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I have heard of lenses with UD elements.

My question is what are UD elements and what benefit do they give?

Can a lens have more than one UD element?

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2  
UD = ultralow dispersion - see this question: photo.stackexchange.com/questions/496/… –  MikeW Aug 28 '13 at 9:40
    
The first result when you google "UD element" is this: canon.com/camera-museum/tech/room/hotaru.html Does that help you at all? –  dpollitt Aug 28 '13 at 17:18
1  
Related question: What are ED elements?. –  thomasrutter Aug 30 '13 at 3:52

3 Answers 3

up vote 3 down vote accepted

When parallel light rays are refracted by a prism, a rainbow-hued spectrum comes out. This phenomenon is called "dispersion". In case of photographic lenses, the dispersion causes color fringes at the edge of subjects, which is called axial chromatic aberration, and as a result, deterioration of image quality of photography. There is a limit to the correction of chromatic aberration, using regular optical glass lens elements only. Some aberrations not corrected by optical glass are called secondary spectrum or residual chromatic aberration or secondary chromatic aberration.

The artificial crystal fluorite lens element, featuring very low optical dispersion index, was developed by Canon to eliminate secondary spectrum. Canon succeeded artificially crystallizing calcium fluoride (CaF2) into fluorite at the end of 1960s. Canon EF lenses are the only interchangeable lenses for 35mm SLR at the time that employ fluorite lens elements.

In the late 1970s, Canon also developed special optical glass lens elements with very low dispersion index called Ultra-low Dispersion (UD) glass lens elements, and in 1990s an upgrade version of UD glass, called Super UD glass, was developed. Fluorite, UD and Super UD glass lens elements are widely used in EF lens series, for super-telephoto L series lenses, as well as in telephoto zoom and wide angle lenses.

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Just a note on factuality...the new Nikon 800mm f/5.6 lens also uses Fluorite elements, and borrows heavily from the design of Canon's newest Mark II EF superteles. –  jrista Aug 28 '13 at 14:51

It is a specialized type of glass that disperses frequencies of light less, thus less issues with color fringes. I don't have my EOS guide book handy, but I seem to recall lenses that have more than one UD element. I'll update my answer later when I get a chance to check and verify which (if any) actually have more than one UD element.

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Heard of ED elements? Same thing.

Well, mostly. While ED stands for extra-low dispersion, UD stands for ultra-low dispersion. In both cases, it means that the refractive index of the lens material is lower than regular glass. UD appears to be a term for low dispersion glass used mainly by Canon.

Low dispersion glass allows lens manufacturers to exploit the difference in refractive index between different lens elements to help reduce colour fringing in a more effective way, with fewer side effects.

  • Different types of glass have different refractive indexes: different amounts by which light will bend when they enter the glass.

  • When light of different colours enters glass, the different colours bend by different amounts. When the resulting image has different colours not lining up properly, this is called chromatic abberation or "colour fringing".

  • A lens assembly has many different lens elements: some where the light enters or exits a convex surface, and some where the light enters or exits a concave surface. At each step, the different colours will either become more spread out or closer together depending on the orientation of that lens surface.

  • It is possible to "correct" for colour fringing to some extent by adding more lens elements, that reverse the colour separation effect of previous lens elements. This is rather complex, because adding other elements also affects the focusing and magnification, too. It's a juggling act.

  • A low dispersion (such as ED) element can help greatly in this, because you can then use the difference in refractive index between the different lens elements to achieve an effect you couldn't otherwise.

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