warped fresnel lens

[Los]

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.

[GPS]

Sure it should lie flat. Before you try to give your money to a pro why don't you check yourself why the lens is warped? Something in the way on the edges?

[Donald Qualls]

Given the Fresnel sheet is plastic, if it's warped as your images show it must be that it's not being compressed as it should be. If that's a factory Fresnel with ridges on one side and matte "ground glass" surface on the other, there should probably be a plain cover glass between the matte side and the photographer (to protect the relatively soft plastic surface from damage as well as to take up the space in the focusing panel); that would press the Fresnel flat against the front frame surface. With exact dimensions, any glass shop should be able to supply a piece of window glass to fit -- and installing it shouldn't put your focus off; rather, it should restore the Fresnel panel to the correct position so the focus on the screen matches that on film in a standard holder.

All of the focusing panels I've examined (a limited number -- 4 or 5), except the homebuilt one on my Aletta, allow removing the ground glass for cleaning or replacement by removing a small number of screws and lifing out a piece of the frame. Yours is likely to be the same.