Upgrade your Letus35 Extreme by adding a back focus adjustment ring. By adding the back focus adjustment ring, you can quickly and accurately adjust the distance between the rear element of the prime lens and the ground glass (film plane). This results in accurate focus marks on the lens and sharp focus with the lens set to infinity. This is a great improvement for Letus35 Extreme design and will allow users to fix back focus issues in the field quickly and accurately. The back focus adjustment ring also incorporates a film plane line and peg on the outside of the adapter. This makes it possible to use a tape measure as an accurate tool for finding focus and also to calibrate the distance markings on the lens correctly.