Certain Oberwerk 25/40x100mm Long-Range Observation binoculars have range-finding reticles installed in the right-side eyepieces. The reticle grid pattern is a series of short horizontal and vertical lines. For most of the focus range, the reticle will be out of focus, and invisible. But at a certain focus point, the reticle will appear in the field of view, as shown in the picture to the right.
The reticle divisions measure “included angle” in mils. The standard lines represent 1 mil, while the group of tightly-spaced very fine lines equal 0.2 mils per division.
The formula to use for measuring distance is-
Distance (meters) = [estimated height or width of target (meters) / included angle (mils)] x 1000
This sounds a little complex, but it’s really quite simple. Here’s an example: If a person in the field of view is about 1 mil (one grid division) in height, we can estimate that person’s height is close to 2 meters (6.5 feet), therefore our formula would be D= (2/1) x 1000. In other words, 2 divided by 1, which is 2, then times 1000. So a person that is one grid division in height is about 2 kilometers away.
Effect of IPD on Reticle
Because the reticle grids are mounted inside the eyepiece turrets, adjusting eyepiece inter-pupil distance will cause the grid pattern to rotate with the prism housing. The grid pattern is aligned to be reasonably straight for average inter-pupil settings, but those with narrower or wider-than-average IPD will see a slightly rotated reticle grid pattern.