I thought of having some fun with ray tracing modeling and this is a nice opportunity.
I though of sharing this 'cause I think some will find interesting.
The objective is this :
and most important
I did a very basic modeling of the camera mirror boxes and also a test model of the mirror's back face.
The dimensions are as close as I could measure them.
For the image circle scenes:
-The light source is positioned at the registration distance of the EF mount(44mm).
-It more closely resembles shooting with very small aperture.
-The light projection was set to the minimum required to cover the FF size sensor.
For the reflection model, I set three lights shooting very narrow beams on three surfaces.
The first is a close approximation to the mirror's back housing with the distinctive horizontal grooves.
The second is a flat surface. This should definitely produce an almost-circular reflection.
The third is the same mirror back but rotated 90 degrees so the grooves are now vertical.
The extra two surfaces serve as a test/verification for the ray tracing accuracy.
Here are the results :
Please note that the bottom and right sides of the mirror box are not included so we can see "inside" easier.
Image circle and the two sensors combined for reference:

40D mirror box:

5DII mirror box:

Observe how the 40D has a lot more mirror area covered by the incoming light. This puts 40D with its smaller sensor and mirror box in a worse situation than the 5D.
And this is the "why this shape" test.

Recognize the shape ?
The way I understand it (it's probably a nice educative discussion) is that reflections from the curved surfaces of the tip of each groove are all mixed and projected on the sensor.
I'm not sure if diffraction plays a major role here.
I could play with this model and deduce many more info, but time is running out.
Observe how the flat surface produces a circular pattern.
The vertical grooves produce a totally different pattern. The non uniformity is because the grooves are not symmetrical.
It takes the horizontal grooves to get what we see in reality.
This may not be the ultimate proof, but (at least for me) very strong evidence as to the mirror causing this issue.
I hope it was an interesting reading
Cheers
George
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