Extension Tube + 400mm lens question

Discussion in '35mm Cameras' started by Eric Miller, Sep 1, 2004.

  1. Eric Miller

    Eric Miller Guest

    Okay, got the Canon 400 5.6L in today. Though its primary use will be
    mounted on my 10D for bird shots at distance, if I wanted to take
    hummingbird photos from about 10 feet, what size extension tube would I
    need? Is there a formula for figuring this out? The minimum focus distance
    on this lens is 3.5 meters. Additionally, I see that Kenko makes a set of
    extension tubes 12mm, 20mm and 36mm. Should I just get a set or will just
    one tube satisfy my need/want? Additionally, am I correct in thinking that
    the magnification from one of these tubes will be the same as just adding
    the length of the tube to the focal length of the lens?


    Eric Miller
    millereric at bellsouth dot net

    Convictions are more dangerous enemies of truth than lies. - Freidrich
    Eric Miller, Sep 1, 2004
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  2. Eric Miller

    Dave Guest

    I have a Canon 12.5 and a 25 mm extension tube. Both work fine, both are
    pretty cheap. I use them only rarely, but your use may vary depending on

    I am not sure if I understand the second question. I get improved short
    focus, but the extra magnification is mostly just secondary to being able to
    get closer to the subject. There are no lenses in those extension tubes,
    just air and electrical connections for the motor/camera. When I used them
    for stacking lenses, they were just added to allow the lenses to fit

    Dave, Sep 1, 2004
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  3. Eric Miller

    Colin D Guest

    Ok, basic formula: 1/u + 1/v = 1/f, where u=object distance from lens,
    v=image distance from lens, f=focal length of lens.

    Using a rearrangement of this formula, at the closest focusing distance
    (cfd) of 3.5 meters, the effective lens extension is 51mm.

    Adding a 48mm extension tube (12mm + 36mm), the effective lens extension
    is 48mm with the lens at infinity, and 99mm when the lens is at cfd.

    Referring to the formula again, object distance with 48mm extension is
    3.733 meters, and 2.016 meters with 99mm extension.

    So, focusing range with a 48mm tube is from 3.7 meters to 2 meters more
    or less.

    Colin D.
    Colin D, Sep 1, 2004
  4. --

    I see that Kenko makes a set of
    I really like the Kenko set of tubes. use them quite a bit. I'm to the point
    now that I have to consider a macro lens because of the time it take to
    figure out the right extension for a project. I would get the Kenko set over
    individual Canon tubes. You have no glass to worry about

    Ray Creveling
    Ray Creveling, Sep 2, 2004
  5. Eric Miller

    Roy G. Biv Guest

    If my calculations are correct, a 25mm tube will allow the 400 to focus from
    8 to 22 feet. Someone posted a formula for the math below. But remember
    that internal focus lenses shorten the focal length at closer distances. So
    focal length plus extension at close up is always equal to focal length at
    infinity. I suggest that you forget the off brand tubes and buy the Canon. I
    haven't checked all the different brands but as far as I can tell the Canon
    will attach to the 400 much more securely - you can check this yourself in
    the store.
    Sorry due to spam I shall not provide a valid e-mail address.
    Please reply to group with questions or comments.
    Roy G. Biv, Sep 2, 2004
  6. Eric Miller

    Colin D Guest

    OK, an update on my previous.

    I belatedly realized that the 400mm probably uses internal focusing, so
    instead of racking the lens out by 51 mm from the film plane, the focal
    length of the lens is shortened sufficiently to focus on 3.5 meters.
    The question becomes, what reduction in focal length will cause a 400mm
    lens to focus at 3.5 meters?

    Since the object distance = 3500mm and the image distance remains at
    400mm, then 1/f = 1/u +1/v, or 1/3500 + 1/400 = 1/358.9, so the
    effective focal length shortens to 359mm approx.

    If we add 48mm of tube in now, the image distance becomes 448mm, so now
    1/u = 1/f - 1/v, or 1/u = 1/359 - 1/448, = 1/1807.1

    Therefore, object distance at cfd is 1.8 meters approximately, so with
    an internal focusing lens, the range is 3.7 meters to 1.8 meters

    Colin D.
    Colin D, Sep 2, 2004
  7. What exactly does this mean?
    David Littlewood, Sep 2, 2004
  8. Eric Miller

    Roy G. Biv Guest

    If I remember the math correctly, (it's been a while since I worked this
    out) at the near focus distance (about 11.5 feet) the 400mm canon lens has a
    focal length of about 360 mm. So in effect you actually have a 360mm lens
    with a 40mm extention tube. Hope this helps.
    Sorry due to spam I shall not provide a valid e-mail address.
    Please reply to group with questions or comments.
    Roy G. Biv, Sep 3, 2004
  9. Well, I think I understand what you are saying. You seem to be asserting
    that the focal length of a lens with internal focussing is always equal
    to focal length at infinity minus the amount of extension form the
    helical focussing mechanism. This implies that the position of the rear
    nodal plane (the point from which image distance is measured) is
    unchanged relative to the position of the lens elements. However, I
    believe this is unlikely to be true in most cases.

    It also assumes the lens has internal focussing only, i.e. no helical
    focus mechanism to bodily move the lens elements away from the body.
    There are lenses of this type, but not all are. If there is a helical
    focus system, the position becomes more complex.

    In all, the rule-of-thumb you put forward may work when it works, but
    could not be taken as being applicable in all cases.

    David Littlewood, Sep 3, 2004
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