The American annual of photography . wn will cut offsome of the light, besides probably giving uneven will also be noted, in Figure 5, that the diameter of theobjective is barely sufficient to admit the beam. A large light- 172 source, then, not only needs a rapid lens for the best results,but one of adequate diameter. All things considered, a large-aperture anastigmat is farsuperior to the Petzval objective. Its field is perfectly flat,and the definition uniformly sharp from center to margin,even with the full opening. One of more moderate aperturewill, of course, answer with
The American annual of photography . wn will cut offsome of the light, besides probably giving uneven will also be noted, in Figure 5, that the diameter of theobjective is barely sufficient to admit the beam. A large light- 172 source, then, not only needs a rapid lens for the best results,but one of adequate diameter. All things considered, a large-aperture anastigmat is farsuperior to the Petzval objective. Its field is perfectly flat,and the definition uniformly sharp from center to margin,even with the full opening. One of more moderate aperturewill, of course, answer with a small light-source. Suchlenses can be obtained to order in focusing mounts or jackets,and a metal iris diaphragm should be stipulated. Some lan-terns have a fine-adjustment screw to the front, which rendersunnecessary any provision for focusing on the lens. With regard to the focal length, a short focus has theadvantage that the distances from lens to easel need not beso great, also the depth of definition is superior should it be. Figure 5. required to tilt the negative and easel in curing distortion. Ifa portrait lens is being used, however, a somewhat longerfocus is advantageous, since, owing to the narrow angle atwhich the beam of light enters, the central portion only ofthe lens is brought into service. The focal length of the condenser and of the objectivemust, in any case, be mutually suitable. A handy rough ruleto remember is that the best focus for the objective will beabout equal to the diameter of the condenser, or within ^in. less or more than the diameter. Thus, a condenser 53^in. in diameter, as used with 3^ in. by 4^ in. negatives, needsan objective of from 5 in. to 6 in. focus. Coming to the correct fixing and adjustment of the objec-tive, it is important that the optical axis of the latter shouldbe perpendicular to both the negative and bromide paper. In afixed-focus box enlarger this is a practical certainty, and it 173 may also be expected in adjustable day
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Keywords: ., bookcentury1900, bookdecade1920, bookpublishernewyo, bookyear1922
