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Thread: Formula for adjusting Focal Length with Close-up Lenses

  1. #21
    Mark Sawyer's Avatar
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    Re: Formula for adjusting Focal Length with Close-up Lenses

    Quote Originally Posted by Kirk Gittings View Post
    I'm curious as to why it matters-just curiosity? I have to say I never even thought about the effect of a CUL on focal length.
    Well, since it affects focal length, and it also affects aperture (as the CU lens magnifies the opening), it affects f/stop, so it's nice to know it for exposure calculation. And for some of us lens-nerd types, it's nice just to understand how things work.
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

  2. #22
    Mark Sawyer's Avatar
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    Re: Formula for adjusting Focal Length with Close-up Lenses

    Quote Originally Posted by xkaes View Post
    All I want is a simple formula to use in the field. I want to do it in my head. A+B=C. So far I have
    NEW FL = OLD FL / (CU diopter / 10)

    So 90 = 100 / 10.
    I'll try one more time:

    the formula is A+B=C

    A = original lens diopter power
    B = CU lens diopter power
    C = combined diopter power

    You can then convert diopter power to mm for focal length with the formula:

    1000/C
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

  3. #23

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    Re: Formula for adjusting Focal Length with Close-up Lenses

    Let's say your widest lens is 90mm. If you want a wider one, just add on a supplementary lens. A #1 supplementary lens converts your 90mm lens to a 82mm lens. A #2 supplementary lens converts your 90mm lens to a 75mm lens. It saves the cost a new lens. Get it?



    Quote Originally Posted by Kirk Gittings View Post
    I'm curious as to why it matters-just curiosity? I have to say I never even thought about the effect of a CUL on focal length.

  4. #24
    Mark Sawyer's Avatar
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    Re: Formula for adjusting Focal Length with Close-up Lenses

    Quote Originally Posted by xkaes View Post
    Let's say your widest lens is 90mm. If you want a wider one, just add on a supplementary lens. A #1 supplementary lens converts your 90mm lens to a 82mm lens. A #2 supplementary lens converts your 90mm lens to a 75mm lens. It saves the cost a new lens. Get it?
    It also creates a lens with a different optical formula that's far less than optimum. Expect a few aberrations to show up, (field curvature, coma, astigmatism, chromatic aberration, maybe some barrel distortion). The formula is:

    A+B=C

    A = a good lens
    B = cheap modifier
    C = piece of shit lens

    Mind you, some of us love our piece of shit lenses! But then, shit is an acquired taste, I suppose...
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

  5. #25
    Kirk Gittings's Avatar
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    Re: Formula for adjusting Focal Length with Close-up Lenses

    Quote Originally Posted by xkaes View Post
    Let's say your widest lens is 90mm. If you want a wider one, just add on a supplementary lens. A #1 supplementary lens converts your 90mm lens to a 82mm lens. A #2 supplementary lens converts your 90mm lens to a 75mm lens. It saves the cost a new lens. Get it?
    I understand that with your example of a "supplementary lens" (good lens becomes crap lens) but you were talking about a close-up lens which I never thought of as changing the focal length.
    Thanks,
    Kirk

    at age 73:
    "The woods are lovely, dark and deep,
    But I have promises to keep,
    And miles to go before I sleep,
    And miles to go before I sleep"

  6. #26

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    Re: Formula for adjusting Focal Length with Close-up Lenses

    OMG. I never thought that this would create a fire storm. All I wanted was a simple formula. And I got it. The question was NOT if it would degrade the quality of the result.

    FYI There are many top-notch CU lenses such as Minolta and Nikon -- with two elements. Most CU lenses are single element, but you are only using the center portion of the image and you will not notice any degradation.

  7. #27
    Mark Sawyer's Avatar
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    Re: Formula for adjusting Focal Length with Close-up Lenses

    Quote Originally Posted by xkaes View Post
    FYI There are many top-notch CU lenses such as Minolta and Nikon -- with two elements. Most CU lenses are single element, but you are only using the center portion of the image and you will not notice any degradation.
    The doublets will be achromatic, getting rid of the color fringing from chromatic aberration, but the other aberrations will remain. You'll also have a smaller image circle with any close-up lens, but even a bit more so with the thicker doublets.
    "I love my Verito lens, but I always have to sharpen everything in Photoshop..."

  8. #28

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    Re: Formula for adjusting Focal Length with Close-up Lenses

    Back to the original thread. Any one have a formula?

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