![]() ![]() aplanatic points.ĭark focus See resting state of accommodation. See confocal principal focus focal line.Īplanatic foci A pair of conjugate object and image points for which an optical system is free of spherical aberration. camera or projector) in order to obtain a sharp image. The centre or starting point of a disease process. The point at which rays of light converge after passing through a convex lens to form a real image ( real focus), or diverge from ( virtual focus) after passing through a concave lens. The star was developed by Siemens & Halske AG (today Siemens) in the 1930s to test the lenses of Siemens narrow-film cameras.1. ![]() (The illustration under Optical transfer function shows spurious resolution caused by blurring.) When looking at the Siemens star with slightly blurred vision, e.g., without spectacles or with defocus from staring, this is seen as a shimmering ring around the Siemens star's center that changes size with viewing distance. (Spurious resolution appears similar to aliasing, but it is a purely optical phenomenon, so it occurs without need of pixels.) This results in inverted polarity of the stripe pattern: black stripes appear in the place of white stripes and vice versa (and further polarity inversions occur further inward). Under optical blur from defocus, a Siemens star (like any periodic pattern) gives rise to the phenomenon of spurious resolution above the resolution limit, i.e. (It is not blurred or aberrated, so it does not show spurious resolution.) It shows aliasing in the center, where the spokes are smaller than the pixel pitch. They are useful in drawing the eye to a point on the page.Ī Siemens star with 128 spokes. Siemens stars are similar to the sunburst pattern used as a background in graphic design, as in the Japanese Naval Ensign, Russian Air Force flag and Jordanian Royal Standard. It is also used during film or video shoots to help setting the focus in special situations. In the field of video production, where it is often called a back focus chart, the Siemens star is widely used to adjust the back focus of removable lenses. Similarly, it can be applied to a camera's optical resolution by taking photographs of a Siemens star printed at high resolution and comparing photographs from different cameras, to see which retained the center detail the closest.Ī Siemens chart filter in a collimator, used for auto-focus calibration of digital still cameras The smallest gap visible is limited by the smallest dot of ink the printer can produce, making the Siemens star a useful tool for comparing two printers' resolutions ( DPI). When printed or displayed on a device with limited resolution, however, the spokes touch at some distance from the center. In concept, the spokes only meet at the exact center of the star – the spokes, and the gaps between them, become narrower the closer to the center one looks, but they never touch except at the center. It consists of a pattern of bright "spokes" on a dark background that radiate from a common center and become wider as they get further from it. JSTOR ( July 2019) ( Learn how and when to remove this template message)Ī Siemens star, or spoke target, is a device used to test the resolution of optical instruments, printers, and displays.Unsourced material may be challenged and removed. Please help improve this article by adding citations to reliable sources. ![]() This article needs additional citations for verification.
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