The back focus ring on the Letus Elite and Letus Ultimate moves the position of the attached SLR lens forward and backward from from the film plain. This allows you to easily adjust precise back-focus.

The process to adjust back focus is as follows:

1. Once you have achieved the focus of the ground glass, attach the lens mount to the front of your Letus35 Elite/Ultimate, tighten the three screws, and attach your SLR/cinema lens. NOTE: For the most precise back focus calibration, use a prime (fixed focal length) lens set to 2 stops higher than wide open. For example, if you have 1.4, 1.8, 2.0, 2.8… markings on your lens, set the lens at f 2.0 or higher. Lenses typically produce their sharpest image 2+ stops past their widest aperture. If possible use something other than a Canon EF lens. Canon EF lenses (especially zoom lenses) are difficult to set proper back focus with.
2. Dial the SLR lens to ∞ (infinity)
3. Point the camera with the Letus35 Elite/Ultimate attached toward some distant objects more than 50+ feet away. If those objects are not in focus loosen the 2 little thumbscrews on the back focus ring and dial the ring until you get the objects in sharp, infinity focus. This is most easily done when attached to an external monitor as you can see precise focus easier.
4. Once you achieve infinity focus in step 3, dial the focus of your lens to some short distance that has the marked number on the focus ring (e.g. 3ft or 1.5m, etc). Hook a tape measure to the Letus35 Elite at the film plane hook and place something at the same distance that you set the lens distance mark to.
5. If you also achieve focus on this close object, your SLR back focus is set. If not, repeat steps 3 and 4 until you get both infinity and close objects in focus. If you cannot achieve this, your lens’ barrel marking may be off and the lens may need to be serviced.

NOTE: The back focus ring only has about 2.5mm of travel distance total. Back focus is a very precise process so not much travel is needed to get it set. The ring will only rotate 2.5 - 3 complete revolutions. Do not force the ring. Once you feel it hit the end of its travel, STOP.

It may not seem the ring is doing much but rest assured, if your lens is moving back and forth, it is working. Very little distance is needed. To confirm everything is moving as it should, look at the hole at the top of the adapter just in front of the back focus ring. Below is an image of the ring turned to both extents to show the total travel.

Ring at rear extent of travel.

Ring at forward extent of travel.