i recently discovered that my ground glass came paired with a fresnel screen from the manufacturer. then i discovered why i was having focus problems during my exercises. i think my fresnel screen is deformed. is replacing the fresnel something i can do myself, or do i need to have a technician do it? i don't want to screw up the back focus. also, if i do install a new fresnel, is it supposed to sit bowed in the frame as it is in the attached photo. i would have thought it should lie flat againt the ground glass.
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