Archived posting to the Leica Users Group, 1999/02/02
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Joe Stephenson wrote: > > As to the use of apo lenses for black and white. I have read that apo > offers advantages in B&W printing because of the apo's lenses ability > to focus light of three (rather than two) wavelengths on exactly the > same plane. If some wavelengths of light are focused at a point behind > or ahead of the film plane, the result will be a fringe or halo around > the area of sharp focus produced by the properly focused wavelengths > of light. When printing color, the result will be a color fringe > around objects. In B&W one will see monochrome fringes. > This doesn't make a whole lot of sense. B&W paper is not sensitive to the whole spectrum, only the bluey-greeny parts of it. APO lenses typically ensure that the red part of the spectrum also focuses on the same plane as the blue-green part. If B&W paper was sensitive to the red part of the spectrum, safelights wouldn't work. Since regular (non-APO) lenses already focus blue-green light in the same plane, there is little gain to be had for an APO-lens when printing B&W. Colour, obviously, is another matter. M. - -- Martin V. Howard, Application Systems Laboratory, | Dept. of Comp. & Info. Sci., Linkoping University, | Just "DOHH" it! SE-581 83 Linkoping, Sweden. Tel +46 13 282 421, +----------------+ Fax +46 13 142 231; marho@ida.liu.se; www.ida.liu.se/~marho