Optical Design

The ability of a riflescope to form an image is called “definition”. The quality of image definition in a scope, determines how
clearly you can see the target, how accurately you can sight and how quickly you can aim and shoot.

How one arrives at the quality of definition in an optical system is dependent on three things;

The way you determine excellence of definition is by resolution. This is a method of actually measuring what the eye can see in each system, and
goes beyond that into the theoretical realm of possible resolution of the instrument. This is because most U.S. Optics “Hi Res” optical systems
“see” better than the human eye on axis, some even off axis. All U.S. Optics “Ultra Hi Res” optical systems are better than the human both off
axis and on axis.

Ultra High Resolution:
Throughout the years, riflescopes have been made with one consideration; “design the optical system to the resolution specs of the human eye,
and no better.” Most companies have not even gone that far.
The average eye will resolve about 40 line pairs per milliradian, which has become the industry standard for acceptance in determining the
maximum resolution of a scope with the human eye. Problem is, their resolution falls off at the edge of field and will not typically do even 35 L.P.
Nine out of ten scopes one sees on the dealer’s shelf, will not resolve 50 line pairs per milliradian at the center of the field of view, let alone the entire
field. Their justification for this has been that”...you only aim with the center of the field, so why worry about the edge?” When reviewing riflescopes
today, most editors are bright enough to use a proper resolution chart to determine full field resolution. Hopefully, they will soon learn to observe and
be able to evaluate the other problems most scopes have, such as coma, chromatic aberrations, light transmission, fogproofing, adequate diopter
adjustment, parallax and usable true eye relief, (true distance of use in which 95% of the field of view can be seen.) The term “Brightness” has no
place in proper scope evaluation.

Resolution depends basically on two factors

Because the above items are apparent, it then becomes necessary to discuss aberrations in order to understand resolution. Once we
understand this phenomenon, we then can understand the concept of ultra-high resolution.

Aberrations

Chromatic Aberrations
The inability of a lens or a system to bring to a common focus light rays of different colors produces chromatic aberrations. Present when images
formed by different colors of light, which make up “white” light, are at different distances from the lens or are of unequal size.
Result
These out of focus images form colored halos around the object you are looking at, and seem to blur them together.
Solution
Chromatic aberration is corrected by the optical design, combining different types of glass which will form several colors at the same point.

Spherical Aberration
Inability of the different zones of a lens to form an image all in one plane at the same distance from the lens
Result
The middle portion of a convex lens has a longer focal length than the outer portions. Makes sharp focus impossible.
Solution
The designer must make sure the proper shapes of the lens to correct this problem does not upset the correction for chromatic and other aberrations.

Coma
A spherical aberration that passes light obliquely through the lens.
Result
Comet shaped image of a distant point. The image appears blurred because each detail appears “smeared”.
Correction
Design must correct this problem.

Distortion
Failure of the lens to form images of straight lines in the field as straight lines.
Result
Distortion occurs because the magnification of the lens system is not consistent throughout the entire field of view. A round object will appear oval,
or a square will have a non-square geometric shape, etc..
Correction
The designer must design an orthoscopic prescription for the lens system.

Curvature of Field
Occurs when the shape of the objective image is not formed on a flat plane, but on a concave area.
Result
The scope can be brought into sharp focus in the center of the field, but the edge of the field will be out of focus, and vice versa.
Correction
The erector lens system in particular, must be corrected properly to achieve necessary flatness of field image.

The U.S. Optics Ultra-HiRes series of optical prescriptions have been thoroughly optimized to make the proper corrections for all aberrations to
achieve the highest resolution possible. Lenses are manufactured, (as all U.S. Optics systems are), to exact tolerances and quality control standards.
Any deviations from tolerances in grinding/polishing of lens components or machining operations produce unwanted aberrations and deteriorated optical performance. The U.S. Optics Ultra-HiRes series of optics undergo extra steps in an already exacting manufacturing process, to ensure that these optical prescriptions are manufactured and assembled as perfectly as possible. These steps include the following:

Optical Terms and their Definitions

The following is a breakdown on how this works

FACTORS TO CONSIDER:

Back