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Thread: Pentax DA 18-250 f3.5-6.3 ED AL IF

  1. #21

    Default Re: Pentax DA 18-250 f3.5-6.3 ED AL IF

    mmm...fengwei's description on magnification got me wondering too.
    As an astronomers, magnification on a telescope is obtained by dividing the focal length of the main objective (main lens) by the focal length of the eyepiece. for the same focal length, you get the same relative sized image on any telescope , everything else remaining the same. For cameras, they are quite so different. I know magnification of a zoom lens is the maximum divided by the minimum focal length. that's how 18-250 yields x13 or a 70-300 yields x4. But the image size can be entirely different for different x4 lenses. And I wonder how you calculate for prime lenses since there is nothing to divide! Its all so confusing...!

  2. #22
    Senior Member creampuff's Avatar
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    Default Re: Pentax DA 18-250 f3.5-6.3 ED AL IF

    Quote Originally Posted by airconvent View Post
    mmm...fengwei's description on magnification got me wondering too.
    As an astronomers, magnification on a telescope is obtained by dividing the focal length of the main objective (main lens) by the focal length of the eyepiece. for the same focal length, you get the same relative sized image on any telescope , everything else remaining the same. For cameras, they are quite so different. I know magnification of a zoom lens is the maximum divided by the minimum focal length. that's how 18-250 yields x13 or a 70-300 yields x4. But the image size can be entirely different for different x4 lenses. And I wonder how you calculate for prime lenses since there is nothing to divide! Its all so confusing...!
    The formula in your example merely refers to the ZOOM RATIO of a lens, a simplistic representation of the zoom range as expressed by the longest focal length divided by the minimum focal length. It has nothing to do at all with the magnification of the lens and for all intents it is a meaningless marketing description.

    Magnification is expressed as 1: x, which is a ratio of the actual size of a subject [1] to the size of the subject image reproduced on the film plane [1/ x]. Therefore, the larger the x value becomes, the smaller the reproduced image on the film plane.

    If you shoot an object that is let's say 60mm big. At 1:1 (1x) it will appear as 60mm in the film plane/sensor. At 1:2 (0.5X) it will be 30mm and at 1:4 (0.25X) it will be 15mm in size.

  3. #23
    Senior Member creampuff's Avatar
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    Default Re: Pentax DA 18-250 f3.5-6.3 ED AL IF

    Just for fun, the zoom ratio of a prime lens is... er...1X
    This just shows how meaningless this zoom ratio marketing jargon is all about...
    Last edited by creampuff; 4th October 2007 at 07:12 AM.

  4. #24

    Post Re: Pentax DA 18-250 f3.5-6.3 ED AL IF

    FYI for all interested...

    It is being offerd in Japan at 64800 yen approx $800+...

    This is the site... I dunno Jap, but i guess from the signs on the site, there is a 10% rebate (or on shipping charges )

    Anyone care to translate?

    < http://www.yodobashi.com/enjoy/more/i/76255586.html >
    May the force be with you...

  5. #25

    Default Re: Pentax DA 18-250 f3.5-6.3 ED AL IF

    I think thats member discount. Tourists without member card can get 5% GST refund off most items except consumables.

  6. #26

    Thumbs up Re: Pentax DA 18-250 f3.5-6.3 ED AL IF

    OIC... Wonder how much would it be when it reaches our shores...
    May the force be with you...

  7. #27
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    Default Re: Pentax DA 18-250 f3.5-6.3 ED AL IF

    i know its been almost a year so i was wondering, has it hit our shores yet?
    anyone know the price?

  8. #28
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    Default Re: Pentax DA 18-250 f3.5-6.3 ED AL IF

    Quote Originally Posted by Grey Knight View Post
    i know its been almost a year so i was wondering, has it hit our shores yet?
    anyone know the price?
    Yes, it's available for quite sometimes already. Some of the bro's here have it. Not sure about the price as could have drop a bit now.

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