DV Info Net - 3 Must Have Lenses

My imagining is that the Nikon to M42 adaptor may add distance. I am not familiar with this arrangement so you should ignore my comments and wait for others to chime in with their solutions.

Is the correct order for assembling the M42 lens to a Nikon camera like this:-

Nikon Camera >> M42 Adaptor >> Your lens?

Is there a correcting optic in the Nikon to M42 adaptor or is the adaptor a simple spacer ring?

If you can get access to a Nikon stills camera, film or digital, try your Nikon adaptor/M42 lens combination on this before you do anything. This will tell you if the combination is good for Nikon at all and you won't waste time with more elaborate testing.

Firstly, the reversable Canon/Nikon mount for the Letus35 is about 1mm too short for Nikon backfocus when pushed fully back into the Letus front housing. Try moving the mount forward in the Letus35 until the flange face of the mount is 10mm from the front face of the Letus35.

If the infinity focus becomes correct, then your problem is more or less solved. If this does not fix it then some more complicated testing will be needed.

Before you go abandoning your M42 lens system in favour of Nikon or Canon, mount the Letus to your videocamera. Set it on a tripod at a table edge with the front of Letus35 just over the table edge.

Take the Nikon mount entirely out of the Letus35.

Choose your M42 mount 50mm lens and fit the Nikon to M42 adaptor. Set the lens focus to 1M and tape the focus ring so it can't move.

If you have a known good stills camera for the M42 system, use this to accurately set the lens focus to 1M by measuring from camera focal plane to a 1M mark.

Set the lens aperture to wide-open when you do this and for the other tests.

Set up a test pattern or Siemens star chart 964.5mm (965mm will do) from the front face of the Letus35 body where the mount goes in. Make sure it is dead centre and at right-angle to the optical centre axis. This makes the chart to focal plane of the Letus35 exactly 1M.

A test chart with resolution lines in block patterns is best. Although your videocamera may not be able to resolve the finest of them, a moire pattern may occur which will enable accurate focus. If you have focus assist or peaking options in your videocamera, select them on.

You will need to make a wooden stand to hold the focus chart upright and maybe a long piece of batten let into the bottom at a precise right-angle with the length towards the camera to easier enable aligment on the optical centre-axis. Propping the chart with heaps of books will not be good enough.

Offer your M42 lens with the Nikon to M42 adaptor fitted towards the Letus body. To make this easier you will need to make a little sled out of a short flat piece of particle board and two upright vee pieces screwed-glued on the ends to hold the lens and sled on the table top.

When moving the lens reaches a point where the test chart is in sharp focus. Measure from the rear flange face of the Nikon to M42 adaptor to the front face of the Letus35. Measure both sides so you know it is is in exact right-angle alignment.

Now, put the reversable mount back on the Letus and put your Nikon to M42 adaptor and 50mm lens back on. Make sure your tape has not fallen off the lens and that the focus has not moved. The test chart will now be out of focus.

Move the video-camera/Letus35 or the test chart. If you are able to get pinsharp focus again, measure the distance between the front of Letus35 housing and the focus chart.

If the new distance measurement is longer compared to your previous measurement, then the lens flange face is too close to the focal plane and has to be moved forward with a spacer.

If only about a millimetre is needed, then this adjustment can be made by moving the Nikon mount forward in the front housing and retightening the thumbscrew. (There are problems with this which I won't go into here.).

If you are not able to get pinsharp focus as you did with the lens sitting on the sled, then measure the distance between the flange face of the lens and the front face of the Letus35. Subtract from this distance, the previous measurement you made from flange face of the lens to front face of Letus35 when you were able to get pinsharp focus.

This will give you now much shorter the reversable mount would have to become to restore the correct flange-to-focal-plane distance.

If the Canon mount on the reversable mount is not needed, you might be able to get away with putting the mount in a lathe and turning off the back of the Canon end of the mount to shorten it and move the flange face backwards towards the focal plane.

You have about 6mm of rearwards shortening available before the back of the Nikon to M42 adaptor begins to interfere on the front of the Letus35 housing. If the back of the Nikon to M42 adaptor has an AI index relay ring on it, then you will only have about 5mm.

If your M42 lens system is good, ie., you have sharp lenses with available apertures of f1.8 or wider, you might be better off having a simpler direct Letus35 to M42 mount made up and forget about using and mutilating the reversable Canon/Nikon mount. This mount can never be a good solution for Nikon without modification.

If you go this route, have the barrel wider than the front of the Letus35 lens mount hole so you end up with a shoulder which will butt against the front of the Letus body.

For fine adjustability you might need to make the shoulder face a half to one millmetre furthur forward so that you have some rearwards adjustment if you need it.

You can then either cut some shim washers to go in between the Letus body and the shoulder face or between the M42 flange face and your lens flange face to establish the final backfocus and achieve a good firm mount into the Letus35.

I have worked this all out in my head from recollections as my own testing was done some time back and all the bits and pieces I cobbled together have been since discarded. So if anyone has corrections to add, please don't keep them to yourself.

For locking the Nikon lenses into the reversable Letus35 mount, I filed away a screw thickness from the "lens-on" end of the longest bayonet sector then drilled and tapped a radial screwhole which comes into that filed off section.

If you go here:-

http://www.youtube.com/watch?v=SB1IN8HTM10

--- the tail end of the clip shows a squareheaded electrical screw at the two o'clock position of the reversable mount when viewd front-on.

The centreline of this screwhole should come down onto the rear edge of the bayonet channel, not in centre of it otherwise the screw's outer edge will come down on the lens flange itself and you won't get a positive lock unless you tension the screw down firm.

With the screwhole in the correct position, fingertight is all that is needed.

You will observe another roundheaded screw at the four o'clock position. This screw simply filled a hole made in the wrong place at the "lens-off" end of the longest bayonet sector.