Archived posting to the Leica Users Group, 2009/02/04
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]>Is that why they call it Double Gauss? A Gauss lens is a lens with a positive meniscus lens on the object side and a negative meniscus lens on the image side. This design was invented by Carl Freidrich Gauss in 1817. The negative lens corrects for chromatic aberration. A *double* Gauss lens is two Gauss lenses back to back so you have two positive meniscus lenses on the outside with two negative meniscus lenses inside them. Traditional designs of this type are symmetrical. >He talks about the Summicron coming from a Taylor Taylor Hobson design. The double Gauss design was radically improved by Paul Rudolph's improvements made while working for Carl Zeiss which consist of replacing the single negative meniscus lenses with cemented doublets (to make a six-element symmetrical lens). Taylor and Hobson's f/2.0 Opic and Speed Panchro designs originated in the 1920s and are evolutionary improvements on Rudolph's designs. Most modern double Gauss designs owe their origins to these revolutionary designs. The original seven-element Summicrons were a further evolution for the type, but were not symmetrical. The later six-element 50mm Summicrons are double Gauss designs derived from Rudolph's design. Sigma have really shaken things up here in an interesing way. This EX DG HSM lens has an aspherical element, like the Leica M 50/1.4 asph, but I don't think it has afloating element which the Leica does. Sigma have moderated some of the aberrations that a floating element corrects for by making the lens BIG. It weighs as much as a Noctilux but the front element is 50% larger. It's a monster all right. The new AF-S Nikkor I tried was disappointing in many ways - it displayed a lot of distortion, didn't seem to be designed for use wide open and has observable focus shift. It's not the lens for my Nikon SLRs I was hoping for. I didn't ever anticipate owning a Sigma lens, but maybe I will. Marty Gallery: http://gallery.leica-users.org/v/freakscene -- Be Yourself @ mail.com! Choose From 200+ Email Addresses Get a Free Account at www.mail.com