Arri 35 Viewfinder Low Budget Modifications

Some information to begin with:

1. Images are formed in different places within a motion picture camera.

A. On the film.
B. On the reflex ground glass.
C. Relayed from the position where an image would be normally be formed on a ground glass, to a position further back as in the Arri 16S.
D. Relayed from the ground glass to right in front of an eyepiece for viewing (Arri IIs and IIIs).
E. Relayed some distance by an eyepiece extension viewfinder.
F. Relayed from a front lens in periscope lens system. These images are viewable with an eyepiece; ground glass, thin piece of paper or other optical system focused on that image.

2. A lens can relay an image from one position to another on the optical axis. It's just like macro photography of an object with a lens extended out of the mount. In viewfinder optical systems the image can be enlarged to facilitate focusing or reduced if needed to view the whole image in the eyepiece. Most viewfinder optic systems are about 1 to 1 in magnification. An image 50-mm distance from a 25-mm focal length lens will form (relay) that image 50-mm the other side of the lens. That is about 4 inches from image to image. Each relay lens group inverts and flips the image side to side. The viewfinder optic tube and eyepiece is a short-range non-erecting telescope.

3. EXTENSION VIEWFINDERS are a 1 to 1 relay lens system with a prism to revert and re-flip the image side to side, so the image remains right side up as it was in the original position. Some monoculars are a good starting point for making an extension finder. They have image erecting prisms and focusing eyepieces already attached. The front lens has to be replaced with a much shorter focal length lens and its distance adjusted to provide the right image size to your eye.

FIELD LENSES bend light rays back into the light path that would be "lost" to the side of a relayed image. The Arri 35-mm ground glass is also a field lens facing away from the ground glass side. These lenses can be simple lenses and not color corrected, as prime, eyepiece and relay lenses must be. This prevents darkening of the image to the eyepiece of wide-angle lenses. If the image is being photographed, as in a periscope lens, the field lens must be clean and free of faults because the faults will record on film or video. In viewfinders this is less important. Solid glass such as prisms and thick lenses extend the light path by about 50%.

Arri S, II C and III optic tubes are not exactly parallel with the taking lens axis. The mirror in the lens turret housing is at about 48 degrees to allow the finder tube to bend 6 degrees away from the camera to provide clearance for the operator's head.

Some modifications:

Making camera doors is a lot of work. Arri II A or B doors can be converted to fit II C cameras by removing some metal and adding some metal and/or epoxy to make them light tight. (Use Seran Wrap between the door and camera when fitting epoxy on an II A door to II C body.) II C doors will work on II A and B's with tape to block light or adding epoxy to build up the camera around the front of the door and lens housing to match a II C. An unmodified II A or II B door will no longer fit a modified II A or II B camera.

Making optics for an Arri II C doors with missing optics is an easily solvable problem with a lathe and some trial and error. II C doors have larger optic tubes than II A and B doors. Optical tubes from II A and B doors will fit II C doors with a simple shim or even tape spacers. The II A/B eyepiece is not removable from the optical tube. The II A/B optic tubes are press fit into the doors.

The II C doors have a 3 cm. (slightly less than 1-3/16 inch) diameter optic tubes so the relay lens has to be small enough to fit inside a 3 cm. O.D. tube. The Arri 16 S optic tube is larger yet and is not adaptable to 35-mm cameras.

The image on the II A/B/C ground glass is inverted (as it is on the film) and has to be reverted to your eye. The image on the ground glass is relayed and reduced some to the rear glass plate of the optic tube where the regular ARRI 16/35 eyepiece is attached. (This makes the 16 and 35-mm eyepieces interchangeable along with the Arri Periscope Finder.) This image can be viewed with an eyepiece or ground glass but you can't see it directly by eye. The eyepiece "looks" at that aerial image right in front of the rear glass plate. This glass is not a lens but keeps the interior of the tube clean. . .

(Note. On the ARRI 16 S an image is relayed and enlarged to a ground glass at the rear of the optic tube on the door. The ground glass is viewed by the removable eyepiece. The door has register pins to insure the ground glass distance stays constant.)

The simplest eyepieces can be slip fit with a retaining screw. The best are ones have circular focusing mounts.

Here are some options for making viewfinder optic tubes:

A replacement II A or B optic. You need a smaller lens to fit inside the II A or B door. The tube is only about in diameter.

A replacement II C or III straight door optic tube can use a 25 mm relay lens and about a 25 mm wide field eyepiece. The tube is 3 cm. in diameter.

A close coupled finder can be made with a 90 degree prism in the II C / anamorphic / light trap opening. A 25-mm lens right outside the light trap and another prism 90 degree to the rear of the camera. The image rotates as with the 16-35 Arri periscope attachments. This puts the camera body closer to the operator's body for hand held work.

I have not made this modification, but considered it and it should work. For a Medical Arri IIC a ground glass holder has to be installed. An II-C ground glass should be used because it contains a field lens. For a short length system put a prism on the swinging door and a 25 mm /+ or- focal length lens as a relay lens next to the prism. The eyepiece would look at the image 50 mm from the lens. This lens would "look" at the ground glass in an installed ground glass holder. Next would be a right angle prism and an eyepiece to look at the image that is relayed from the ground glass. It will be erect and correct side to side. A 25 mm (1 in.) lens in an approximately 1 to 1 imaging situation would relay an image of the ground glass 50 mm (2 in.) from the lens projected 50 mm on the other side of the lens. (In this case you can blow up or reduce the image to suit. Arri reduces the aerial image so the same eyepiece would work with the Arri 35 as with the 16 S.)

Another solution might be a porro prism at the first prism and a roof prism to rotate the image in the image tube between the relay lens and eyepiece. This would be a manual correction. Some accessory manufacturers may have done this. So, where to get relay lenses? Try any old 16-mm camera lenses to prototype. In a 1 to 1 application they will cover a larger 35-mm image. Edmund Scientific may have some corrected lenses. Rolyn Optics has them.

Prisms are easier to mount than mirrors and are available from astronomical suppliers as are eyepiece optics.

One issue about a short focal length relay lens is some wide-angle lenses will appear darker in the corners in the viewfinder because some light rays hitting the groundglass will be "lost" and not picked up by the relay lens. These rays will be on the film and properly exposed. This also applies to tilt and shift lenses. Putting all this together requires a lathe, some metal and knowing how to use the lathe. I am not averse to using plastics, Delrin, PVC, and plumbing parts. Soon for the site I will have "Telescoping Pipe and Tube" which gives the dimensions of many available types of pipe and tube.

The "Fallini Door" for the II C used a 50mm relay lens. It slid back and forth in the door to give a magnified position like the Mitchell 35 Rack Over viewfinders.

With a ground glass removed you can see part of the image that was formed on the ground glass. It is very bright, but you can't use this image for focusing because your eye also focuses and would correct for any adjustment you make in camera lens focus. You would be looking at an aerial image. Side finder 16-mm zoom lenses do this with a small ground glass in the middle for focus.

Some issues. You don't want an image that appears too large to the eye. I feel it is hard to compose/digest an image that appears too large. You never look at a picture too close. There is a comfortable distance in relationship to the size of the picture that it is comfortable to view it.

The Arri 16 optic tube is not usable for the 35-mm cameras because it blows-up the image too much, is too large in diameter and contains a ground glass.

Duplicating the Arri eyepiece threads is a metric lathe-threading job.

Astronomical prisms are already mounted and viewfinder optics are relatively cheap. The erecting prisms can be used for extension eyepieces.

I have used an old 75-mm f 2.0 lens as an eyepiece. It gives a reduced and bright image.

If you have a budget and need more professional looking equipment, try www.jurgen.com. Jurgen makes very fine doors for most Arri cameras.

If you are using the camera upside down with a periscope lens that inverts image to the viewfinder, you can remove the viewfinder optic tube and replace it with a simple magnifier that looks directly at the ground glass. The image will be small, but erect.
© Copyright 1999-2004 Ron Dexter. All Rights Reserved.