Can some kind contributor tell me exactly what APO and ASPH mean both optically and in practice ?

-- Tony Brookes (gdz00@lineone.net), July 26, 2001

Answers

APO stands for "apochromatically corrected". In most lenses, optical design concentrates the focus of blue light and green light into a single plane, but red light falls slightly into another plane of focus. Red subjects, therefore, would be ever so slightly out of focus compared to blue and green subjects in the same frame. Not sure you'd ever notice though, the difference is so slight. This is the same basic principle that requires you to shift the focus for infrared photography, related to the wave length of red light. In APO lenses, the design and expense has been put in to making red light focus on the same plane as blue and green. Under a microscope you would see that all light subject is now in focus, creating a sharper image overall. Many manufacturers offer APO designs, but in most of these only the very center of the lens is APO corrected. Leica prides itself on making most of the frame APO corrected.

ASPH stands for "ashperic design". Most lenses have a spherical design - that is, the radius of curvature is constant. These are easy to manufacture by grinding while "spinning" the glass. This design however restricts the number of optical corrections that can be made to the design to render the most realistic image possible. ASPH lenses, however, involve usually 1 element that does *not* have a constant radius of curvature. These elements can be made by 1) expensive manual grinding, 2) molded plastic, 3) Leica's patented "press" process, where the element is pressed into an aspherical ("non-spherical") shape. This design allows the manufacturer to introduce corrections into compact lens designs that weren't possible before. Practically, the lens performs "better" (up to interpretation) due to increased correction of the image, in a package not significantly bigger than the spherical version.

I'm sure there are many experts on this site that can "enhance" my layman's explanation.

-- Ken Shipman (kennyshipman@aol.com), July 26, 2001.

A minor addendum to the layman's explanation above.

In practice, APO and ASPH mean "more expensive."

-- Mike Dixon (mike@mikedixonphotography.com), July 26, 2001.

Thanks a lot Ken. Very clear and very helpful.

-- Tony Brookes (gdz00@lineone.net), July 26, 2001.

Some logical consequences of all this: 1) With APO lenses and infrared photography, you do *not* have to shift the focus, because the red light focus has already been corrected. 2) Only longer lenses can make good use of APO correction. Normal and short (wide angle) lenses have enough depth of field to make the focus shift moot. 3) Any lens stands to gain from ASPH elements as part of the design, but this adds to the expense of manufacture (as does APO correction). The manufacturer has to strike a balance between the cost increase and image quality to be gained.

-- Ken Shipman (kennyshipman@aol.com), July 26, 2001.

4) If you put a non-APO teleconverter behind your expensive 100mm APO-R or 180mm APO-R lens, you just negated the APO correction you spent so much for. If you must teleconvert, an APO lens needs an APO teleconverter.

-- Ken Shipman (kennyshipman@aol.com), July 26, 2001.

These subjects have been explained to me countless times, but I've never heard or read a better or more clear explanation than Ken's in this thread. Thanks, Ken!

-- Tony Rowlett (rowlett@mail.com), July 26, 2001.

So, Mike, let's see if I've got this straight:

APO = Always Priced Outrageously

ASPH = Also Specially Priced High

Right?

-- Bob Fleischman (RFXMAIL@prodigy.net), July 26, 2001.

As far as "in practice", I've noticed that well designed APO and aspherical lenses are in general sharper wide open than there more traditionally designed counter parts. Don't own any Leica lenses labeled as such, but have read the 135mm Tele Elmar is pretty much an APO design.

-- Andrew Schank (aschank@flash.net), July 26, 2001.

I agree with Tony...

...I knew this Vivitar converter had something to do with the new bouque I was getting, what to do I convenced all my clients it was great, now they all want the new buquue...

-- r watson (al1231234@hotmail.com), July 26, 2001.

Consequence #5: many times there are complaints that the current 90mm APO-Summicron-M ASPH is "too sharp" for portraiture. Human skin reflects a lot of red light. The APO correction specifically works to better focus red light. Thus skin blemishes are in sharper focus with any APO lens. Before laying out extra money for this lens, consider what you will use it for.

-- Ken Shipman (kennyshipman@aol.com), July 27, 2001.

Just to add a bit to what's been said; it's not enough to just have APO correction in lenses that are of long focal length. Designers realized that the uncorrected colors in an APO design using conventional glass would still color fringe. So, most APO designs today also use low dispersion glass as well. Even superachromatic lenses use low dispersion glass to reduce fringing from the uncorrected colors. See the Zeiss 250 and 350mm lenses of this type. APO lenses that use conventional glass have much more spherochromatism (change in spherical aberration with wavelength)than APO low dispersion types. With shorter focal length(90,100, 135) lenses, the designer can probably get by without color problems when using an APO design without LD glass. An exception is the short view camera lens in which it will be used far off the optical axis at imes and lateral color needs to be controlled.

To have no separate infrared mark on a lens, there needs to be low spherochromatism and therefore just having an APO designation is not a guarantee of this.

Also, if a lens is APO, that does not mean that the three corrected colors are corrected in all zones of the lens. Those colors will focus in front or behind the film if they pass through the uncorrected zones of the lens. This was the problem with conventional glass. LD glass greatly improved this so that the uncorrected zones would control the focus deviation from the film plane much better.

Not only do aspheric surfaces correct out spherical aberration, they are useful in controlling distortion. Check out the display of this at the Solms plant near Wetzlar.

-- Steve Rasmussen (srasmuss@flash.net), July 28, 2001.