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Thread: Extension Tube...

  1. #1
    rpcrowe's Avatar
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    Richard

    Extension Tube...

    I have seen charts listing the reduced MFD and increased image to subject ratio using extension tubes with various focal length lenses but, I cannot seem to find one on the Internet. Can anyone supply a link to such a chart.

    I have a fairly complete selection of photo gear including a Tamron 90mm f/2.8 Macro and a Canon 300mm f/4L IS lens. I am mentioning these two lenses because of the Macro capability of the 90mm f/2.8 and the very decent close-up capability of the 300L. The close-up capability of the 300mm lens is increased when I add my Canon 1.4x TC which increases the focal length to 420mm @ f/5.6. My 7-200mm f/4L IS lens really doesn't have a great close-up capability at all.

    I like to see the increased magnification I will get using the 1.4x TC on my macro lens. I cannot however add my Canon TC to my 90mm macro lens without using an extension tube which will provide space so the TC and lens elements can fit.

    A extension tube will also provide somewhat closer focus for my 300mm lens and will also allow closer focusing with my 400mm f/5.6L which has a fairly long MFD and my 70-200mm f/4L IS. It would be nice just to include an extension tube in my travel kit when I don't want to carry the macro lens...

    I did not want to spend the money on a complete set of tubes because I really don't think I will use them that often. However, I found a single Kenko 20mm extension tube on eBay for quite a reasonable price. I think that it might be fun to play with.
    Last edited by rpcrowe; 16th July 2012 at 02:39 PM.

  2. #2
    DanK's Avatar
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    Re: Extension Tube...

    Richard,

    I have lost the equation, which came with my Kenko tubes. However, I recall two things: for a precise answer, I think you need the native magnification factor of the lens, and for macro lenses at MFD, the conventional equation understates the additional magnification. However, I think you can get a VERY rough idea by considering the ratio of extension to FL. If I recall correctly, the approximation is (extension+FL)/FL times the original magnification. At MFD with a macro lens, you get more. For the latter, if you google extension tubes and Lord V, you should find some tests that Brian Valentine posted. Som, the bottom line is that 20mm would give you a modest boost at MFD with your macro lens but not a whole lot with a long lens.

    I'll post two examples. One is my standard example of a 100mm macro and a 36 mm tube. The second is my 70-200, racked out to 200, with the full 68mm of the Kenko set, at MFD. If I recall correctly, neither is highly cropped.

    Dan

    Extension Tube...

    Extension Tube...

  3. #3
    ktuli's Avatar
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    Bill S

    Re: Extension Tube...

    I don't know if this will be of assistance...

    http://eosdoc.com/jlcalc/

    - Bill

  4. #4
    DanK's Avatar
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    Re: Extension Tube...

    Richard,

    I realized after posting that I had bookmarked the posting by "Lord V" that I mentioned. This is it.

    BTW, his macro work is absolutely outstanding. You can find some of his work and some tutorials by him here.

    Dan

  5. #5

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    Re: Extension Tube...

    The simplest thing to play with is a magnfiying glass big enough to cover the front of your largest lens and use it to overcome the limited focusing of most long lense that are not designed as macro beasties. When finished playing consider getting a quality CU lens of moderate power [ say 2 dioptre ] that is coated and mult-ielement. But for really close you need a bellows and lens with aperture control. The cheap extension tube is probably just that and needs a manual lens to work with it ... so buyer beware is the thing to be. The longer the lens the less effect an extension tube has on it. I have about 270mm of possible extension and with my 135mm it keeps me away from the subject nicely.
    Extension Tube...
    In addition to the bellows I also have two M42 tube sets and an EOS set picked up over the years .

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