How do you switch the lens mount- Canon/Nikon

[Matt St. Charles]

Just got a used Letus35A with swappable mount, and I can't figure out to reverse the mount. It's on the Canon FD side now, and I can see that the Nikon side opposite it, but I can't get it out.
Is there a manual for the Letus series?
Thanks!

[Bob Hart]

You need to unscrew the thumbscrew on front of the Letus body until the mount is free to move. You then slide the mount forward out of the body, face it around and re-insert it with the Nikon mount facing outwards.

This reversable mount is an innovative convenient feature but has limitations on the Nikon side. It does not positively retain the lens with a pin lock so the lens can inadvertently be unmounted by moving the focus.

You are advised to examine the dedicated Nikon mount from Quyen Le.

For correct backfocus or the correct relationship between the flange face and the focal plane, the reversable mount needs to be about 1mm forward of its most rearward travel inside the Letus35 body. Ultra-wide lenses will be most adversely affected otherwise.

Moving the lens mount forward for correct backfocus makes it unstable in the front body. The first tendency to fix it is to over-tighten the thumscrew which may strip the thread which is cut in the aluminium tube of the body.

A workaround is to precisely cut a cylinder of thin walled PVC pipe about 5mm broad and cut a section out of it so that it can be squeezed inwards to fit snugly between the back of the mount and a small shoulder immmediately in front of the dust excluder glass.

For final precise adjustment, cut some thin shim washers to go between the plastic cylinder and the dust excluder glass. Paper or cereal packet material is adequate. The total combined spacer is about 6mm.

This spacer insert helps to stabilise the mount from rocking on the pressure points of the screw tip and the body 180 degrees opposite in the hole and maintain squareness of the mount in the body.

You may find the thumbscrew walks the mount forward as you tighten it. The screw tip falls partially over the groove cut for the Canon lenses.

I added two more screws to stabilise the mount for heavier lenses but cutting and tapping these risks damaging the battery wiring.

The screws tend to crush metal into the cCanon mount groove and the lens mount position becomes unmanageable.

I also milled flat bottomed axial channels in the mount for all the screws to bed down onto to fix this problem.

If you do this you have to replace the thumbscrew as it becomes too short.

It is an awkard business this workaround, which is why I recommend you examine the dedicated Nikon mount.