Aspherical so non spherical what is it actually?!

[jip]

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Hello guys,

 

My question for today: What are aspherical surfaces in lenses, and what do they do exactly?

[jdlaing]

Aspheric lens - Wikipedia, the free encyclopedia

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[adan]

To expand a bit:

 

Spherical lenses have a surface that is a section of a sphere. That is, they have a constant radius. This makes them easy to grind and polish, especially in mass production.

 

http://johnsrolleionlypage.homestead.com/Rollei_12_Optics_Polishing.jpg

 

However, a spherical surface is not the best way to form a photographic image, especially with large-diameter (i.e. "fast") lenses. It introduces "spherical aberration" - light coming through the outer edges is not focused at the same point as light passing through the center.

 

This requires additional glass elements that correct the error of the first element, but introduce their own aberrations, and more elements to correct those...etc. etc.. However, for quite a long time, it was still cheaper to pay for the extra bits of glass than to produce one-off aspherical surfaces.

 

When Leica first produced aspherical surfaces (Nocitlux f/1.2, 35mm f/1.4 "Aspherical," not "ASPH") they had to be hand-made one at a time, with a fairly large failure rate (toss the lens and start over).

 

But since about 1990, better (cheaper, mass-production with a low loss rate) techniques have allowed the addition of aspheric elements throughout the industry. Leica and Hoya jointly developed a "blank-pressing" or molding process whereby soft hot glass is pressed against a metal mold already shaped to the aspheric curve. So that now even inexpensive P&S lenses often have aspheric elements.

 

What they do depends on what the designer wants them to do: improve performance without adding elements, allow fewer elements without reducing performance, allow other elements to correct something other than spherical aberration (chromatic aberrations, astigmatism, etc.)

 

Erwin Puts says one aspheric surface does the work of two spherical surfaces, so two aspheric surfaces can eliminate one entire element (two sides, both spherical). Or improve the lens without adding one whole additional element.

Edited January 10, 2015 by adan

[jip]

Thanks for the explanation!

[bruniroquai]

great Adan, thanks!

[epand56]

Extremely clear explanation. Thank you Andy.

[Son of Helm]

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To expand a bit:

 

Spherical lenses have a surface that is a section of a sphere. That is, they have a constant radius. This makes them easy to grind and polish, especially in mass production.

 

 

 

Erwin Puts says one aspheric surface does the work of two spherical surfaces, so two aspheric surfaces can eliminate one entire element (two sides, both spherical). Or improve the lens without adding one whole additional element.

 

 

Inexpensive lenses reduce aberrations by adding lots of lens elements, but this increases flare. Using asphericals reduce the number of elements which may improve flare suppression

[Bill W]

The 35 f1.4 "Aspherical" will cost you anywhere from $10,000 to $15,000 today. There are several on ebay if one is interested.

[james.liam]

The 35 f1.4 "Aspherical" will cost you anywhere from $10,000 to $15,000 today. There are several on ebay if one is interested.

 

Is that double aspherical "all that"? Wondered what the big deal was all about, besides rarity.

[rodoleica]

To expand a bit:

 

Spherical lenses have a surface that is a section of a sphere. That is, they have a constant radius. This makes them easy to grind and polish, especially in mass production.

 

http://johnsrolleionlypage.homestead.com/Rollei_12_Optics_Polishing.jpg

 

However, a spherical surface is not the best way to form a photographic image, especially with large-diameter (i.e. "fast") lenses. It introduces "spherical aberration" - light coming through the outer edges is not focused at the same point as light passing through the center.

 

This requires additional glass elements that correct the error of the first element, but introduce their own aberrations, and more elements to correct those...etc. etc.. However, for quite a long time, it was still cheaper to pay for the extra bits of glass than to produce one-off aspherical surfaces.

 

When Leica first produced aspherical surfaces (Nocitlux f/1.2, 35mm f/1.4 "Aspherical," not "ASPH") they had to be hand-made one at a time, with a fairly large failure rate (toss the lens and start over).

 

But since about 1990, better (cheaper, mass-production with a low loss rate) techniques have allowed the addition of aspheric elements throughout the industry. Leica and Hoya jointly developed a "blank-pressing" or molding process whereby soft hot glass is pressed against a metal mold already shaped to the aspheric curve. So that now even inexpensive P&S lenses often have aspheric elements.

 

What they do depends on what the designer wants them to do: improve performance without adding elements, allow fewer elements without reducing performance, allow other elements to correct something other than spherical aberration (chromatic aberrations, astigmatism, etc.)

 

Erwin Puts says one aspheric surface does the work of two spherical surfaces, so two aspheric surfaces can eliminate one entire element (two sides, both spherical). Or improve the lens without adding one whole additional element.

Very clear ex^lanation. Thank you.

[adan]

Is that double aspherical "all that"? Wondered what the big deal was all about, besides rarity.

 

A decade ago, I was lucky enough to get a 35 f/1.4 "Aspherical" and an "ASPH" together in the same place at the same time, and do a little test.

 

Quick test - rare "Aspherical" 35 f/1.4 - Photo.net Leica and Rangefinders Forum

 

At f/1.4, the Aspherical was a bit sharper in the center and softer in the corners - the ASPH had more even sharpness over the whole image. Net - it would be hard to say one was "better" than the other overall, although the Aspherical might be a bit better for photojournalism (corners don't matter) and the ASPH better when the corners DO matter.

 

I didn't test for focus shift, however. Wasn't on my radar screen at the time.

 

So I'd say the price difference is based primarily on rarity/collectability.

[pico]

An aspherical front lens is easily identified. It has the profile of a premature boob job.

[luigi bertolotti]

Just to add a bit of personal experience about the magic of "asph"... in the automotive industry is normal to have aspherical surfaces in the reflectors of front lamps : the need to accord this with style-driven constraints makes the manufacturing of moulds for those components a very delicate and highly specialized job.... btw, CNC machines do not perform, mathematically strict speaking, aspherical tool paths, but very small arcs, continuosly tangent one to the other, with very small variations in radius... tolerancing fixes it all.

Edited January 23, 2015 by luigi bertolotti

[jdlaing]

An aspherical front lens is easily identified. It has the profile of a premature boob job.

 

I'm going to need to see some photographic evidence of this phenominon.

[mca]

great info and explanation Andy, thank you!

[CheshireCat]

Is that double aspherical "all that"? Wondered what the big deal was all about, besides rarity.

 

In general, a double-aspherical lens is supposed to be better corrected than a single-aspherical lens. But this may not be the case. Technology always advances, and an improved/better design could use less asphericals for similar performance.

 

It is interesting to note the following.

50mm lenses group:

- The Noctilux 50/1.2 has 2 asphericals

- The Noctilux 50/1 has 0 asphericals

- The Noctilux 50/0.95 has 2 asphericals

- The SLRMagic 50 T/0.95 has 0 asphericals

35mm lenses group:

- The Summilux Aspherical 35/1.4 has 2 asphericals

- The Summilux FLE 35/1.4 has 1 aspherical

- The Nokton 35/1.2 has 3 asphericals

 

Each group is ordered by release date.

And when these lenses are compared, it is clear that what really matters is the overall lens design, rather than the number of asphericals.

Also, for some people, lens "character" matters even more than lens performance.

For some other people, lens "rarity" is the way to go.

 

The logical conclusion is: which lens is best for you has not much to do with the number of asphericals

Edited January 25, 2015 by CheshireCat

[ashfaque]

Thanks for all the easy explanations. Just of out of curiousity, can you please share your thoughts and analysis on Zeiss ones - more specifically Sonnar and Otus lenses. I know some of them are meant for DSLRs.

 

Bests,

Ashfaque

Edited January 27, 2015 by ashfaque

[jip]

Thanks for all the easy explanations. Just of out of curiousity, can you please share your thoughts and analysis on Zeiss ones - more specifically Sonnar and Otus lenses. I know some of them are meant for DSLRs.

 

Bests,

Ashfaque

 

 

You're hijacking my thread.

[CheshireCat]

Thanks for all the easy explanations. Just of out of curiousity, can you please share your thoughts and analysis on Zeiss ones - more specifically Sonnar and Otus lenses. I know some of them are meant for DSLRs.

 

Quickly, without hijacking the thread too much

The "Sonnar" name is for a lens design, while the "Otus" name is for a high-end lens line (with different lens designs, currently a Distagon and a Planar).

 

About the latest and greatest Zeiss DSLR lenses, as far as I know:

- The APO Sonnar 135/2 has 0 or 1 aspherical (it is reported in the lens description but not in the diagram).

- The Otus APO Distagon 55/1.4 has 1 double-aspherical element (i.e. 2 aspherical surfaces).

- The Otus APO Planar 85/1.4 has 1 aspherical.

 

Note that the APO Sonnar 135/2 is not part of the high-end Otus line, but it is in my opinion the best 135 lens ever made.

You can adapt all the above Zeiss lenses to an M camera.

 

If you are interested in Zeiss lens design, I strongly recommend searching the Internet for articles by Dr. Nasse.

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Hallo,

Look here: Leica M

[ashfaque]

Thanks CC. I've skimmed through them whilst reading through the naming history of Zeiss lenses.

 

Anyway, no more 'hijacking' from me.