Extension tubes - how much effect

Discussion in 'Digital SLR' started by Musty, May 25, 2005.

  1. Musty

    Musty Guest

    I have been looking at these extension tubes, to see if maybe I can use my
    70-200 for macro shots (and thus avoid buying a macro prime). The 70-200
    minimum focusing distance is 1.4m (not good enough for macro of bugs). If I
    used the following extension tube, how close will I be able to focus?

    Canon Extension Tube EF 25 II - $129 (also supports EF-S, not that I have
    any)

    Seems like a cheap way to add macro capability to my lovely L zoom. Do I
    lose stops? Is there any other side effect?

    Thanks
    Musty.
     
    Musty, May 25, 2005
    #1
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  2. Musty

    Stacey Guest

    I've been amazed at the quality of the results from the 2 element nikon
    diopters. They work great with a zoom in that range, no light loss and the
    zoom fuction controls magnification. Also works on any lens/system with
    rings. With a tube you lose stops and may or may not be as good as a
    diopter depending on the lens being used. Zooms normally aren't that great
    on a tube but some are. Given the nikon 2 element diopters (like the 4T or
    the 6T) are half what this tube is or less, I'd try that route.
     
    Stacey, May 25, 2005
    #2
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  3. Musty

    Crownfield Guest

    stacey, the light loss is a function of magnification.
    as the image is bigger, the light loss is greater.
    its hard for me to understand how a closeup lens
    can magnify without light loss.
     
    Crownfield, May 25, 2005
    #3
  4. You need extension equal to the focal length to get 1:1 (life-size
    images on the sensor). You may not *need* to get all the way to 1:1,
    but that's a useful rule to remember. So you'd need at least 70mm of
    extension, or nearly 3 times that tube.
     
    David Dyer-Bennet, May 25, 2005
    #4
  5. Musty

    Paul Furman Guest

    That's funny because I use a Canon Diopter on my 70-200 Nikon <g>.
    Supposedly the more expensive two-element diopters are better than the
    typical cheapies. I got the Canon because it's the only one that fits
    the large 77mm thread but if you only use it zoomed, a smaller step-down
    ring situation ought to work fine.



    The diopters do not lose light, the extension tubes do. The diopters do
    not extend the focal length, just the closest focussable distance. The
    diopters add more glass though. And I think the problem with this
    approach is too many pieces of glass in a zoom then adding a couple more
    with the diopter. The advantage is using the VR (IS) and a good lens.
    But the "real" macro lenses are all primes and the extension tubes don't
    add any glass so give the best results.

    Also note that metering doesn't work with a diopter but probably not
    with a tube either.
     
    Paul Furman, May 25, 2005
    #5
  6. Musty

    Stacey Guest

    Not always..

    Another photography myth (#11?).

    http://medfmt.8k.com/mf/diopter.html#beat


    Why wouldn't it? And it should also work with a tube as both are TTL
    metering.
     
    Stacey, May 25, 2005
    #6
  7. Musty

    AustinMN Guest

    Paul Furman wrote:
    This has some truth, some half-truth.
    This is total nonsense.

    Austin
     
    AustinMN, May 25, 2005
    #7
  8. Musty

    Paul Furman Guest


    OK you caught me <g>. I think it's the flash that doesn't meter with my
    setup I have to go to manual.

    I also get the impression that really close macros tend to have very
    sensitive magnification of the lighting effects. I'm pretty unclear
    about this, something I've seen more with strange concoctions of
    reversed lenses & such.
     
    Paul Furman, May 25, 2005
    #8
  9. Musty

    Paul Furman Guest


    Hmm thanks, very interesting link! Still that chart shows a diopter on a
    normal prime (not a zoom) beating extension tubes on the same prime and
    the macro prime beat them all (except for central sharpness). Ah I see,
    the macro lenses big difference is compromises to get even sharpness
    edge to center & minimize distortion. I agree that's not important for
    natural objects, only as a copy stand device.


    Yeah, see my other reply, I was confused. I think it was flash I had
    trouble with. It seems metering does not work with (most) extension
    tubes & bellows though, right?
     
    Paul Furman, May 25, 2005
    #9
  10. Musty

    Norm Dresner Guest

    A supplementary (closeup) lens actually changes the effective focal length
    of the lens. While the magnification is a factor, the change in focal
    length just compensates and the effective F-stop remains the same. It's
    magic.

    Norm
     
    Norm Dresner, May 25, 2005
    #10
  11. Musty

    Paul Furman Guest

    Heh, this has me really confused. The link above even gives a formula
    though I don't understand
    http://medfmt.8k.com/mf/diopter.html#beat
    Scroll to the next topic "Diopters to Change Focal Length of Lenses"

    According to that my 400mm setup becomes 1200mm with a +2 diopter. They
    also mention some affect on aperture that I don't get and needing to
    stop down to maintain sharpness. Can anyone explain further?
     
    Paul Furman, May 25, 2005
    #11
  12. Musty

    Paul Furman Guest


    Hmm on that same page search for "Ok, I'll try this one more time with a
    simple example." and there is yet another formula that puts a 100mm lens
    with a +1 diopter at 90.9mm thus converting f/11 to f/10. Then he does
    some calculations for the same amount of magnification using extension
    tubes and the f/11 becomes f/12.1 so that's a point for extension tubes
    being more useful even if the DOF improvement is relatively slight.
     
    Paul Furman, May 25, 2005
    #12
  13. Aperture automation doesn't work through many tubes and bellows; so
    full-aperture metering doesn't work. So you have to know how to do
    stop-down metering on your equipment, if it even supports it at all.
     
    David Dyer-Bennet, May 25, 2005
    #13
  14. Musty

    Crownfield Guest

    the closeup lens does not lose light.
    the extension tubes do not lose light.
    the MAGNIFICATION loses the light.

    look at any rep ratio chart
    showing magnification vs stops compensation.
    tubes or lenses are never mentioned.
    magnification is the controlling variable.

    on any camera that I have ever used,
    ttl metering always works perfectly with either method.

    external metering also works perfectly
    when you compensate for magnification.
     
    Crownfield, May 26, 2005
    #14
  15. Musty

    Norm Dresner Guest

    Back to the closeup lens. Each time you put one on, you reduce the
    effective focal length of the combination. For the mathematically minded
    (that's me and maybe one or two others out there reading this NG):

    Each supplementary lens is numbered as the fraction of a meter its focal
    length is.
    +1 is 1/1 = 1 meter
    +2 is 1/2 = .5 meters
    +4 is 1/4 = ,25 meters
    etc.
    When you combine lenses, the focal length of the combination goes as the sum
    of the reciprocals:
    1/Fcombination = 1/F1 + 1/F2
    So adding a +1 lens to a 100mm prime, you get 1/100mm + 1/1000mm = ...
    1/90.9090...mm
    (Actually it's not quite that simple because it also depends on the distance
    between them but this is a good approximation for screwing a closeup lens
    onto the front of a normal lens).
    Since the diameter of the diaphragm hole doesn't change, the Fstop does
    because it's the ratio of the focal length divided by the diameter of the
    hole. In this case, Fstops decrease in magnitude by 90.9090/1.1...
    BUT ... as you move the lens away from the film (increase the separation
    between the lens and its focal plane), the effective Fstop also changes. If
    you have a real macro lens, most modern cameras actually show this change as
    you focus closer. It turns out that, to a good approximation, the changing
    distance causes a change in the effective Fstop that just matches the change
    from adding the supplementary lens. I've actually done the exact
    calculations (only once!) and it's really true.

    That's why you don't need to compensate for adding a close-up lens but do
    for adding an extension tube.

    Norm
     
    Norm Dresner, May 26, 2005
    #15
  16. Musty

    Paul Furman Guest

    OK so it reduces the focal length (slightly) & the other calculation I
    was looking at is something else.

    Maybe my problem is I don't really understand what "focal length" is.
    There are two concepts I can think of:

    1. distance in mm between the film/sensor & the front element (though I
    realize it's a bit more complicated than that, something vaguely in that
    ballpark, measured in mm.

    2. Closest focusing distance, generally measured in meters, not mm. The
    same 200mm "focal length" lens can sometimes focus closer or further at
    the close end.

    Sorry if I'm just hopelessly lost beyond hope of clarification short of
    going back to college for this stuff. I am quite thoroughly lost.
     
    Paul Furman, May 26, 2005
    #16
  17. Musty

    Don Guest

    I have used Canon extension tubes on my 70 - 200 L and as someone previously
    noted, it makes little or no discernable difference to the light and the
    E-TTL 11 metering on my 20D. If you wish to see the effects of the tubes on
    focusing distances for all the Canon lenses etc you can find a pdf chart on
    various Canon home pages (obviously depending where you are). In simple
    terms you get closer focusing but loose your infinity focus. I find them
    and diopters to be a cheap solution if you don't have a macro lens but don't
    let anyone tell you that they can perform either optically or practically as
    well as a good macro. This simply isn't the case.

    regards

    Don from Down Under
     
    Don, May 26, 2005
    #17
  18. Musty

    Stacey Guest

    It really has nothing to do with that unless it's a simple lens like a
    tessar. On one of those its a measurement from the center of the lens to
    the film. With a telephoto or a retrofocus wide angle these direct
    measurements don't apply.
    Not that either.


    The main things to consider it what is the field of view and what
    magnification it provides, which is based on the focal length and the
    focusing distance. If you decrease the distance using the same focal
    length, you increase the mag as does increasing the focal length at the
    same distance.
     
    Stacey, May 26, 2005
    #18
  19. Musty

    Pete D Guest

    Some cameras will do stop down metering though so this is not a problem.
     
    Pete D, May 26, 2005
    #19
  20. Musty

    Pete D Guest

    I know that the Pentax D-SLR's do, makes it easy.
     
    Pete D, May 26, 2005
    #20
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