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Martin Brown 04-02-2013 07:12 AM

Re: Perfect lens
 
On 02/04/2013 09:08, Alfred Molon wrote:
> Just wondering, if money played no role how good could lenses be?


Lenses are typically made of glass and their chromatic aberrations can
only be corrected exactly at two (achromat) or three (apochromat)
wavelengths. If you allow the lens to be slow enough then it is possibel
to make completely diffraction limited optics beyond about f5 these days
for moderate to long focal lengths.
>
> No chromatic aberrations, no geometric distortions, huge sharpness from
> corner to corner even wide open, or are there some physical constraints
> which prevent from producing a perfect lens?
>

Mostly the fact that all glasses have a dispersive component. If you do
away with the requirement of a flat focal plane then you can do much
better with Schmidt camera designs of wide field extremely fast survey
telescopes with something like a focal ratios of f3 although a few have
been made with focal ratios less than 1.


http://en.wikipedia.org/wiki/Schmidt_camera

A derivative of this by Baker-Nunn was used to track and image
satellites during the cold war. His obituary for super lenses:

http://www.utsandiego.com/uniontrib/...1j17baker.html

A notable new all mirror design of telescope with a very fast ratio and
essentially no aberrations is down to Willstrop of Cambridge using a
three mirror design to get an f1.6 wide field diffraction limited scope.

http://www.ast.cam.ac.uk/about/three-mirror.telescope

That is about as close to a cost no object perfect lens as it is
presently possible to manufacture. No use at all for an SLR though...

A review (in French) of some of the fastest lenses ever made is online:

http://www.dg77.net/photo/tech/fastex.htm

Some of them are close to perfect diffraction limited performance when
stopped down to normal working apertures.

--
Regards,
Martin Brown

Wolfgang Weisselberg 04-02-2013 02:00 PM

Re: Perfect lens
 
Martin Brown <|||newspam|||@nezumi.demon.co.uk> wrote:
> On 02/04/2013 09:08, Alfred Molon wrote:


>> Just wondering, if money played no role how good could lenses be?


> Lenses are typically made of glass and their chromatic aberrations can
> only be corrected exactly at two (achromat) or three (apochromat)
> wavelengths.


one, two, three wavelengths --- tell me why four, five, six
aren't possible!


>> No chromatic aberrations, no geometric distortions, huge sharpness from
>> corner to corner even wide open, or are there some physical constraints
>> which prevent from producing a perfect lens?


> Mostly the fact that all glasses have a dispersive component.


http://en.wikipedia.org/wiki/Negativ..._metamaterials
does seem to open a few new ways.


-Wolfgang

RichA 04-02-2013 03:53 PM

Re: Perfect lens
 
On Apr 2, 2:12*am, Martin Brown <|||newspam...@nezumi.demon.co.uk>
wrote:
> On 02/04/2013 09:08, Alfred Molon wrote:
>
> > Just wondering, if money played no role how good could lenses be?

>
> Lenses are typically made of glass and their chromatic aberrations can
> only be corrected exactly at two (achromat) or three (apochromat)
> wavelengths. If you allow the lens to be slow enough then it is possibel
> to make completely diffraction limited optics beyond about f5 these days
> for moderate to long focal lengths.
>
> > No chromatic aberrations, no geometric distortions, huge sharpness from
> > corner to corner even wide open, or are there some physical constraints
> > which prevent from producing a perfect lens?

>
> Mostly the fact that all glasses have a dispersive component. If you do
> away with the requirement of a flat focal plane then you can do much
> better with Schmidt camera designs of wide field extremely fast survey
> telescopes with something like a focal ratios of f3 although a few have
> been made with focal ratios less than 1.
>
> http://en.wikipedia.org/wiki/Schmidt_camera
>
> A derivative of this by Baker-Nunn was used to track and image
> satellites during the cold war. His obituary for super lenses:
>
> http://www.utsandiego.com/uniontrib/...1j17baker.html
>
> A notable new all mirror design of telescope with a very fast ratio and
> essentially no aberrations is down to Willstrop of Cambridge using a
> three mirror design to get an f1.6 wide field diffraction limited scope.
>
> http://www.ast.cam.ac.uk/about/three-mirror.telescope
>
> That is about as close to a cost no object perfect lens as it is
> presently possible to manufacture. No use at all for an SLR though...
>
> A review (in French) of some of the fastest lenses ever made is online:
>
> http://www.dg77.net/photo/tech/fastex.htm
>
> Some of them are close to perfect diffraction limited performance when
> stopped down to normal working apertures.
>
> --
> Regards,
> Martin Brown


I've seen one of the Baker Super Schmidts in the flesh. 12"
correctors, like two inverted punch bowls and a 24" primary mirror.

Martin Brown 04-03-2013 12:21 PM

Re: Perfect lens
 
On 02/04/2013 15:00, Wolfgang Weisselberg wrote:
> Martin Brown <|||newspam|||@nezumi.demon.co.uk> wrote:
>> On 02/04/2013 09:08, Alfred Molon wrote:

>
>>> Just wondering, if money played no role how good could lenses be?

>
>> Lenses are typically made of glass and their chromatic aberrations can
>> only be corrected exactly at two (achromat) or three (apochromat)
>> wavelengths.

>
> one, two, three wavelengths --- tell me why four, five, six
> aren't possible!


Four is possible but it is rarely done in practice. You are into the law
of diminishing returns and every air to glass surface loses some
contrast. When it is they are called superachromats and typically extend
to near infrared eg.

http://en.wikipedia.org/wiki/Superachromat
(delta f ~ 0.001

For realistic choices of lens material there is little or nothing to be
gained by using more than three wavelengths. You are stuck with the
dispersive properties that the clear glasses happen to have.

The last big thing was using calcium fluorite and now similarly behaved
exotic glasses without the brittle handling characteristics of fluorite.

Apochromats are usually as good as you need to get for photography

http://en.wikipedia.org/wiki/Apochromat
(delta f ~ 0.002 on one corrected for near IR as well

Achromat is the basic first order correction right in red and blue.

http://en.wikipedia.org/wiki/Achromat
(delta f ~0.005

They do tend to show purple fringing on high contrast highlights.

>>> No chromatic aberrations, no geometric distortions, huge sharpness from
>>> corner to corner even wide open, or are there some physical constraints
>>> which prevent from producing a perfect lens?

>
>> Mostly the fact that all glasses have a dispersive component.

>
> http://en.wikipedia.org/wiki/Negativ..._metamaterials
> does seem to open a few new ways.
>
>
> -Wolfgang


They may but they are unlikely to ever be used for optical components in
the visible band.

--
Regards,
Martin Brown

Wolfgang Weisselberg 04-03-2013 05:37 PM

Re: Perfect lens
 
Martin Brown <|||newspam|||@nezumi.demon.co.uk> wrote:
> On 02/04/2013 15:00, Wolfgang Weisselberg wrote:
>> Martin Brown <|||newspam|||@nezumi.demon.co.uk> wrote:
>>> On 02/04/2013 09:08, Alfred Molon wrote:


>>>> Just wondering, if money played no role how good could lenses be?


>>> Lenses are typically made of glass and their chromatic aberrations can
>>> only be corrected exactly at two (achromat) or three (apochromat)
>>> wavelengths.


>> one, two, three wavelengths --- tell me why four, five, six
>> aren't possible!


> Four is possible but it is rarely done in practice. You are into the law
> of diminishing returns


Money plays no role. (Nor does weight or carryability.)

> and every air to glass surface loses some
> contrast.


Check out how many surfaces one of the 70-200 IS zoom lenses
have ...

> When it is they are called superachromats and typically extend
> to near infrared eg.


> http://en.wikipedia.org/wiki/Superachromat
> (delta f ~ 0.001


> For realistic choices of lens material there is little or nothing to be
> gained by using more than three wavelengths. You are stuck with the
> dispersive properties that the clear glasses happen to have.


That's maybe a common sense and certainly a cost limitation;
the OP threw out that concern out of the window.

And the wikipedia link you gave says it's "highly beneficial"
for certain uses.


> The last big thing was using calcium fluorite and now similarly behaved
> exotic glasses without the brittle handling characteristics of fluorite.


> Apochromats are usually as good as you need to get for photography


.... currently.

But we're not concerned here with what you need, but what if
you had infinite money.


>>>> No chromatic aberrations, no geometric distortions, huge sharpness from
>>>> corner to corner even wide open, or are there some physical constraints
>>>> which prevent from producing a perfect lens?


>>> Mostly the fact that all glasses have a dispersive component.


>> http://en.wikipedia.org/wiki/Negativ..._metamaterials
>> does seem to open a few new ways.


> They may but they are unlikely to ever be used for optical components in
> the visible band.


Just as unlikely as people using transistors for photography.

-Wolfgang

Martin Brown 04-04-2013 09:53 AM

Re: Perfect lens
 
On 04/04/2013 10:29, Neil Ellwood wrote:
> On Tue, 02 Apr 2013 16:00:55 +0200, Wolfgang Weisselberg wrote:
>
>> Martin Brown <|||newspam|||@nezumi.demon.co.uk> wrote:
>>> On 02/04/2013 09:08, Alfred Molon wrote:

>>
>>>> Just wondering, if money played no role how good could lenses be?

>>
>>> Lenses are typically made of glass and their chromatic aberrations can
>>> only be corrected exactly at two (achromat) or three (apochromat)
>>> wavelengths.

>>
>> one, two, three wavelengths --- tell me why four, five, six aren't
>> possible!


> We can normally see only three wavelengths (red, green and blue) and to
> correct for more wavelengths would increase the cost and complexity by a
> great deal.


Actually the cones in our eyes are sensitive to roughly blue, green and
yellow. Our perception of red is computed in the brain as yellow-green
which means that you can play tricks on its perception of colour by
using a strong minus yellow bandpass filter. See for example:

http://openwetware.org/wiki/BIO254:Phototransduction

A few people do have abnormal extended red sensitivity that allows them
to see a difference between live and dead plant foliage. If you wrap
some unexposed colour slide film against your eyes and wait for dark
adaption to bring the scene back into colour you can boost the relative
amount of IR detection (black slide film is transparent in the near IR
and dense for visible). The result is you see live foliage as lighter.

A good camera lens has to bring all light to a decent focus as otherwise
you get the characteristic purple haloes around highlights. The latter
being a signature of basic achromat designs at best focus.

--
Regards,
Martin Brown

Wolfgang Weisselberg 04-05-2013 07:24 PM

Re: Perfect lens
 
Neil Ellwood <cral.elllwood2@btopenworld.com> wrote:
> On Tue, 02 Apr 2013 16:00:55 +0200, Wolfgang Weisselberg wrote:
>> Martin Brown <|||newspam|||@nezumi.demon.co.uk> wrote:


>>> Lenses are typically made of glass and their chromatic aberrations can
>>> only be corrected exactly at two (achromat) or three (apochromat)
>>> wavelengths.


>> one, two, three wavelengths --- tell me why four, five, six aren't
>> possible!


> We can normally see only three wavelengths (red, green and blue) and to
> correct for more wavelengths would increase the cost and complexity by a
> great deal.


That's may be an argument that it's not economic, not that it
can't be done.

In addition, while you may have only 3 colour sensor types in
your eye, each of them react to a fairly wide frequency range.
The same is true for your digital consumer camera sensors ---
in fact, red pixels also (need to) react to blueish frequencies.

So since we're not seeing and recording 3 single (or very
narrow) wavelengths, both perfectly corrected and ... less
perfectly corrected wavelengths overlap in any image formed
even by apochromatic lenses ...

-Wolfgang

Paul Ciszek 04-07-2013 09:55 PM

Re: Perfect lens
 

In article <MPG.2bcb40db622705a698c9c0@news.supernews.com>,
Alfred Molon <alfred_molon@yahoo.com> wrote:
>Perhaps another question: would they make a mirror lens for a camera and
>the shape of the mirrors were perfect (assume zero error), no
>displacement errors etc., would such a lens be perfect? Unlimited corner
>to corner sharpness wide open, no geometric or chromatic aberrations?


Undergraduate optics was a long time ago, but I seem to remember the textbook
saying something along these lines: Even a mathematically perfect lens or mirror
could only do one, not-very-useful thing perfectly: Bring a point source of light,
on-axis, to a single point focus, also on-axis. If you use small-angle approximations
and ignore the higher order terms, it turns out you can also sort of focus other
points a little bit a way from the optical axis and thereby form an image. But
imaging is by its very nature deviating from the mathematical idea into the realm
of "how good is good enough?". I think our teacher said something to the effect that
a non-spherical lens designed to perfectly focus an on-axis point source was actually
*worse* for imaging purposes than a spherical lens; I don't recall what he said about
mirrors specifically, but it was the same theme: Imaging is an apporoximation.

The one thing that a mirror *will* do for you is kill chromatic aberration. That is
why Newton was able to get such good results with his first attempt at a reflector
telescope.

--
"Remember when teachers, public employees, Planned Parenthood, NPR and PBS
crashed the stock market, wiped out half of our 401Ks, took trillions in
TARP money, spilled oil in the Gulf of Mexico, gave themselves billions in
bonuses, and paid no taxes? Yeah, me neither."


Martin Brown 04-08-2013 06:52 AM

Re: Perfect lens
 
On 07/04/2013 08:38, Alfred Molon wrote:

> Perhaps another question: would they make a mirror lens for a camera and
> the shape of the mirrors were perfect (assume zero error), no
> displacement errors etc., would such a lens be perfect? Unlimited corner
> to corner sharpness wide open, no geometric or chromatic aberrations?


There are always some small aberrations off axis but they can be kept
very well controlled iff you are prepared to sacrifice having a flat
focal plane. The best wide field diffraction limited optics all have one
thing in common - a curved focal plane (usually spherical).

Here is an example optimisation for a Schmidt telescope design:
http://sitemaker.umich.edu/anupam.ga...inal_final.pdf

(you may not like the maths)

Sharpness is always diffraction limited by the finite aperture of the
lens - you can't get around that without an infinite lens. It is for
this reason that astronomers are always building bigger telescopes.

The atmosphere also limits resolution in long telephoto lenses.

--
Regards,
Martin Brown

Wolfgang Weisselberg 04-10-2013 09:44 PM

Re: Perfect lens
 
Alfred Molon <alfred_molon@yahoo.com> wrote:
> Perhaps another question: would they make a mirror lens for a camera and
> the shape of the mirrors were perfect (assume zero error), no
> displacement errors etc., would such a lens be perfect?


Take a look at the bokeh of a mirror lens to be cured of that
idea. :-)

Of course, if your light rays are coming in perfectly parallel
(i.e. infinite distance) it means there's no closer or further
and thus no bokeh problems.

-Wolfgang


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