Archived posting to the Leica Users Group, 2004/11/03
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]Due to the wavelength dependancy of refraction lenses do not focus all colours in the same plane at once. Acromats are corrected such that two colours are in focus in the same plane at once, apocromats are corrected such that 3 colours are in focus at the same plane at once. Other wavelengths are not in focus causing colour fringing and lack of sharpness. The extent to which the other colours are OOF is lens design dependant. Apo lenses should be sharper than non apo. Frank On 4 Nov, 2004, at 06:59, Slobodan Dimitrov wrote: > I wouldn't go so far as to say that APO is a term projecting absolute > quality. It's more in the line of a relative value. > It's my understanding that apo lenses focus all the color sources at a > specific focus distance set by the manufacturer. The other ranges, > more or > less, work like any other optic. > I'll gladly stand corrected if I'm wrong. > S. Dimitrov > > >> From: Frank Dernie <Frank.Dernie@btinternet.com> >> Reply-To: Leica Users Group <lug@leica-users.org> >> Date: Thu, 4 Nov 2004 06:30:16 +0000 >> To: Leica Users Group <lug@leica-users.org> >> Subject: Re: [Leica] LHSA convention cluster bombs Part One The >> Beginning. >> >> Hi William, >> I do think it is fair to say that APO tells you how sharp a lens is >> and >> is an absolute statement of quality. It does not give a clue as to >> whether you will like the boke. You need the actual MTF curves to >> evaluate the boke (My limited experience convinces me that the >> explanation in the "Lens Work" book is the first I have seen to be >> right in every case on boke). >> Asph on the other hand tells you nothing. Most lenses made today are >> "asph" including many spectacles and pretty well all P&S and digicam >> lenses. Leica use asph lenses for the same reason as others, to get >> the >> quality at a lower price due to fewer lens elements. When asph >> elements >> were hand ground they were extremely expensive. Ground asph lenses >> still are very expensive but hot pressed ones, as used by Leica and >> others, are no longer prohibitive. >> Frank >> >> On 4 Nov, 2004, at 03:12, William G. Lamb, III wrote: >> >>> >>> Dan, >>> >>> Sorry, twelve-hour day in the office today and am just getting to my >>> mail... >>> >>> "Bokeh" is one thing, "brittleness" is another. I'm not saying that >>> APO >>> correction generally means that some images in harsh side-light will >>> be brittle. I'm observing that LEICA lenses in the 90mm - 100mm >>> range >>> appear to have this tendency, whatever their reasons may be, the 90 >>> APO >>> ASPH in particular. >>> >>> Since 2000, the direction Leica has been going in lens design as >>> exhibited >>> by the 28 ASPH is a GOOD THING in my view. The results aren't harsh >>> and the corrections produce images in contrasty light which would >>> have >>> been almost impossible a few years prior. As I said earlier, I have >>> ordered >>> a 50 ASPH in the hope that this trend in lens design continues. I'm >>> just >>> suspicious of the APO correction Leica did in the past and won't buy >>> a >>> lens on blind faith with this designation until I've seen many >>> examples of >>> its output. Trust this clarifies... >>> >>> William >>> >>> At 08:06 AM 11/03/2004 -0500, you wrote: >>>> I've missed something here. I can understand why someone might not >>>> like >>>> the ASPH correction in a lens; I *think* it is residual spherical >>>> aberation which is the basis for lovely "bokeh" which people get >>>> soft >>>> and >>>> fuzzy over (or conversely might be the cause of "brittleness"), and >>>> which >>>> is highly corrected in aspherical designs. APO design merely >>>> reduces >>>> chromatic aberation, which I can't see having an unintended >>>> detrimental >>>> effect on a len's performance. >>>> >>>> -dan c. >>>> >>>> At 07:48 AM 03-11-04 -0500, William G. Lamb, III wrote: >>>>> >>>> [snip] >>>>> >>>>> So let's not get too wound up here, O.K.? I would LOVE to fill that >>>> 75mm >>>>> frame on two of my M's. If it's an ASPH, that could be very good. >>>>> If >>>> it's >>>>> got APO correction, I'll be more inclined to wait and see some >>>> results >>>>> before springing for one. That's all... >>> >>> >>> _______________________________________________ >>> Leica Users Group. >>> See http://leica-users.org/mailman/listinfo/lug for more information >>> >> >> _______________________________________________ >> Leica Users Group. >> See http://leica-users.org/mailman/listinfo/lug for more information > > _______________________________________________ > Leica Users Group. > See http://leica-users.org/mailman/listinfo/lug for more information >