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.
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.
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...