Archived posting to the Leica Users Group, 1998/12/28
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]PSchiemer@aol.com wrote: [lots snipped, mostly about how much better off we'd be if we stuck to describing lens performance in terms of resolution, evaluated in line pairs per unit of linear distance...] >Everybody gets better, gray area fades and contrast gets brighter. Contrast, ah yes. THAT is a (not "the") crux of the matter, as Erwin Puts has pointed out so clearly, over and over. As Erwin points out, the old DR Summicron 50 can resolve well over 100 lp/mm but the Summicron-M is a much better performer, due in large measure to its superior contrast at 10-20 lp/mm. See, for discussion: http://www.imx.nl/photosite/leica/r/tests/r14-50.html As a biologist whose professional work revolves around optical microscopy, perhaps I can add to this discussion. With the green light that's often used, an optical microscope with perfect diffraction-limited optics can resolve two points about 1 x 10^-7 m apart. But we can *detect* structures substantially smaller than this. In fact it is easy to detect polymeric protein filaments that are 0.06 x 10^-7 m in diameter. With more sensitive detectors, single molecules of the jellyfish green fluorescent protein (yes, that is what it is called) can be detected by optical microscopes. And needless to say, our microscopes are *not* diffraction limited. The critical issue here is signal-to-noise ratio, or effective contrast. For this reason almost all very high resolution microscopy is now done with fluorescent stains that emit light, rather than with absorptive stains. Flourescent stains simply have a far higher SNR, allowing us to detect (and resolve) much smaller structures. Furthermore, SNR is the reason why so many biologists lust after $200,000 laser scanning confocal microscopes that have linear resolution no better than is achieved with typical $20,000 research microscopes. The confocal 'scopes allow the researcher to exclude out-of-focus information, resulting in dramatic increases in contrast. For an example of a fluorescently stained sample imaged using an ON TOPIC Leica confocal laser scanning microscope, see: http://www.webcom.com/alexey/news.html High resolution is a desirable parameter in a camera lens but it is by no means the only, or even the most important, parameter. Resolution is necessary, but it is not sufficient! Contrast, distortion, tonality, and rendering of out-of-focus information all are significant contributors, but a lp/mm figure explicitly addresses none of these qualities. Maybe that's one reason why people in this forum don't generally rely on lp/mm figures as the ultimate arbiters of image quality? .......................................................................... Alexey Merz | URL: http://www.webcom.com/alexey | email: alexey@webcom.com