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#11
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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
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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
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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
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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
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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
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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
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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
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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
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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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