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Thread: Lens' glass element-group question

  1. #1

    Default Lens' glass element-group question

    A little question over here, does more glass elements in a lens mean higher quality?

    Higher quality not in term of pictures taken but lens quality.

  2. #2

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    Some lenses have more elements to correct for chromatic aberrations (achromatic/apochromatic). I dunno if it's the general trend to say that more elements = higher quality.

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    Default ...eerrr nope.......not always.....

    Hiee....

    To my knowledge, the number of elements have its plus and minus. In most cases, the size of the lens, the range(e.g zoom) influences the choice for the designers to decide the number of glass going into the lens. Glass itself is a material(medium) . As per general uderstanding...more medium means light have to pass through more "things". THis if not carefully accounted for, will influnces the "speed" of the lens.

    In some cases, for abberation correction, asperics and APOchromatic glasses are used to reduce distortions for long lenses. These amy be used with other glasses like LD(low Dispersion etc. Well, at the end of the day, More Glass meay also mean higher Cost ===== which doen't mean that they are superior in quality etc.

    As for IF(internal focus lenses), engineers design thesese lenses with creative mechanical movements of dedicated group of glasses internally to get the desired zooms fo example. So if you get to see on the net, some lenses use or share the basic chassis(body) but internally these glass arrangments and shapes determines the type of images thst you will get......Me no expert on these but just from knowledge shered among photography ppl.

    By the way, now lenses are also made from plastics - or even Hybris lenses where both glass and plastics used - reducing overall weight of the lens.

    Well.....hope this helps.....

    regards,
    sulhan

  4. #4

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    plastic lens? Not good right?

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    I think more elements == smaller lens body, thanks to multiple refraction. APO make the refr higher hence either less glass per element or less elements...
    quality wise for the same type of glass more is bad due to air cushioning and over refracting... giving CA...
    think of a gear train. more gears == less power.
    "I'm... dreaming... of a wide... angle~
    Just like the ones I used to know~"

  6. #6
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    Default

    Yamashita,

    Your question is a bit complex and isn't as straightforward as better or worse. I'll deal with that after dealing with a few of the things that have already been mentioned in reply.

    Sulhan,

    The size of the lens has no real direct relation to the number of elements and groups. Instead, it is really the optical complexity of the lens. Telephoto prime lenses which are relatively simple to correct optically require few elements and groups, despite their vast physical size. A small superwide angle lens can require two to three times the number of elements and groups as a telephoto with the same aperture 60 times its weight because of the difficulty in correcting wide angle lenses.

    Zoom lenses, by nature of their complex optical formula in needing to correct aberrations across a range of focal lengths, mean that they all tend to have a relatively large amount of elements and groups. It follows that the larger the zoom range, the higher the number of elements and groups as a general rule. Zooms that cross different focal length groupings (wide to normal to tele) also have an abnormally high number of groups and elements because optically, a wide angle lens is very different from a telephoto lens.

    Sulhan is right in that a lens with more elements and groups can be a slower lens. An aperture is a physical measurement of the lens iris that is (or should be) accurate from lens to lens. However, the amount of glass surfaces present in the lens will limit the amount of light transmitted to the film plane, and also reduce contrast of the resulting image. As with filters, each element will entail light passing from air to glass back to air again, with light loss to minor transmission flaws, reflection, and refraction. Depending on how well an element is coated, the loss can range from a negligible amount, to about 3% of the light reaching the lens element.

    Multiply that figure by 15 times in your average zoom lens, and you can see how a lot of light can be lost, compared with a lens that only has 6 elements.

    Aspherical elements are not really needed to correct for distortions in long lenses, unlike what was suggested by Sulhan. They are more helpful in dealing with the distortion caused by wide angle lenses.

    Apochromatic and Low Dispersion glass are actually the same thing. Tamron uses LD to designate their lenses with apochromatic glass in it.

    More glass doesn't mean high cost, not in the sense that we are discussing here (elements and groups). Better glass, however, does mean drastically higher cost. While a bundle of small elements and groups (in your average 28-200mm superzoom costing $450 for example) can be relatively cheap, the same amount of elements and groups in a good quality wide angle prime lens, say in the 18-20mm area, can cost twice as much if not far more (the Nikon 18mm for example).

    Internal Focus has nothing to do with creative mechanical movements. The issue is not in getting the mechanics to drive an internal part of a lens, that is straightforward. The big difference is designing an optical formula whereby the focusing group is inside the lens. This means a whole rework of a lens' optical formula, and is not as simple as simply taking the glass off the front or the back, putting it in the middle, and then finding some creative mechanical movement to be able to move that group of elements.

    Be careful about what you read on the Net (including this post for instance), don't accept everything as gospel truth, and more importantly, when you read something, make sure you're understanding it properly and avoid the broken telephone syndrome, so you're not passing on spurious information.

    Lens barrels are made from polycarbonate, yes. Lenses are still by and large made from glass, so I wouldn't worry too much about it. The main area where plastic is used is in the manufacture of hybrid ashperical lens elements. Molded and ground glass aspherical elements can be notoriously expensive to manufacture (and as already stated elsewhere is at least one reason explaining the relative expense of the Nikkor 20-35mm f2.8 compared to the AF-S 17-35mm; in fact the aforementioned Nikkor 18mm also uses either a ground or molded aspherical element, exactly which eludes me at the moment but a quick look at a Nikon source will reveal).

    Of course, it helps if you understand what an aspherical lens is to start with, which will allow you to understand why they are so costly to mold or grind.

    As a result, cheaper lenses like the 28-200 utilise hybrid (not hybris [sic]) aspherical elements (not lenses), which is basically a standard optically designed piece of glass, which is mated to plastic piece that produces the required aspherical geometry.

    Denizenx,

    I'm not altogether sure you're making sense, certainly I am struggling to understand you, but of course it could (honestly) just be me.

    Now to answer the original question. More pieces of glass increases the chances of optical imperfections etc. It also reduces light transmission as mentioned. However with some lenses, you need to have many elements and groups to correct the lens optically, particularly with zoom lenses. So it really is a trade off.

    Essentially, I wouldn't worry too much about the number of elements and groups in a lens, as the engineers would have made all the design decisions with lots of experience and backed up by computers with a large amount of information at their disposal. Chances are the elements and groups your lens has are the best solution for the given price and focal length.

  7. #7

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    Thanks Jed for your detailed explaination.

  8. #8

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    Originally posted by Jed

    Denizenx,

    I'm not altogether sure you're making sense, certainly I am struggling to understand you, but of course it could (honestly) just be me.
    Nope Jed, it's not just you.

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    Default Hieeee....

    I spend some ime studying the cross cection internal layout of lenses through the Sigma and Minolta lens catalogue.....from there, it seem that many lens share same bodybut the internal elements may be alittle diffrent to change the light path and zoom ranges.......

    For example.....the Minolta 24-85mm and the new 24-105mm(D). The newer 24-105mm(D) cost lower than the fromar 24-85mm.

    The 24-85mm(415g) though have more elements and weigh little more than the 24-105(395g).

    The extra cost would then be corelated to the extra glass in this case.

    Another comparison is the Kenko MC4 and MC7 2x teleconverter, The numbers explains the number of elements and the MC4 coat is lower and smaller in size. Performance wise bothe are the same.


    Here is a linl to popular photography website explaining on how the Different lens maker practice in lens design....

    pop photo


    hope this helps....
    regards,
    me

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    Default Re: Hieeee....

    Yamashita you are most welcome. I hope it helps.

    Originally posted by sulhan I spend some ime studying the cross cection internal layout of lenses through the Sigma and Minolta lens catalogue.....
    Heh. Good pastime! I'm a little partial to the Nikon catalogue myself... drool.

    from there, it seem that many lens share same bodybut the internal elements may be alittle diffrent to change the light path and zoom ranges.......

    Your point being? They might well use the same lens barrels... lens barrels don't produce the image, they just house the glass that produces the image. You're just stating what I would take to be common knowledge. I'd be surprised if you could change the light path and zoom ranges by just changing the body (sic, barrel) if the lens.

    The extra cost would then be corelated to the extra glass in this case.

    I've already extensively explained above that cost is largely determined by quality of glass and difficulty of manufacture. There is no direct proportion to the number of lens elements and groups as is being discussed here. I don't doubt that a greater mass of glass will, ceteris paribus, result in greater expense. But a 6 element, 5 group 300/2.8 costs a lot more than a 18 element, 16 group 28-200mm. I know, as I said, there is a much larger mass of glass, but we're discussing elements and groups here.

    Another comparison is the Kenko MC4 and MC7 2x teleconverter, The numbers explains the number of elements and the MC4 coat is lower and smaller in size. Performance wise bothe are the same.

    The first part if spot on, you're certainly right that the numbers relate to the number of elements each teleconverter possesses.

    However, to say that performance wise they are bothe (sic) the same is absolute nonsense and I can say this as a fact.

    Actually, that's a very good example with which to answer the original question. The 7 element version has more elements and yields better optical quality because it has more elements to correct the conversion optically. The 4 element version on the other hand will be (very marginally) "faster" in the sense of losing less light to transmission loss.

    Ultimately the 7 element version is a better buy optically if you can afford it.

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    Default

    heh, to summarize, I was just saying the use of more elements is to reduce the lens size, APO is my mistake should be aspherical.

    so by using more elements the amount of glass needed is less (weight lighter) and aspherical pieces can be stacked closer together (dimensions shorter). LD is to make sure there is less CA when u use too many elements..

    that's all lor...
    "I'm... dreaming... of a wide... angle~
    Just like the ones I used to know~"

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    the gear train thing is just to say that if u put too many gears inbetween a motor and its resultant movement u lose power lor... in thiss case image quality lah... haha my 60 seconds analogy lor...
    "I'm... dreaming... of a wide... angle~
    Just like the ones I used to know~"

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