lens question

Discussion in 'Photography Beginners' Forum' started by Shakka Brah, Nov 20, 2006.

  1. Shakka Brah

    Shakka Brah TPF Noob!

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    Under the specs for lenses there is usually a construction part. This part says something like 6 elements in 5 groups. What does this mean and how important is it?

    Thanks in advance.
     
  2. Big Mike

    Big Mike I am Big, I am Mike Staff Member Supporting Member

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    From what I can tell...it doesn't matter all that much...unless you know a lot about lens construction...which most of us probably don't. I've even seen on Canon's web site (I think) they show a section view drawing of the lens to show the elements and groups...I still have no idea what the benefits or downfalls of more or less groups/elements are.

    If you really want to know...I'm sure you could research it...but it's probably not that important.
     
  3. GSDMan

    GSDMan TPF Noob!

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    The whole of my lens design knowledge is contained in the folowing sentence.

    The first piece of glass in a lens forms the image and the rest fix the problems of the first.

    Translation - it only takes one simple lens to form an image. Every other lens, or group of lenses, is there for color correction, field flattening, changing effective focal length, etc.
     
  4. toastydeath

    toastydeath TPF Noob!

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    It matters in a few different ways.

    The optic pack has to account for and remove the various aberrations that occur when you're trying to focus multiple wavelengths of light, and at the same time focus the light onto a flat plane after putting your light through curved surfaces.

    One aspheric, achromatic lens element can take the place of several traditional lens elements. This is why high power laser optic assemblies use aspheric elements. Of course, small (camera-sized) aspheric and achromatic lenses are more expensive to produce when compared to using (same sized) many elements to preform an optical correction.

    Does it matter in the end? I certainly wouldn't buy a lens based on the number of elements it had. I don't care if it's built with glass or plastic (optical 'resin'). If it does the job well and is of sturdy construction, then it's a good lens.
     
  5. fmw

    fmw No longer a newbie, moving up!

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    GSD has an succinct but accurate view of the whole business. I'm not a lens designer, but I have studied lens design and I find the diagrams interesting because it shows me how the designer attacked the problems. There are many ways to "skin the cat."

    As a rule of thumb, simplicity is king. Whenever you add elements to correct something, you always add more issues to correct. Basically, the trick is to get it done with as much simplicity as possible. Someone mentioned aspheric elements above. That is an example of using simplicity because aspheric elements can replace several traditional elements.

    Classic 35mm designs like the Nikkor 105 f2.5 or the Leica 90mm f2.8 are simple - just a handful of elements. The problem today is that people want zoom lenses and therein lies the difficulty for the lens designer. It is hard enough to correct a single focal length - it is a matter of selecting compromises to correct one with a range of focal lengths. So the zoom lenses don't have more elements because they are better. They have more elements because they are more complicated. The complication shows up as more compromise in the design and performance of the lens. It is a tradeoff between flexibility and performance. When I see that a lens has 15 elements I scratch my head and wonder that the thing can make photographs at all. It can, of course, but it takes a computer to do all the calculations that go into making one of these monsters.

    The very highest performing lenses are those that do not need to zoom and do not need to focus (focusing is handled by the camera rather than the lens itself.) Examples are view camera lenses and others like the Mamiya RZ fixed lenses which are really short view camera lenses with a smaller image circle. These lenses do not need to zoom or focus. Nothing moves inside the lens barrel. They just need to project a well corrected, sharp image and they do. They are very simple compared to 35mm sized zoom lenses. You simply can't believe what they can do until you use one.

    So don't view the complement of elements as a mark of quality or performance, view it as the mark of complexity and the designer's desire to lessen the impact of the compromises.
     
  6. Alex_B

    Alex_B No longer a newbie, moving up!

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    i agree with most of the things said above ... plus the more elements the more prone to flare/reflections a lens will be under difficult light. and it gets heavier.
    a zoom lens needs certainly more elements, but their number is not useful for comparison.

    to judge a lenses optical quality .. see what images it produces, and not how it is constructed. :mrgreen:
     

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