What's main different of APO and non-APO lens

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Can you please advise the major different or impact at APO feature

-- KL (klrltl@yahoo.com), February 14, 2002

Answers

APO = apochromat(ic). All glass refracts (bends) light to a certain degree, but unfortunately does not bend all colors of the visible light spectrum equally. Hence, you can get a "prism" effect or color fringing on point-sources of light in an image. This problem is present in all lenses, but is more noticeable in long lenses due to their higher magnification. An "APO" design uses combinations of special glass in their design to bring the three primary colors of visible light (red, green and blue) back together at the plane of focus, hence eliminating the color fringing...

:-),

-- Jack Flesher (jbflesher@msn.com), February 14, 2002.


exactly as above - -whether a profound effect or not, and wehter some of the best non-APOs are close enough, is another issue. The more recent APO lenses are just great, though some other aspects are differnet as well.

You will see non-APO evidence in a few circumstances -- taking a picture of a sheaf of ruffled papers, or thin white threads, you will see a very soft fringing -- red on one side,perhaps, blue on the other (you may have to look 10-30X magnitfication, or, t may be obvious). No matter how severe, this leads to a some loss of definition and contrast (the worse the lens, the worse the effect, and in the best (e.g., current 90 mm 2.8, maybe almost none -- but Leica is supposed to be very tight with allowing the APO nomencalture on its lenses)).

In more subtle comparisons, I have found a 90 summicron (last one) compared to a 90 APO, while very close in some areas, to show a distinct difference in sharpness on the edges depending on the color for the older one (the APO is better, anyway, but tiny 3-5 mm letters shot at 12 feet are lost if blue, present if black/red, etc...)

Used to be said that B&W images were immune (they aren't, focus and focus spread is just that). However, if you put a filter to restrict the color spectrum getting through, you will overcome a littel of the non-APO spread -- assuming you focus according to the selected color. APOs may also ahve less focus shift with infrared - I beleive the best of the new ones suggest NO focus shift when using filtered infrared -- that gives you an idea of the magnitude of the focus shift wih color spectrum of some of the older lenses.

-- Lacey Smith (lacsm@bellsouth.net), February 14, 2002.


I don't think anyone mentioned that non-APO lenses are generally corrected for two primary colors, so the difference isn't as great as correction for all three colors vs. only one, or none at all.

-- Bob Fleischman (RFXMAIL@prodigy.net), February 14, 2002.

A lens which brings two of the primary colors into focus at the same plane is referred to as an "acromat(ic)". Since blue light bends more than green, which bends more than red, these lenses were generally corrected to balance either the green-red or green-blue.

The earliest lenses did not balance any of the colors. As a result many photographers (with their B&W film) would use red, green or blue filters, depending on the scene, to improve the sharpness of thier images. This worked well since the flter allowed only one of the primary colors through and blocking the two other out-of-focus colors. Similarly, as I understand it, one of the reasons the "minus blue" (strong yellow) filter became so popular with B&W photographers was that it blocked blue, hence rendering images shot with the common green-red acromats sharper; it was simply an added plus that the skies were also rendered darker.

:-),

-- Jack Flesher (jbflesher@msn.com), February 15, 2002.


And the next step up from Apochromat is the Superchromat. I wonder if we'll ever see such a lens for the Leica?

-- Frank Horn (owlhoot45@hotmail.com), February 15, 2002.


Virtually all modern APO lenses use low dispersion glass. Herzberger's brainchild(the superachromat)was very useful when convention glass was used in designs for longitudinal chromatic control. But, it is more important to have low dispesion glass than to correct for 4 colors. The uncorrected colors fall so close to the film plane in an APO design that uses low dispersion glass, that going to a superachromatic design will yield only a slight improvement. I doubt that Leica will follow what Zeiss has done in this area.

-- Steve Rasmussen (srasmuss@flash.net), February 15, 2002.

I'm surprised no-one mentioned the term "Refractive Index". I was sure leica owners would know this inside out- unlike myself. I would know this if I had to plunk $$$$ for krautenese glass.

-- Mike DeVoue (karma77@att.net), February 15, 2002.

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