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  1. #1
    altair's Avatar
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    M42 extension tube questions

    Hi all.

    Just got myself a set of Pentacon M42 extension tubes. There are 3 tubes, measuring about 10mm, 17mm and 31mm. I plan to use it with one of my M42 bodies mounted on a Fujinon 50/1.8 lens. The tubes didn't come with a manual or chart, and this is my 1st time trying out extension tubes, so I have some questions:

    a) How do I know how much exposure compensation I need to apply? Does the compensation required differ with which tubes I use, e.g when I only use the 10mm tube and when I use all 3 tubes together? I'm guessing it does..but by how much?

    b) The tubes I have doesn't have an M42 'pin', in which case, how does it stop down the lens then? I tried it just now using the Fujinon lens, and beyond f5.6 or so, the lens won't stop down anymore, no matter what fstop I set it to. When used wide open at f1.8, it functions fine. Will I have to use it wide open all the time, then? I noticed that there are other types of M42 tubes that have a regular M42 pin.

    Thanks in advance, would appreciate any feedback/help.

    -Dani-

  2. #2
    Rol_Lei Nut's Avatar
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    Hello Dani,

    1) Many M42 lenses have a switch to change from manual to auto diaphragm. If yours has one, then set it to manual for the diaphragm to close properly.

    2) Using extension tubes with a purely manual diaphragm really is a PITA! It's worth getting some with the "pin".

    3) The longer the tube, the more the exposure has to be compensated. Others will give you the exact formulas for that, though if your camera has a through the lens meter, you don't need to compensate (the meter "sees" the actual light coming through).
    M6, SL, SL2, R5, P6x7, SL3003, SL35-E, F, F2, FM, FE-2, Varex IIa

  3. #3
    Tony-S's Avatar
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    With your 50mm lens set at infinity, the number of stops of increased exposure should be about:

    10mm = 0.5 to 1
    17mm = 1.5
    31mm = 2 to 2.5
    41mm = 2.5
    48mm = 3
    58mm = 3.5

    You'll get better latitude if you use negative versus slide film.

  4. #4

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    10mm, 17mm, 31mm Extension Tube Set with 50mm Lens at Infinity Focus

    If the camera has through the lens metering, then the meter will give the correct reading through the tubes and lens combination. If you’re using a hand held meter, then a calculation is needed.

    Rather than bore you with the calculations,

    with 50mm lens at the “infinity” setting, you get the following results in the form:

    Tube or combination of tubes, time multiplication factor, number of additional f-stops required.

    10mm, 1.44x, 0.53f

    17mm, 1.80x, 0.84f

    31mm, 2.62x, 1.39f

    (10mm + 17mm), 2.37x, 1.24f

    (10mm + 31mm), 3.31x, 1.73f

    (17mm + 31mm), 3.84x, 1.94f

    (10mm + 17mm + 31mm), 4.67x, 2.22f

    If you focus the lens closer, the exposure requirements increase slightly, but not by much for a 50/1.8 normal lens with its relatively short maximum extension.

  5. #5
    Tony-S's Avatar
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    Hmm. Ian's numbers are a bit different from what I can gather from my Pentax bellows manual.

  6. #6

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    The difference in f-stops is

    Δf = 2*ln(E/f)/ln2

    where E = f + tube length and f = focal length of the lens.

    The time factor = 2^Δf.

    Example 1: Tube length = 50mm and we’re using a 50mm lens at infinity.

    Then E = 50mm + 50mm = 100mm.

    Δf = 2*ln(100mm/50mm)/ln2 = 2 stops

    Time factor = 2^2 = 4.


    Example 2: Tube length = 0 (no tube used).

    Then E =50mm + 0 = 50mm.

    Δf = 2*ln(50mm/50mm)/ln2 = 0 stops—which is correct. No compensation is needed.

    Time factor = 2^0 = 1. The metered time is multiplied by 1, i.e. no time change.

  7. #7
    Tony-S's Avatar
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    Ian, I'm sure the math is right but I'm curious why the Pentax manual has different numbers. Does the distance from the flange to the film plane make a difference in the calculation?

  8. #8

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    No. The geometric center of a (symmetric) lens focused at infinity is the distance f from the film plane measured on the central axis of the lens. When you add an extension tube of thickness t, you’re simply moving the lens forward by t.

    This is precisely the same as displacing a lens forward by extending the bellows. The exposure calculations are the same for an SLR with extension tubes or a view camera with bellows extended.

    I wonder if the manual you referred to presumes a REVERSED lens. Many SLR makers recommend reversing the lens to optimize image quality at very close focusing distances because of the retrofocus design made necessary to provide the clearance for the swinging reflex mirror.

    That could position the center of the lens at a different point and result in a different calculated result.

  9. #9
    Tony-S's Avatar
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    It doesn't say anything about reversing the lens. It does refer to the 50mm f/1.4 Super Takumar. It has two distances listed for each, the bellows length and the film-to-subject distance. For instance, with a bellows length of 50 mm the FTSD is 194 mm, a magnification of 1 and exposure factor of 3 stops.

  10. #10
    altair's Avatar
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    Thanks for the replies everyone!

    Rollei_Nut: Right, I forgot all about that A/M switch on the lens! The Fujinon lens I used doesn't have one, but I have a Takumar that does. I'll use that instead next time. The camera I used does have TTL meter, but I prefer using a handheld one for incident readings. Thanks!

    Tony & Ian: Wow, thanks very much for providing the compensation needed. Those maths equations and formulas really boggled my mind! Haha. Anyway, I did a quick test just now with a 12-frame roll of film. Used both the 10mm & 17mm tubes together, mostly set at infinity. Shot quickly through the roll, developed it and from the negatives, I can see that the exposure compensation that the 2 of you provided does seem to be correct.

    One other question: Do the compensation factors given change with the format & type of lens used? I.e I have another set of extension tubes coming for my Mamiya 645. Mean to use it with the 80/2.8. Do the factors given above still apply?

    Thanks again!

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