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Archive 2010 · Secretly aspherical lenses

thrice
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p.1 #1 · p.1 #1 · Secretly aspherical lenses

A lot of manufacturers advertise in their marketing or technical documents when a lens has an aspherical element or 2 (or 3 or 4).

What about those magical lenses that have a secret aspherical element? Not mentioned anywhere in marketing or technical documents, sometimes not even in lens diagrams.

If you're curious whether a lens has any aspherical elements, simply look at the reflections in the lens, particularly if you have a nice big light like a fluorescent behind you. Move the lens slowly and you should see the reflection distort like this:

Dec 20, 2010 at 05:17 AM

thrice
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p.1 #2 · p.1 #2 · Secretly aspherical lenses

Three such lenses I know of:

Leica 35mm Summilux-R
Zeiss ZM 18mm Distagon
Voigtlander 50mm Color-Skopar

I struggled to get photos of the reflections of the aspherical elements, as they're an inner element, but they're definitely not spherical.

Dec 20, 2010 at 05:19 AM

denoir
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p.1 #3 · p.1 #3 · Secretly aspherical lenses

Another way of seeing if a lens has aspherical elements is to look at bokeh 'blobs'. Aspherical elements leave a signature in the form of onion rings:

Zeiss 35/1.4 Distagon Rollei:
http://peltarion.eu/img/comp/35/asph-1.jpg

Leica 35/1.4 Summilux ASPH:
http://peltarion.eu/img/comp/35/asph-2.jpg

Dec 20, 2010 at 07:49 AM

thrice
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p.1 #4 · p.1 #4 · Secretly aspherical lenses

Not so easy with very wide or relatively slow lenses.

Dec 20, 2010 at 08:10 AM

AhamB
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p.1 #5 · p.1 #5 · Secretly aspherical lenses

thrice wrote:
I struggled to get photos of the reflections of the aspherical elements, as they're an inner element, but they're definitely not spherical.

It's very difficult to get pictures that show what lens reflection look like to the eye, isn't it? The much deeper (perceived) DOF of the eye may play a role. Maybe the brain uses focus stacking.

Dec 20, 2010 at 08:26 AM

Bifurcator
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p.1 #6 · p.1 #6 · Secretly aspherical lenses

denoir wrote:
Another way of seeing if a lens has aspherical elements is to look at bokeh 'blobs'. Aspherical elements leave a signature in the form of onion rings:

Zeiss 35/1.4 Distagon Rollei:
http://peltarion.eu/img/comp/35/asph-1.jpg

Leica 35/1.4 Summilux ASPH:
http://peltarion.eu/img/comp/35/asph-2.jpg

I thought that was only true of lenses containing aspherical elements that are formed by more than one piece of glass? No?

To explain, some manufacturers make the aspherical surface by glueing two or more pieces together instead of grinding and polishing a single piece. AFAIK, all manufacturers who do this count the constructed element as a single element - so it won't show up in a element diagram or the group/element count spec..

I read somewhere that only those kinds make the onion-ring patterns.

Edited on Dec 20, 2010 at 10:27 AM · View previous versions

Dec 20, 2010 at 10:24 AM

thrice
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p.1 #7 · p.1 #7 · Secretly aspherical lenses

The 35mm Summilux-M ASPH has a standalone aspherical element. Moulded aspherics are what cause the onion rings. Typically, hand-ground aspherical elements don't show the onion pattern.

Dec 20, 2010 at 10:27 AM

Bifurcator
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p.1 #8 · p.1 #8 · Secretly aspherical lenses

Cool, thanks for the confirmation!

Dec 20, 2010 at 10:29 AM

Bifurcator
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p.1 #9 · p.1 #9 · Secretly aspherical lenses

AhamB wrote:
It's very difficult to get pictures that show what lens reflection look like to the eye, isn't it? The much deeper (perceived) DOF of the eye may play a role. Maybe the brain uses focus stacking.

A good way to do this is (or has been for me) to use a very fast lens set to f1.2 or f1.0 or whatever, and then take a series of pictures into the lens you want to know about - focusing a little farther down each time. The DOF will take out the reflections from the elements that are not in focus at each point.

I use this method to find out which surfaces need cleaning or that may be damaged or whatever... But it (usually) also reveals the aspherical surfaces too.

If your fast lens doesn't have close focusing a close-up filter will do!

Neat thread idea BTW Thrice.

Edited on Dec 20, 2010 at 10:40 AM · View previous versions

Dec 20, 2010 at 10:35 AM

denoir
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p.1 #10 · p.1 #10 · Secretly aspherical lenses

Interesting. Does anyone still hand grind optics? I.e is there some modern aspherical design production lens that won't show onion rings?

Dec 20, 2010 at 10:39 AM

Bifurcator
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p.1 #11 · p.1 #11 · Secretly aspherical lenses

I dunno such insider info well, thrice may. But as I understand it single component aspherical elements are just as or more common than layered ones... Only they're more expensive to produce. And I have to guess that thrice didn't actually mean they were ground by hand... But rather by a grinding machine... as opposed to glueing parts together.

Dec 20, 2010 at 10:44 AM

AhamB
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p.1 #12 · p.1 #12 · Secretly aspherical lenses

thrice wrote:
Moulded aspherics are what cause the onion rings.

More specifically: the (concentric) machining marks in the mould cause the aspherics created with them to show onion rings.

@Luka: Early Noct-Nikkor batches are supposed/claimed to have had hand-grinded aspherics. I doubt that any modern consumer lenses have them.

Dec 20, 2010 at 01:08 PM

Toothwalker
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p.1 #13 · p.1 #13 · Secretly aspherical lenses

AhamB wrote:
More specifically: the (concentric) machining marks in the mould cause the aspherics created with them to show onion rings.

More specifically ... such aspherics are one of several possible causes of onion rings.

Dec 20, 2010 at 01:20 PM

Makten
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p.1 #14 · p.1 #14 · Secretly aspherical lenses

Toothwalker wrote:
More specifically ... such aspherics are one of several possible causes of onion rings.

Yes, I've seen "onion rings" from the Nikkor 105/2.5, which has no aspherical element. My guess is that it could also come from diffraction patterns for certain designs.

Dec 20, 2010 at 01:33 PM

Toothwalker
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p.1 #15 · p.1 #15 · Secretly aspherical lenses

Makten wrote:
Yes, I've seen "onion rings" from the Nikkor 105/2.5, which has no aspherical element. My guess is that it could also come from diffraction patterns for certain designs.

Fresnel diffraction at the aperture, reflections between neighboring surfaces in cemented groups ... analysis of interference patterns is not easy. IMHO, diffraction at the lens aperture is the only mechanism that can account for the onion pentagons and onion hexagons of which we have seen examples in this forum.

Dec 20, 2010 at 01:44 PM

Steve Spencer
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p.1 #16 · p.1 #16 · Secretly aspherical lenses

thrice wrote:
The 35mm Summilux-M ASPH has a standalone aspherical element. Moulded aspherics are what cause the onion rings. Typically, hand-ground aspherical elements don't show the onion pattern.

Hi Dan,

I don't think it is that simple. There are a bunch of ways to make aspherical lenses, which are just a lens that is not spherical in shape. One way to make them is to grind and polish a spherical glass lens into an aspherical shape. Another way is to heat glass (or plastic if it is lower quality) and mold it into an aspherical shape. Yet another way is to glue on more glass (or add plastic or polymer) to make the aspherical shape.

The method of making the aspherical lens does matter. Generally gluing on extra glass or building up aspherical surfaces with polymers is cheaper, but produces at least somewhat inferior results. Molded glass asphericals can also be excellent. For example the Pentax Limited 31mm has this type of apshericals and it is generally regarded as an excellent lens. Ground glass asphericals can be quite nice, but a lot depends on the quality of the grinding and polishing. So, I don't think you can make any sweeping statements about the best way to make asphericals, except that the quality of the process probably matters.

Also, as I understand it the onion rings results when the polishing doesn't completely remove the results of manufacturing process whether it is by grinding, molding, or gluing. I know that some lenses with ground asphericals can have onion ring type bokeh if you look close. For example, the Canon FD 85mm f/1.2L definitely does.

Here are a couple links that I have found helpful on asphericals:

http://www.mir.com.my/rb/photography/opinion/html/aspherical.htm
http://en.wikipedia.org/wiki/Aspheric_lens
http://cpn.canon-europe.com/content/education/infobank/lenses/fluorite_aspherical_and_ud_lenses.do

Dec 20, 2010 at 02:38 PM

sebboh
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p.1 #17 · p.1 #17 · Secretly aspherical lenses

Steve Spencer wrote:
Also, as I understand it the onion rings results when the polishing doesn't completely remove the results of manufacturing process whether it is by grinding, molding, or gluing. I know that some lenses with ground asphericals can have onion ring type bokeh if you look close. For example, the Canon FD 85mm f/1.2L definitely does.

you can see them in luka's example from the summilux (though they are much subtler than the from the zeiss), which dan said was hand ground as well. to my eye the pattern in that shot actually looks like a spiral rather than concentric rings. am i imagining it?

Dec 20, 2010 at 02:51 PM

Makten
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p.1 #18 · p.1 #18 · Secretly aspherical lenses

Toothwalker wrote:
Fresnel diffraction at the aperture, reflections between neighboring surfaces in cemented groups ... analysis of interference patterns is not easy. IMHO, diffraction at the lens aperture is the only mechanism that can account for the onion pentagons and onion hexagons of which we have seen examples in this forum.

I'm actually refering to wide-open shots with the 105/2.5. But if you can se "onion octagons" or any other shape than round, it should be proof for not being caused by an aspherical element.

Dec 20, 2010 at 02:57 PM

rsrsrs
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p.1 #19 · p.1 #19 · Secretly aspherical lenses

the Vario-Sonnar® T* 3.5-4.5/24-85mm has "aspheric surfaces."
but don't know how much, surfaces is plural ....

gruss
reinhard

Dec 20, 2010 at 03:09 PM

Keith B.
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p.1 #20 · p.1 #20 · Secretly aspherical lenses

thrice wrote:
A lot of manufacturers advertise in their marketing or technical documents when a lens has an aspherical element or 2 (or 3 or 4).

What about those magical lenses that have a secret aspherical element? Not mentioned anywhere in marketing or technical documents, sometimes not even in lens diagrams.

If you're curious whether a lens has any aspherical elements, simply look at the reflections in the lens, particularly if you have a nice big light like a fluorescent behind you. Move the lens slowly and you should see the reflection distort like this:

This is not a guaranteed method of determining the presence of aspheres.

Dec 20, 2010 at 03:37 PM

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