Bob Hart

Stuart.


I'm covering this point in a separate post.

You have referenced a discrepency between the witness mark info on the side of the SLR camera lens on front of the Extreme and the real distance between the subject and the focal plane.

Just where you place the Letus Extreme relative to the Fujinon lens is totally divorced from the SLR lens focal plane position and its relationship to the SLR lens on front.

You should regard the SLR lens on front of your Letus Extreme to the groundglass as a separate camera.

If there is a discrepancy between lens witness marks and real distance between the focal plane which is the groundglass and the subject, then this is a separate issue to the Fujinon focus issue and a separate problem to resolve.


There will be four causes :

The lens witness marks are incorrect. (Sometimes the witness mark barrel can move on the internal focus barrel and can be re-adjusted by slacking off some small grub screws with the lens mounted on a reference instrument and resetting the position of the outer ring.)

The lens internal collimation is off.

The mount is incorrectly positioned in the Extreme.

The groundglass is out of position.


If you move the Letus body furthur forward of the Fujinon lens, you may find a little variation in the groundglass area you see for a given zoom setting on the Fujinon but that and an altered relay focal distance are the only other variations.

On the Extreme, the genuine Letus pillar which goes under the front tube of the Extreme body is normally attached so that the vertical upright which connects to the bridge is between the red thumbscrew and the flip enclosure, not forward of the red thumbscrew.

If it is in this correct rearward position, then the rear face of the pillar represents the focal plane and is what you hang your tape measure off.

So far as I know, the collimation or backfocus relating to the SLR lens mounted in front of the Extreme should be pretty close to correct with the Nikon mount and the others slipped fully home into the housing.

There may be a few millimetres discrepancy between real distance and lens witness marks. My concern is to have infinity focus available on the lenses. I want the backfocus to be correct as this is a critical issue for wide and especially ultrawide lenses which will simply go softer at all focal positions.