Alan Chan http://www.pbase.com/wlachan
It starts to darken with longer focal lengths the longer the more pronounced. The effect is due to the geometry of the light path. It's not blocked, the path is actually off the bottom of the mirror, it's hard for me to explain without diagrams. On my MXs the darkening is apparent but there is still some image at the edge even at 600mm. Needless to say you don't see this effect with cameras equipped with longer mirrors such as the LX. By the way I just tried to mount a LX screen in a MX and I had no problems. I hate to think you might be having more equipment problems.
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