A Precise Method of Focusing Telescopic Sights

American Rifleman, vol 69, No 16, 1 May 1922 - page 4

BY J. W. Fecker, Optical Engineer

OPTICAL INSTRUMENTS have been developed for sighting ordnance material in order to increase the accuracy with which a piece can be laid upon a given target. In order to derive the full benefit of this valuable invention, it is a prime requisite that the instruments be correctly adjusted. This adjustment may be divided into two parts.

First, the instrument must be so assembled that the lenses and sighting mark within the instrument are properly spaced and aligned.

Secondly, the instrument must be attached and aligned to the gun so that the shot strikes upon the target when the sighting mark in the telescope is superposed upon the image of the target. I shall here deal with the adjustment of the first kind, as the second adjustment is already familiar to all users of telescopic sights.

In order that a fixed telescope shall always point directly at a fixed target no matter what the position of the eye, it is essential that the image of the target, as formed by the objective and erecting system, is exactly in the plane of the cross wires or other sighting mark. The eyepiece must be so adjusted that its focal plane exactly coincides with the plane of the cross wires. This being done the eye may be moved clear across the field without there being any displacement of the image from the cross wires. If an instrument is not so adjusted, when the eye is moved away from the center of the lens, the cross wires and image separate, i.e., the cross wires appear to jump over to one side, producing what is called parallax, a defect which makes it impossible to duplicate shots, as a very slight displacement of the eye from the center of the lens causes the image and cross wires to assume different relative positions, with resulting displacement of the shots on the target.

In adjusting any telescope with cross wires, the first operation is always to point the telescope to the clear sky or any uniformly illuminated surface, and move the eyepiece in or out until the position is found where the cross wires appear blackest and most sharply defined. In some telescopes the eyepiece is fixed and focusing is accomplished by moving the cross wires. In this case move the cross wires until position of maximum sharpness is obtained. The telescope should then be put in a steady rest and sighted upon the target, at the distance at which it is to be used, and it is important that the telescope be held firmly fixed.

The scope is now focused upon the target by moving the objective lens in or out until the target is sharply defined. Due to the large range of accommodation of the human eye it is not possible to tell exactly when the position of best focus is reached by judging the image alone. The final setting of the objective is accomplished by the parallax method. The head is quickly nodded slightly up or down, or from right to left, and the image observed carefully to see if the cross wires appear to jump. The objective is now moved very minute distances in or out, until a position is found at which the cross wires jump the least. Now move the eyepiece in or out a very little at a time, until a position is found where the jumping or parallax entirely disappears. The optical system is now in correct adjustment, and the position of the eyepiece and objective should be securely clamped.

When the sight is to be used on several widely differing ranges the adjustment may be made for the shortest range and the objective focused for the longer ranges and the position of the objective locking device marked carefully upon the tube with a very fine line and the range marked opposite.

After the scope is once adjusted for any range the eyepiece should never be disturbed, all focusing for different ranges to be done by moving the objective only.

In some cases the shooter will meet with a. condition in which he will find one wire will show parallax, while the other will not move at all. This condition is caused by astigmatism, which may be either in the observer's eye, or in the lenses of the scope.

In order to determine whether the astigmatism is in the telescope or the eye, observe carefully which wire has parallax, and rotate the telescope 90 degrees, and test for parallax. If the scope has astigmatism, the wire which appeared to jump the first time will remain quiet, while the other wire will show parallax. If the same wire appears to jump in both cases the trouble is in the shooter’s eye. Astigmatism in the eye can not be overcome by any amount of focusing.

It can only be corrected by the shooter having glasses fitted to his eyes to correct his vision to normal. A shooter who has astigmatic eyes must either wear his glasses when shooting, or have one of his spectacle lenses trimmed down and properly mounted in the eyepiece of his scope. A shooter who is near or far sighted only, can shoot with out his glasses, as he can focus the eyepiece for his eyes when adjusting the scope.

If the scope itself shows astigmatism, the only remedy is to have the lenses corrected.

This phenomena of one wire remaining apparently quiet and the other apparently jumping is known as cross parallax. It is caused either by the surface of some lens being cylindrical instead of spherical, or by strain in the glass. This latter may be caused by improper annealing, or some lens being mounted too tightly in its cell. Lenses should not be too closely mounted, as contraction of the mount in extreme cold will often strain the lenses sufficiently to cause cross parallax and affect the sharpness of the image.

This method of adjusting a sight seems very long and difficult when written thus at length, but in practice it is short and precise. An experienced observer can adjust a sight in a surprisingly short time, and during the war experienced adjusters frequently adjusted fifteen or more scopes per hour.