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Photometric redshift (photo-z) is an estimate of an object's redshift using photometry. The expected appearance of various objects at various redshifts under various filters of some photometric system are gathered or modeled and compared to observed bodies for a match. Spectrography has the advantage of revealing spectral features with well-known rest wavelengths (e.g., specific spectral lines) for precise redshift-determinations, but photometry has advantages both regarding the number of objects simultaneously observed and also in requiring less light, thus handling faint (distant) sources. The former advantage makes photometric redshifts a reasonable means to discover targets for spectrography. Modern light-efficient spectrographs, and multi-object spectrographs have narrowed photometry's advantages.