>If the
>image on the screen is the same as will go on the film, why does
>metering off the screen not work?


Hi-- The meter reads a ground glass through a simple condenser lens and a Fresnel lens - works fine when the optics are centered, but decentering
the lens on the ground glass moves the hot spot away from the center of the frosted screen (important, if the metering is center-weighted, as most are
these days) and away from the center of the simple condenser lens and Fresnell lens, also producing unevenness in illumination (this is quite obvious when looking through a PC lens while shifting it - one side of the screen becomes
quite a bit darker than the oposite side). David