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Thread: Optics puzzle

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    Default Optics puzzle

    Following from my doubts between choosing a long macro-lens or a shorter one with extender (http://www.birdphotographers.net/for...2.8-VR-with-TC), someone I know who owns the Nikkor 105mm micro and the TC20E-II performed a test to determine the working distance of this combination at 1:1.

    Working distance of the lens+TC combination at 2:1 is 15 cm, equal to the working distance of the lens only at 1:1. Following from the formula of the magnification ratio (which is the image distance divided by the working distance), the working distance of the 105mm+TC combination at 1:1 should be twice the working distance at 2:1, i.e. 2*15 cm=30cm. The test, however, showed that the working distance at 1:1 was only 21 cm.

    So my question now is: did we mess up the test or do the general calculations for the magnification ratio do not hold for a lens+TC combination?

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    Quote Originally Posted by Jerry van Dijk View Post
    So my question now is: did we mess up the test or do the general calculations for the magnification ratio do not hold for a lens+TC combination?
    Jerry,

    The answer is neither. You used a simple lens formula. The macro lens is a compound lens to start and you made it a more complex compound lens when adding the TC. The TC adds magnification on the sensor side, so does not change anything on the subject side (meaning the distance).

    But note, image quality can be affected by diffraction at high magnifications adding TCs on macro lenses. With the small pixels in many of today's digital cameras, it doesn't take much to hit the diffraction limits. For example, the diffraction spot diameter of an f/5.6 lens is just over 7 microns for green light, and many DSLRs have pixels smaller than this.

    Roger

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    Can't one say that a compound lens can be reduced to single element? For if you do ray tracing, you are only interested in how the incident light ray is deviated from front element to that of the rear element. (aka a transfer function). I think would make sense from even the transformation matrices standpoint in that you are in effect multiplying them together to get the equivalent single matrix. Or from a more practical point of view this is useful for establishing the real "focal length" of say a reflex/mirror lens.

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    Quote Originally Posted by Chris Korman View Post
    Can't one say that a compound lens can be reduced to single element? For if you do ray tracing, you are only interested in how the incident light ray is deviated from front element to that of the rear element. (aka a transfer function). I think would make sense from even the transformation matrices standpoint in that you are in effect multiplying them together to get the equivalent single matrix. Or from a more practical point of view this is useful for establishing the real "focal length" of say a reflex/mirror lens.
    Chris,

    For a graphical ray tracing exercise, one needs to use the principle planes, which are generally located inside the lens for compound lenses, and not the front and rear elements.

    You can't replace a compound lens with a simple lens because the distances will not be the same. In a telephoto lens, for example, the principle plane to focal plane distance is almost always significantly smaller than the focal length. Add a 2x TC and you don't move the focal plane position much, for example.

    In your macro lens setup, a couple of things changed when you added the TC. 1) magnification increased (and thus the focal length increased). 2) the sensor plane was moved further from the lens.

    Roger

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    Hi Roger,

    Thank you for your explanations. The risks for IQ of adding a TC make a lot of sense.
    Like Chris, I assumed that the compound lens could be considered as a single element for calculations, but suspected that this would be too much of a simplification.
    The changes in focal length and the change in distance of the sensor plane can be included in the various calculations, but don't really play a role in the test performed, since they are constant for the calculations at 2:1 and 1:1.

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    The discrepancy in working distance can be explained if this is an internal focusing lens. As the lens focuses to closer distances it changes to a wider focal length.

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