Replace ACL Viewfinder With One From NPR

Using an NPR Orientable Viewfinder with the ACL

NPR and CP-16 orientable viewfinders can be adapted to the ACL, with some slight modification to the finder at the point where it attaches to the camera, machining the appropriate mount for the camera , and the introduction of a small diopter to bring the finder into the correct range for the ACL.

Adapted to the ACL, the NPR finder is 2 inches longer than an ACL finder. Therefore it is not suited to shoulder holding the camera — use the non-orientable ACL 1 finder instead for shoulder holding. BUT FOR EVERYTHING ELSE, it’s like having a built-in 2″ extension tube!

Below is one approach to adapting the NPR finder — it requires a modest modification, and the creation of a custom machined adapter. There may very well be shops that could create an adapter to fit the NPR finder perfectly with no mods, and then screw onto the ACL — I’ve never struggled with the math. But this is how my own NPR finder was made to work an ACL, yielding an image identical to the ACL 1.5 Angenieux orientable.

PHOTO 1: The rear of the NPR Finder needs metal filed away to fit in an ACL-type adapter, (an adapter very similar to the adapter used on the ACL 1.5).  (left: special NPR adapter; right: standard ACL 1.5 adapter)

PHOTO 2: An adapter very similar to the ACL 1.5 adapter needs to be created for an NPR finder — but there ARE differences. From overhead, they look almost identical. A metal plate shaped like the area on the ACL that the adapter will screw into has a thread attached on top. This plate starts out as 30mm x 38mm, and then is contoured to the correct shape. Note that the threads will need to be dug into on the sides to allow the screws to enter the plate. Also, the “key” on the inside of the threads (this “key” is used to orient the finder) is 90 degrees different between the two … on the NPR adapter it is adjacent to a screw hole (hard to see in the photo on the left). On the normal 1.5 adapter this “key” can be seen at the top of the right photo. Also, the hole bored in the center of the threaded-piece is 19mm in diameter on the factory 1.5 adapter . For the Custom NPR adapter this needs to be 20mm in diameter.

PHOTO 3: A side view shows the major difference between the two adapters. On the custom adapter, the black plate that screws into the camera is 6mm high, (the normal 1.5 adapter plate is 2mm), and the threads are 6mm high (the normal 1.5 adapter threads are 9mm). The normal 1.5 adapter has an extension “baffle” on the base to enter the camera body to act as a light seal.

PHOTO 4: For the custom NPR finder adapter, the rear of the metal plate is bored into, and a diopter is epoxied in place to bring the NPR finder’s diopter adjustment into the correct range for the ACL. This can be a small round diopter typically used in the viewfinder of a 35 SLR camera for custom eye correction. I experimented to find the right diopter, and have long since forgotten which I used. Start with a +3 and a -3, and find out which direction to go in. One of those will get you pretty close … then finesse the number if necessary.

(Optical Electro House in Culver City, CA is a good place to do this kind of modification. Take these plans in.)

Of course, if you happen to have a spare standard adapter used for the ACL 1.5, you could probably convert it by boring the 19mm diameter to 20mm, repositioning the key, and cementing a diopter onto the end of the light baffle. The above plans are provided on the assumption that no such factory ACL 1.5 adapter is sitting on the shelf.