Archived posting to the Leica Users Group, 1998/08/04
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Dominique wrote: >RE : the extender is a divergent system, more precisely a Barlow lens >according to Leica (Gunter Osterloh, angewandte leica technik, p.157 :"d= ie >Barlow-linse ist eine starke negativ-linse"). >Suppose your lens has an angle of view of 27=B0 (summicron 90). With the >2X-extender behind the lens the angle of view remains the same but the >divergent system doubles the size of the projected image on the focus. S= o >the resolution of the image is divided by 2. If your lens at f:2 has a >resolution of, say, 20 lp/mm ; with a perfect extender, we'll have exact= ly >10 lp/mm. BTW as the size of the projected image is multiplied by 2, the >surface is multiplied by 4 and then the aperture is reduced by 2-stops. >Q.E.D. Dominique, in this case Eric is basically right. Adding a complex optical system to another complex optical system results in a third complex optic= al system, which can be less, or even possibly more than the sum of its part= s. A system can be optimized for lens plus converter, so that removing the converter will actually reduce performance. If there were a converter whi= ch just 'doubled' everything, without other regard to optical properties, yo= ur conclusion might be justified, but adding optical systems such as a converter can actually improve certain aspects of optical performance, an= d actual performance losses are often a lot less than you surmise. A good converter often causes a loss of only 10 percent in lines per millimeter. Since there are a lot more things to optical performance than lines per millimeter, the overall loss can be greater, but hard to quantify. In the case of short, fast teles used wide open, flare and loss of contrast are common in all lens plus converter combinations I have tried. As the converter in question is not part of an optimized system but a general purpose converter, the quality of lens plus converter will usuall= y be less than that of the lens alone, which is an optimized system on its own. That is about all you can predict in general; the degree of degradation is not fixed. * Henning J. Wulff /|\ Wulff Photography & Design /###\ mailto:henningw@archiphoto.com |[ ]| http://www.archiphoto.com