Archived posting to the Leica Users Group, 2000/08/21
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]At 8:56 PM +1000 8/21/00, A.H.SCHMIDT wrote: >Subject: Re: [Leica] a summicron by any other name > >> Date: Sun, 20 Aug 2000 20:11:01 EDT >> From: Krechtz@aol.com >> Reply-To: leica-users@mejac.palo-alto.ca.us >> To: leica-users@mejac.palo-alto.ca.us >> >> In a message dated 8/20/00 3:11:28 PM Eastern Daylight Time, >> mark@rabiner.cncoffice.com writes: >> >> << It's my impression that current Leica lenses are nicely color matched. >> >> >> >> Probably so, but what about matching lenses from different periods? Also, >> >> how good is the match between APO lenses and others? >> >> Joe Sobel > >Joe, > > There is no reason why an APO lens should match any of the other other >lenses. As the name implies, an APO lens should be completely color neutral. >That is, it should have no chromatic aberrations >- a main contributor to biasing a lens to give a more or less warmer or >colder appearance.- >The coating should also be completely colour neutral and not favour any >colour. >If this does not apply, then it is not an APO lens. > >The main function of these lenses is not in general photography, but in >process work. >Process cameras used APO lenses for years already. > >Before coating, and in the early days of coating, most lenses had a warmer >tone. >This lenses where primarily corrected for the blue rays. This rays where the >sharpest at the focal point on the negative. The yellow and read where not >as sharp and gave a slight yellow or reddish hue. lenses which where >corrected for the red or yellow spectrum had a cooler appearance for the >opposite reason. Coating also contributed somewhat to the warmth, because >the blue ray did not scatter as much in the glass. The blue ray therefore >went exactly where they where supposed to go and no bluish tint was >apparent. I'm sorry, Horst, but this isn't true. Apochromatic correction of a lens has nothing to do with the overall spectral response. Neither does it have anything to do with coatings, or flare level, or distortion or any one of a number of other aberrations. All it means is that three wavelengths in the visible region of the spectrum will focus on the same plane. It does not even mean that the rest of the wavelengths are focussed within a small distance of the same plane. Of course, most manufacturers of general use lenses try to do a good job of other aspects of aberration correction and quality design when going to the trouble of making an 'Apo' lens, but that is a different matter. * Henning J. Wulff /|\ Wulff Photography & Design /###\ mailto:henningw@archiphoto.com |[ ]| http://www.archiphoto.com