[joze]

popo / just to understand you right: you think that the mirror box vignettes front or back-focused spots? Here's a sketch of what I understand:

1. The lens is focused to on object O1 such that we get a sharp image on the sensor (blue rays)
   
2. The object O2 (red rays) is out-of focus, in fact it's in front of the focus
   
3. The image I2 of O2 lies then behind the sensor
   
4. The top rim rays get vignetted by the mirror box
   

Is this what you believe?

 

If this theory is true, I see the following consequences:

• out-of-focus objects which are BEHIND the focused plane don't get vignetted --> true, at least to my test images which would confirm your theory
  
• highlights in front of the focus would always generate this ugly diamond shape for a given body / aperture rendering the body / aperture combination pretty much inappropriate for this use case (if you like roundish bokeh). But maybe most of the real world use cases are such that you've the highlights behind the sharp object anyway. I'd consider that a major design flaw of the body.
  
• what seems strange to me is that JoJu's body (Nikon) shows the same behaviour as mine (Canon). If it was the body I'd have expected to see one body having the problem and the other not (unless the mirror box is built conformant to some standard).
  

I've to say that this is the most plausible explanation I've seen here.

 



 

If there's an asymmetriy of this kind of mirror box vignetting -- eg. top field points are vignetted differently from bottom field points -- the hypothesis could be easily proven by turning the camera over the top and reapeating the shot.