Archived posting to the Leica Users Group, 1998/08/02
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]>From: Eric Welch <ewelch@ponyexpress.net> >Date: Fri, 31 Jul 1998 19:41:30 -0500 >Subject: Re: [Leica] 2x extender R disappointment > >At 09:37 PM 7/31/98 +0200, you wrote: > >>Comment : remember the converter's law :when you use a 2X-extender, Apo= or >>not, Leica or not, you double the size of the image and you divide by t= wo >>the resolution of the lens. > >It takes the center of the image circle, and transmits it. So your math >above doesn't add up. It certainly doesn't cut it in half. If that were >true, then the best lenses would be one element lenses, because if you a= dd >a second one, it would cut the resolution of the first one in half, then >the third would cut it in half again. A converter just creates a differe= nt >optical configuration. It doesn't cut the quality in half. > >Imagine how bad my 70-180 would be with its 13 elements.=20 > > >- --=20 > >Eric Welch RE : the extender is a divergent system, more precisely a Barlow lens according to Leica (Gunter Osterloh, angewandte leica technik, p.157 :"di= e 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. So 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 exactl= y 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