Replace ACL Ground Glass with one from NPR

It is possible to replace the ACL ground glass with one from an NPR camera. To use an NPR ground glass to replace the ACL prism, is is necessary to construct a metal frame that is almost the same size as the ACL prism itself, except that the top and bottom pieces extend out to be over the top of the L-shaped side piece. There are 5 pieces:

1) The rear metal piece that allows the ACL Ground Glass to be screwed into the camera (the actual ACL part). The side of this plate that doesn’t have the screw holes needs to have a piece of 1mm thick first surface mirror glued onto it.

2) Top and bottom pieces that are identical to the measurements of the ACL prism except for one dimension, they extend further forward than the ACL prism surface to be overtop of the L-shaped side piece..

3) Two of the L-shaped side pieces are required (there is only one used in mounting the ACL prism), with small rectangular holes to allow the ends of the NPR Ground glass to fit into these side pieces securely.

You’ll also need some tiny screws, a tap the size of those screws, and black epoxy.

ILLUSTRATION 1: The assembly as seen from above: The top metal piece is screwed into the rear plate that attaches the assembly into the camera (this plate is the actual ACL part). Two tiny screws attach the top piece to the rear plate (you’ll have to tap the plate). A first surface mirror 1mm thick has been glued to the inside of the rear plate.

ILLUSTRATION 2: In addition to the standard L-shaped side piece on the left, a second such piece has been attached with epoxy on the right. Each of these pieces has a small rectangular slit added to accommodate the end of the NPR ground glass. NOTE: Don’t epoxy the right L-shaped piece into place until the ground glass is in place! Also, note how the top and bottom pieces extend beyond the surface of the ground glass, to be overtop of the side pieces.

ILLUSTRATION 3: The screw that normally holds the left side piece in place is eliminated, and this side piece is epoxied into place. This expoxy also covers the slit where the end of the NPR ground glass enters the side piece, holding the ground glass firmly in place.

ILLUSTRATION 4: It is difficult to figure out what this is at first. You are seeing the assembly from the side that would typically have a convex lens on the ACL prism. In this case, the side is completely open, allowing you to see the mirror that is glued to the rear plate. The end of the mirror not required to bounce an image to the finder is painted flat black (as is the other end of the mirror). NOTE: The mirror may not fit all the way to the left edge of the rear plate, because the side piece may occupy some room. Don’t glue on the mirror until you have first taped the whole assembly together to judge that it is fitting together correctly.

If you have made all pieces perfectly square, and lined up all the pieces perfectly square, then the ground glass will be perfectly parellel (at a right angle) to the camera’s aperture plate.

This assembly will jut out closer to the shutter than the normal ACL prism, and a little filing will likely be necessary to get everything to fit. But it can be done. The resultant image should be a little brighter than the ACL prism (though that alone isn’t a good enough reason to go through all this trouble).