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A Siemens star is a device used to test the resolution of optical instruments, printers and displays. 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. Theoretically 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. When printed or displayed on a device with limited resolution, however, the spokes touch at some distance from the center. 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). 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.
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. It is also used during film or video shoots to help setting the focus in special situations.
Under optical blur from defocus, a Siemens star gives rise to the phenomenon of spurious resolution above the resolution limit, i.e. towards the center of the Siemens star. 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). 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.
- ISO 15775 chart (pdf) resolution test chart featuring a vector siemens star
- Siemens star with n=314 sectors for high resolution and easy calculation (vector graphic in PDF)
- Strasburger, Hans; Bach, Michael; Heinrich, Sven P. (2018). "Blur Unblurred – A Mini Tutorial". I-Perception. 9 (2): 204166951876585. doi:10.1177/2041669518765850. PMC 5946648. PMID 29770182.
- Smith, George (1982). "Ocular defocus, spurious resolution and contrast reversal". Ophthalmic and Physiological Optics. 2: 5–23.