The American annual of photography . m) ; (6) Curvature of field. Other aberrations existbut they only appear in objectives of exceedingly large diam-eter. Chromatic aberration, though the first fault to be corrected(chronologically speaking) is still extremely troublesome ow-ing to the fact that glass does not disperse all parts of thespectrum equally, and it is quite impossible to correct any lensfor all colors. The ordinary photographic lens of short ormedium focal length is quite well enough corrected for anyreasonable purpose, but in the huge telescope this variation ofthe foci of various
The American annual of photography . m) ; (6) Curvature of field. Other aberrations existbut they only appear in objectives of exceedingly large diam-eter. Chromatic aberration, though the first fault to be corrected(chronologically speaking) is still extremely troublesome ow-ing to the fact that glass does not disperse all parts of thespectrum equally, and it is quite impossible to correct any lensfor all colors. The ordinary photographic lens of short ormedium focal length is quite well enough corrected for anyreasonable purpose, but in the huge telescope this variation ofthe foci of various colors may be a matter of inches and provevery troublesome indeed—particularly when it is desired tophotograph with them. In microscope objectives it is possible to use fluor spar andobtain almost complete elimination of this troublesome error,but perfect pieces of fluor spar are never found of size suffi-cient for use in photographic lenses, so that recourse must behad (when especially fine color correction is imperative) to 246. JOY. Hand Coated Platinum on Japanese Vellum. MYERS R. JONES. 247 a variety of glass which contains calcium fluoride. This glasspossesses to some degree the dispersive peculiarities of fluorspar, but is not nearly so effective. Spherical aberration of direct pencils can be corrected socompletely that in the paraxial zone (or even an angle ofperhaps ten or even fifteen degrees) the image will bear enlarge-ment until diffraction breaks it down, i. e. The standard ofdefinition is equal to the theoretical diffraction image. Thisstandard of definition can be more easily obtained if largerangles than those named are not demanded, but in many of themodern lenses not only an angle of sixty degrees or more mustbe covered, but also they are required to work at relativeapertures far greater than were deemed practicable in formeryears. This condition frequently makes necessary the sacrifice ofsome of the fine central definition in order to obtain fair mar-gina
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Keywords: ., bookcentury1900, bookdecade1920, bookpublishernewyo, bookyear1922
