What does APO mean optically and practically ?

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

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.

 

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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.

 

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

layman's explanation.

A minor addendum to the layman's explanation above.

 

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

Thanks a lot Ken. Very clear and very helpful.

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.

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.

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!

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

 

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APO = Always Priced Outrageously

 

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ASPH = Also Specially Priced High

 

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Right?

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.

I agree with Tony...

 

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...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...

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.

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.

 

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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.

 

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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.

 

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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.