseless remarks?

The field of view on an FF camera at 400mm is about 6 degrees. I can't see that ever vignetting.

Seems fairly impossible to me, too. A similar end result may be possible if the wrong hood is attached, though, though I'm not sure if this would properly be called vignetting.

I recall reading somewhere that very long lenses may suffer from mirror-cutoff. SLR mirrors usually do not extend all the way down to the bottom of the mirror-shutter chamber in the camera. Some of the light from a very long lens may pass beneath the mirror and not be reflected up to the viewfinder.

This phenomenon would affect only the image seen in the viewfinder, not what is seen by the film or digital sensor.

This may, or may not, be the issue discussed in the other forums.

The term "vignetting" is being misused by those that made this statement.

The term is "light fall off" ,. and especially with Digital FF it is indeed a problem.
Most lenses have measurable fall off to some degree,. but the way that light hits digital sensors exacerbates this problem,. since the farhter from center of the sensor the light hits at more of an angle,. it increases the fall off. Film was simply less sensitive to the angle that light hits it.

Having never owned an FF Digital though I do not have experience with it first hand,. but I most certainly do see fall off on my 1.3x framed 1D MkII with long lenses.

It is subtle,. and not ever a concern for me.. but it is perceptible.

You are the only one to get this right so far.
Lenses of 600mm and over do show some cut-off with SOME bodies because of the mirror size.
Canon made a point of stressing in the 5D specifications that there was no cut-off with the 600mm - where that leaves the 1200/5.6L I don't know!
And as you say there is no image cut-off it is just a finder problem.

... ? ...

I realize the phenomena you refer to is also a real one,. but light fall off is certainly a real phenomena.. it is tested for in reviews.

As the OP mentions "with max aperture" I am pretty certain that the subject he was referring to is light fall off .. which occurs at large apertures and diminishes as you stop down.

All else being equal (ie on a zoom lens) The longer the focal length the more one needs to stop down to eliminate light fall off.

The Mirror cut off issue is a different beast all together and I do not think it is eliminated by stopping down.