Putting a Nikon Mount on Rack-Over Mitchells

Mitchell 35-mm Standards and High-Speed cameras are good choices for shots needing pin registration and a built in shutter. If a true reflex is not needed they are adaptable to many stop motion, printer and animation uses. The Mitchell turret will adapt to almost any mount, including the Nikon mount. Used Nikkor lenses are getting more expensive, but are still a bargain if you consider Zeiss primes, etc. Before trying this conversion, make sure you have basic drill, tapping and machine experience or work with someone who does.

You can make a whole new front for the camera, but I prefer using the turret and retain the raising/falling front feature and keep the other 3 lens holes in the Mitchell turret. Racking the turret over to an empty lens hole is an easy way to check for dirt in the aperture rather than remove film and aperture plate to look for a hair that might have fallen out in the process. The other lens holes can be used for lenses or attaching a parallax video assist that clay animators using the "Lunch Box", which allows viewing work in progress in real 24 or 30 fps time. A side by side non-reflex video camera gives a much better picture than a video assist looking at the camera's ground glass. There is a parallax problem, but with a locked-off camera, this is a minimal problem. I use 1-7/8" diameter plastic control knobs retained with a Delrin ring for an adjustable mount in the lens hole above the taking lens. Animators like to see more on the video than the camera records. The Lunch Box is from www.animationtoolworks.com. 877-625-6438.

To attach a Nikon mount it is best to find a Nikon K1-/K5 Extension Tube Set. Mounts removed from Nikon camera bodies are harder to use, but not impossible. (Use the extra 52-mm extension tubes with the K set for lens shades.) The lens hole that you select needs a slight relief shoulder machined into the lens hole to make room for the ears on the Nikkor lens. (1.935 " in diameter and .050" deep.) The lens mount and shim will fit OK on the turret face, but clearance for the lens ears is needed. Remove only the amount needed. You need room for the mounting screws in the Nikkor mount. (Note: a 10" lathe will not swing a Mitchell turret. You will have to use a fly cutter, Criterion or rotary head in a mill or use a larger lathe with small enough chuck openings. Or make a mandrel to fit the lens opening.) Note, Mitchell turrets are all slightly different and there might be metal or air in different places.

To lay out the holes I recommend making a register plug the inside diameter of the lens hole (1.800") stepped down to the minimum diameter of the Nikkor lens mount (1.730"). This will insure perfect centering of the mount in the hole. There isn't much room for error. You can end up trying to drill and tap holes into thin air if too close to the edge of the lens hole.

WARNING / Very Important! Do not attach a lens to a mount that has any burrs, screw heads or pins that do not retract fully. The lens will jam on the mount and make destruction of the lens, turret and/or mount necessary. If all is well, but the lens release lever does not retract the pin quite far enough you can make it retract further with a small screwdriver. Important! Run your finger over the mount before attaching a lens to check for high spots, screw heads or burrs.

Be careful about the orientation of the new mount. The lens release lever has to clear the Mitchell turret detent device and should be flush with the center circle of the turret. Using the Nikon K-3 ring will put the release lever at about 5:30 O'clock looking at the front of the turret. This puts the focus and iris marks on the lens to the side of the camera at 3:00 O'clock. To prevent drill or tap breakage, you should mill out behind any holes in the back of the turret casting, where the drill will come through metal AT AN ANGLE. If the 4 holes in the Nikon mount are lined up right, this will happen only with one screw hole.

(If you use the K1 ring you will have to remove more metal from the lens hole and place the release lever in a different place. The witness marks on the lens will also be different.)

I make .075" to 083" shims to correct the flange to film distance from Mitchell to Nikon standard. Some say that the Mitchell turrets flange to film distances are not all the same. I haven't found that so with 6 cameras. Turn the shim from Delrin, Brass or hard aluminum. I turned a tube to fit the inside shoulder diameter (1.925") and outside diameter (2.225") of the Nikkor mount. Make enough tube for several shims. Then cut them to length while still in the lathe with a sharp cut-off tool bit from the chuck side of the tube. I measure the length of the shim from the face of the tube to the cutting tool with the depth gauge end of calipers. You can sand the spacer down if too long, but keeping it parallel is an issue. If you make one too big, try again. You can also turn a smaller ring to fit the ID of the shim and remove small amounts. A small 6-jaw chuck is very helpful. A soft collet would also work.

I recommend carefully removing the lens locking lever and pin with a good Phillips screwdriver to help fitting the mount screw holes in the turret. Don't lose the little pin or screw. The pin can fall out when the mount is removed from the factory 52-mm spacer. You can now fit the lens and shim and check for flange to film distance.

Using the formula (d = f squared divided by a-f ) will give you the distance the lens is out of the mount when the lens is focused at infinity. d = displacement of lens out of mount at infinity. f = focal length of lens. a = distance of sharp focused object. You can use inches or metric measures. Note, d = 0 when a = infinity. Example: for a 50 mm lens (2 inch) set to infinity but is focusing at 10 feet. 2" squared/ 10x12"-2 =4/120"-2"=4/118"=.034" so the lens mount is .034" too far out of the mount. Shorten the mount by .034". Better yet, try .030 and check again.

You can also use a smaller zoom lens to check your film to mount distance. If the focus moves closer as you zoom wider, the mount is too long and visa-versa. To check if the camera flange distance agrees with the ground glass, you need an accurate depth gauge or auto collimator. (See Collimators and Auto Collimators in Professional Equipment Advice.)

Hold the lens firmly against the mount and see if it focus' to infinity. If the lens doesn't you have to shorten the shim. If it focuses beyond infinity, the shim is too short and you should make a new longer shim or try to cut some brass shim shock or try Scotch tape on the shim.

I used #1-72 x 1/4" long flat head Phillips screws. You could use 1.6 mm screws as in the Nikon K set, but the original screws won't be long enough and you will need new longer screws. You will have to countersink the 1-72 flat head screw heads in the Nikkor mount. Use a small countersink and not a drill as the drill can grab the brass and eat a much larger hole in the mount. (Don't let any screw or burr stick up on mount, it will jam the lens on the mount!)

To locate the holes in the turret, use a body clearance drill (# 50 for 1-72 screws) into the Nikkor mount and shim. Drill a small starting detent in the turret face for alignment of the tapping drill (a # 53).

Once you have decided where the mount will go, put in the register plug described above and locate, drill and tap one hole before doing more. Put in a screw. Then locate, drill and tap a second hole, put in a screw and then drill and tap the last two holes.

Once you have the correct flange to film distance, reattach the release lever and mill a hole for clearance of the lens release lever. Some turrets are thinner here and you might mill through the metal and you may have to epoxy (J-B Weld) a backing for the hole.

You will have to carefully remove material from the shim to clear the Nikkor lens release lever. Do it with a small file and not a saw.

I have a 100 ea. #1-72 screws and will share if you E-Mail me.

© Copyright 1999-2004 Ron Dexter. All Rights Reserved.