Logically, because for one line to exist, it must have a background. If it is thought of as a black line then it has a white background. If it is thought of as a white line then it has a black background. By saying that the "pair" is a black line and a white line, then it is not possible to draw one line pair!
No one draws a black line on a white piece of paper and claims it's a line pair! No one draws two black lines parallel together on a sheet of paper and claims it's four lines! No one draws 100 black lines parallel together on a sheet of paper and claims it's 200 lines! I'm starting to become convinced that the idea of "line pairs per mm" started as the result of some anal retentive who, for the sake of argument alone, claimed that one could think of the resolution target as white lines on a black background just as well as black lines on a white background (true enough). Either he or others decided that it was somehow "more scientific, more intellectual or whatever" to start calling both the black area and the white area as lines. To me, this leap is nonsense. I am quickly beginning to believe that "line pairs per millimeter" is nothing more than a pseudo intellectual attempt to sound more knowledgeable about the topic, and (in fact) is really the same as lines per millimeter. Indeed, a 10^100 search of the web shows that the terms are often used interchangeably and refer to the same number. To me, this means that lp/mm = l/mm, and that nonsense speak has crawled into our photo jargon even at the highest levels just as it has into the jargon of my engineering profession. The following link discusses the best resolution one can theoretically achieve with any lens given it's f/stop: http://www.zeiss.de/de/photo/home_e.nsf/0/73d528c09b620a11c125697700548cd6?O penDocument Note that for 200 line pairs per millimeter, they give the rounded approximation of f/8 for a white light spectrum of even energy distribution. This corresponds very well with my earlier calculated estimate of f/7.3 for green (monochromatic) light which was relative to lines (on a background) per millimeter. Whether you say "lines per millimeter" or "line pairs per millimeter" the number is the same. Other noteworthy excerpts from this Zeiss article: "Today's high quality color films do reach resolutions in the region of 140 line pairs per millimeter with Kodak Ektar 25 leading the field at 200! The full resolution potential of these films cannot be utilized with existing depth-of-field concepts nor f-settings of f/11 and beyond. On the other hand all real lenses on the market today are limited not only by diffraction, but by lens errors also. Some of them quite heavily." > From: J. C. O'Connell [mailto:[EMAIL PROTECTED] > > One black line and one white line is a "PAIR" > of lines to me. Why would you need 4 lines > to make up a pair? >
