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This is WHY the best lenses today only use molded aspherical elements



 
 
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  #11  
Old October 7th 07, 06:24 PM posted to rec.photo.digital.slr-systems
Doug McDonald
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Default This is WHY the best lenses today only use molded asphericalelements

Robert Coe wrote:


Help an old man out here, folks. Is "aspherical" just the current term for
non-spherical conic sections, i.e. parabolas and hyperbolas?



Not quite, as applied to modern camera lenses. Its a general
term for a cylindrically symmetric non-spherical lens. There
is no implication that it is a conic section. A general
asphere on one lens surface is sufficient to produce
zero spherical aberration at one wavelength and one object
distance (and one zoom length for a zoom) **to all orders**.
A conic section is not sufficient to all orders.


Doug McDonald

  #12  
Old October 7th 07, 06:28 PM posted to rec.photo.digital.slr-systems
RichA
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Default This is WHY the best lenses today only use molded aspherical elements

On Oct 7, 1:24 pm, Doug McDonald wrote:
Robert Coe wrote:

Help an old man out here, folks. Is "aspherical" just the current term for
non-spherical conic sections, i.e. parabolas and hyperbolas?


Not quite, as applied to modern camera lenses. Its a general
term for a cylindrically symmetric non-spherical lens. There
is no implication that it is a conic section. A general
asphere on one lens surface is sufficient to produce
zero spherical aberration at one wavelength and one object
distance (and one zoom length for a zoom) **to all orders**.
A conic section is not sufficient to all orders.

Doug McDonald


Here is some basic info as they are used in camera lenses.

http://images.google.ca/imgres?imgur...3Doff%26sa%3DG



LENS TECHNOLOGY

by Klaus Schroiff

Special Lens Elements
Aspherical Elements

Aspherical elements are meant to reduce the level of distortions and
to improve the sharpness at the image borders. Basically aspherical
elements can optimize the focus of edge and center light rays (see
picture below). It is very popular to name a lens e.g. "28-80
aspherical" in these days but the word "aspherical" does not reveal
the effectiveness and quality of the aspherical element.

There're three types of aspherical elements:

1. grounded
2. molded
3. hybrid

Hybrid asphericals are quite cheap to produce and widely used for
cheap consumer grade zoom lenses. Basically it's a spherical glass
lens with an additional plastic surface which forms the aspherical
shape.
Ground aspherical lenses consist of glass only (obviously). The
grinding process of a spherical element and the following polishing
process is very expensive so you only find them in Pro-class lenses
(e.g. Canon 17-35L, Nikkor 20-35 etc.) and a few older lenses.
Molded aspericals are produced by directly molding glass in a molding
machine by incorporating an aspherical metal die. These elements are
often used in good amateur class lenses.

The gain of quality by using hybrid lenses seems to be quite limited
while lenses with true ground aspherical lenses often produce
outstanding results, molded aspericals are in between quality-wise.
Apart from the sheer material and production aspects the positioning
is also very important. The best wide-angle lenses use a large
diameter aspherical element in the front group (probably for a better
correction of light rays with steep entry angle ?), rear aspherical
elements (=smaller and therefore cheaper) are less effective.
However it should also be mentioned that the maybe very best wide-
angle zoom, the Carl-Zeiss T* 28-85/3.3-4, doesn't feature any
aspherical elements at all so the game plays also in the quality of
the general lens construction (that should be obvious anyway).
In general asperical elements are used in ultra-wide and wide-angle
fixfocals & zooms as well as standard and so-called super zooms.

  #13  
Old October 8th 07, 03:05 AM posted to rec.photo.digital.slr-systems
Robert Coe
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Posts: 4,901
Default This is WHY the best lenses today only use molded aspherical elements

On Sat, 06 Oct 2007 04:01:40 GMT, philippe wrote:
: Actually, I kinda like some of the wines you guys produce.. but when it
: comes to beer, you have to have at least *some* alcoholic content..

You may not like the taste of American beer. (Most Europeans don't, or at
least profess not to.) But you're wrong about the alcoholic content. Some
states have laws restricting the percentage of alcohol to about half the
normal level; but in those that don't (including most of our large cities),
it's about the same as that of most European beer.

Bob
  #14  
Old October 8th 07, 01:22 PM posted to rec.photo.digital.slr-systems
Robert Coe
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Posts: 4,901
Default This is WHY the best lenses today only use molded aspherical elements

On Sun, 07 Oct 2007 11:59:47 -0400, Robert Coe wrote:
: On Fri, 05 Oct 2007 12:24:37 -0700, RichA wrote:
: : On Oct 4, 5:45 pm, Rita Ä Berkowitz ritaberk2O04 @aol.com wrote:
: : RichA wrote:
: : And the cheapo lenses today use plastic molded aspherics.
: : Because there is NO way anyone would PAY for properly hand-ground
: : aspherical elements.
: :
: : DAMN!! I wonder how well that would work on the old Mk III? I am guessing
: : it will not perform any better than the old 58/1.2 Noct Nikkor on any
: : camera.
: :
: : Rita
: :
: : I read an article in a Leica magazine about that lens. The aspherics
: : help considerably to control the huge amount of spherical aberration
: : (which manifests itself as the defocusing blur you see wide open in
: : such lenses) that such a speed would generate. If the Nikon wasn't
: : similarly equipped, the Leica would clobber it wide open.
:
: Help an old man out here, folks. Is "aspherical" just the current term for
: non-spherical conic sections, i.e. parabolas and hyperbolas? I thought (glass)
: parabolic elements had been common for quite a few years and that most good
: film SLR lenses had them. They introduced tiny amounts of coma, but prevented
: a lot more spherical aberration.
:
: Or is this something entirely new that I've missed? I suppose it's possible
: that the world has simply passed me by. :^|
:
: Bob

Rich & Doug,

Thanks for the interesting and informative replies.

Bob
  #15  
Old October 8th 07, 07:22 PM posted to rec.photo.digital.slr-systems
Wolfgang Weisselberg
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Posts: 5,285
Default This is WHY the best lenses today only use molded aspherical elements

Robert Coe wrote:
: RichA wrote:


: And the cheapo lenses today use plastic molded aspherics.
: Because there is NO way anyone would PAY for properly hand-ground
: aspherical elements.


Note: RichA has a almost sexual fetish of abhorring plastics
in public, yet is completely unable to distinguish between a
plastics bag from the shopping mall and an optical precision
element, or a high-strength camera exterior.

Help an old man out here, folks. Is "aspherical" just the current term for
non-spherical conic sections, i.e. parabolas and hyperbolas?


aspherical means what it says: not spherical.

Since surfaces from a sphere are easy to calculate and I understand
easier to make, aspherical also has the meaning "something special"
for lenses --- most of the surfaces are, after all, spherical.

On the other hand, my glasses are aspherical, and high-refraction
plastic (glass would nearly trriple the weight and be _very_
thick).

And no, my glasses lenses are not conical, they basically have 2
different radii orthogonally to each other, instead of one radius.

I thought (glass) parabolic elements had been common for quite a few
years and that most good film SLR lenses had them. They introduced
tiny amounts of coma, but prevented a lot more spherical aberration.


Aspherical lenses are, indeed, used to combat aberrations,
and you will find them in most good lenses.

-Wolfgang
  #16  
Old October 9th 07, 02:13 AM posted to rec.photo.digital.slr-systems
David J. Littleboy
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Posts: 2,618
Default This is WHY the best lenses today only use molded aspherical elements


"Wolfgang Weisselberg" wrote:

Sensible stuff snipped, of courseg

Aspherical lenses are, indeed, used to combat aberrations,
and you will find them in most good lenses.


I object to the "most" in there. The new Zeiss Ikon rangefinder lenses have
no aspherical elements and outperform the comparable Leica aspherics
(according to the reviews in both the Japanese magazines I read). The Zeiss
Ikon lenses are under half the price of the Leitz lenses, but still
outrageously expensive.

And, of course, the classic medium format lenses (Biogon, Planar, Sonnar and
the Japanese knock-offs such as the Mamiya 7 lenses) had (and still have!)
no aspheric elements and remain some of the best lenses ever made.

David J. Littleboy
Tokyo, Japan



  #17  
Old October 9th 07, 03:22 PM posted to rec.photo.digital.slr-systems
RichA
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Posts: 2,544
Default This is WHY the best lenses today only use molded aspherical elements

On Oct 8, 2:22 pm, Wolfgang Weisselberg
wrote:
Robert Coe wrote:
: RichA wrote:
: And the cheapo lenses today use plastic molded aspherics.
: Because there is NO way anyone would PAY for properly hand-ground
: aspherical elements.


Note: RichA has a almost sexual fetish of abhorring plastics
in public, yet is completely unable to distinguish between a
plastics bag from the shopping mall and an optical precision
element, or a high-strength camera exterior.


There are no precision plastic optical elements. Precision implies
diffraction limited performance, 1/4 wave surface accuracy in green-
yellow light, peak to valley.
I think plastic is a cheap compromise, used when glass and metal are
cost-prohibited and never otherwise.


  #18  
Old October 10th 07, 03:01 AM posted to rec.photo.digital.slr-systems
â–€Slack
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Posts: 39
Default This is WHY the best lenses today only use molded aspherical elements

On Thu, 04 Oct 2007 15:15:32 -0700, Rita Ä Berkowitz ritaberk2O04
wrote:

Pboud wrote:

Is there anywhere else in the world worth selling to? Hell, I
restrict my sales to the continental US. Screw Hawaii and Alaska
since they are worse than Canada.

Yeah.. no beer...


Nonsense! There's plenty of beer!
http://www.geocities.com/ritaberk2007/sacrafice.htm


Rita



All that proves scratch, scratch, Rita, is you got crabs!
--
Slack
  #19  
Old October 13th 07, 07:22 PM posted to rec.photo.digital.slr-systems
Charlie Self
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Posts: 236
Default This is WHY the best lenses today only use molded aspherical elements

On Oct 4, 3:24 pm, RichA wrote:
And the cheapo lenses today use plastic molded aspherics.
Because there is NO way anyone would PAY for properly hand-ground
aspherical elements.

http://cgi.ebay.com/Leica-50mm-f1-2-...int-M2-M3-M4-M...


And you're saying a hand ground lens is better than a machine ground
one on computer controlled gear?

 




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