Understanding and choosing optics Part II: how to weed out or accept the undesirable traits

Guns Magazine, May, 2008 by Jacob Gottfredson

In Part 1 of Understanding and Choosing Optics, we talked about resolution, contrast, the importance of exit pupil and the difference between Porro prism and roof prism binoculars. We also included a Zeiss test chart and showed you how to evaluate an optic for resolution and contrast. We only scratched the surface.

The Zeiss test chart allows us to test image quality and resolution and contrast are easily. The various blocks on the test chart decrease in size, allowing us to test the resolving power of the optic in a standardized way. Contrast can be gauged by how black the black is and how white the white is as opposed to the whites appearing gray and the blacks not being sharply black.

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While this can be accomplished using the print on a newspaper, the Zeiss test chart is precisely made and lets you know the resolving power in a standardized and measurable way you can express to others including an optical scientist. Also, the slant of some of the bars is not duplicated in newspaper print. If you tried to do the same using a newspaper, you'd have to send that same newspaper to all whom you wished to impart the information along with the comparison of another optic of similar attributes. Even so, determining the exact resolving power and the presence of astigmatism, for example, would be difficult. There are other more comprehensive charts available, and you can obtain them online. For now, however, let us climb a few foothills before tackling the mountain.

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Astigmatism

The chart allows us to test various aberrations found in optics in a predetermined manner. Let's start with astigmatism. Astigmatism is an extremely important matter in an optic. If the binocular, riflescope, or spotting scope has astigmatism, the image quality will be poor and will lead to eyestrain. To evaluate a binocular, for example, place the chart 15' away in good light, farther for more powerful optics. Put the binocular on a tripod and look at the chart. Focus the binocular the best you can on the vertical bars, then on the horizontal bars, then the diagonal bars. If you cannot focus them all at the same time, the optic has astigmatism. Astigmatism is not always seen when viewing common objects like a mountainside or the walls and shelves in your local sporting goods store. Although we may not be conscious of this aberration, it will frustrate you wondering why things don't seem quite right, and your eyes always seem to be strained. Resolution is never as good as it should be, and you will wonder why.

Even if you take the Zeiss chart to your favorite sporting goods store along with your tripod, many local sporting goods stores do not carry some of the better optics. When you begin testing them, you may become frustrated if the less expensive brands will not meet these tests. Amazingly, some in the $200 to $400 range nowadays will. Apart from that, you will have to rely on the opinions of writers and other resources. A Web site called "Better View Desired" evaluates binoculars and spotting scopes for bird watchers and does an excellent job. The site www.6mmbr.com does so for hunting and competition shooters. And then of course, we do our best here.

Remember: Evaluate optics with like powers and objective lens diameters. Don't evaluate an 8x32mm binocular against a 10x50mm, for example.

Spherical aberration and curvature of field are two other things to look for you can find using the Zeiss test chart. Curvature of field causes the image formation plane to become curved like the inside of a shallow bowl, preventing the lens from producing a flat image of a flat subject. When the center of the image is in focus, the periphery is out of focus, and when the periphery is in focus, the center is out of focus. The degree of curvature of field is largely affected by the method used for correcting astigmatism. Good correction of astigmatism usually results in small curvature of field. If the center of the field is excellent, as many are, the optic is not always bad. However, I prefer to look at all the field in the image without having to move every object viewed to the center of the lens.

Spherical aberration causes parallel light rays passing through the edge of a lens to converge at a focal point closer to the lens than light rays passing through the center of the lens. There is a limit to the degree of correction possible using spherical lenses. With large aperture lenses, the only effective way to thoroughly compensate spherical aberration is to use an aspherical lens element.

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Distortion

The lack of distortion is another trademark of a good optic. One of the requirements of an ideal lens is that "the image formed by the lens has the same shape as the subject." Distortion is a type of aberration causing straight lines to become curved (distorted) in the image, thus preventing this ideal condition from being fulfilled. Distortion stretching the shape in the diagonal direction is called pincushion distortion, and that which compresses the shape in the diagonal direction is called barrel distortion.

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Understanding and choosing optics Part II: how to weed out or accept the undesirable traits

Guns Magazine, May, 2008 by Jacob Gottfredson

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