Extender tubes and exposure

Discussion in 'Phototechnical questions' started by derekphl, Oct 9, 2003.

  1. derekphl

    derekphl New Member

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    Greetings all, apologies if this has already been covered, but I couldn't find anything specifically. I have two extender tubes (55 & 21), and was wondering what roughly the exposure compensations should be.

    Please bear in mind that I am not so much of a "by the book" photographer, and I also brackett 4 times. I am using an 80mm and 60mm lenses and I'm really after a rough estimate depending on the different variations. I have lready done a number of shots with them and the results have been great, but it's using them both together that's the key.

    And yes, I will one day get round to buying a close-up calculator!! :eek:p
     
  2. marsu

    marsu New Member

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    I am sure purists will correct me, and maybe dear Peter C. Patton will have something to say about it too, but I use 1/2 stop with the 21 and 1 1/2 stop with the 55. Or 2 stops for both combined.

    Manu
     
  3. toona

    toona New Member

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    >Derek

    Exposure compensation is given on the H'blad website for their latest lenses: You must be logged in to see this link

    Doesn't have the 21 anymore, but it can be calculated from the figures there. Remember that racking out the focus on the lens also adds extension (there is 9mm on an CF 80mm lens, from memory, but it might be more on a CFE). This is why there are two figures on the table published, for min focus (infinity) and max close up focus. A lens focused at infinity has no extension.

    Actually there is a neat relationship if you can find it (don't have a text book here, but empirically it is extension divided by the focal length times 2). A total of 80 mm extension for the 80 mm lens, matching the lens focal length, will give life size reproduction at the cost of two stops (your 21and 55 tubes, plus 4 mm of lens extension). 40mm extension gives 1 stop etc. 20 mm (close enough to 21) gives 0.5 stops. 55 (now changed to 56 with the newer tube) is 1.3 stops according to the table, but it calculates at 1.375 for the pedants.

    Piggybacking tubes should just mean adding compensation together. Remember to add the compensation for the lens if relevant.

    Hasselblad do not recommend wide angle lenses for extension. This presumably applies to the 60 mm, but certainly to the 50 and 40. Presumably this is why there are no figures for the 60 on the table published.

    The exposure calculator is good for a lot of things, but only gives compensation in 1/2 stops. The table mentioned above is more precise.

    I hope this makes sense.

    Nick

    PS Perhaps Peter can comment as well (in fact he will, as I am sure that it is an automated reply problem. A lesson for all...do not use an automated 'I am out of the office' message if on a list!
     
  4. qnu

    qnu Banned

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    The set of simple formulae to use is:

    1) Magnification = Extension / Focal length

    2) Aperture factor = 1 / (Magnification + 1)

    3) Shutterspeed factor = (Magnification + 1)^2

    4) Correction in stops = log(Shutterspeed factor) / log(2)

    Extension includes the built-in extension of the lens barrel, i.e. the one you add when turning the focussing ring from inifinity setting. With the lens set to infinity, it is the length of the tube added.
    Maximum built-in extension is not small enough to ignore. So don't ;-)

    Things get a bit more complicated (and more precise) when considering that most (if not all) lenses are not symmetrical. A tele lens, for instance, will lose light faster with than the above formulae would suggest.
    To take asymmetry into account, we need to know the sizes of the entrance and exit pupils of the lens. The ratio of these sizes must be used as a factor in the above formula 1). This ratio (let's call it the asymmetry factor, AF) is

    5)AF = diameter exit pupil / diameter entrance pupil.

    1) should be transformed to 1a)

    1a) Magnification = Extension / (Focal length * AF)

    But *only* to calculate exposure compensation. Not when determining image size!

    The sizes of entry and exit pupils can be found in the Zeiss Lens Data Sheets, available on-line on Zeiss's website (You must be logged in to see this link. Click your way to photography, select Hasselblad and the appropriate lens line, pick the lens you're interested in, and click to see the technical information (PDF-format)).

    These simple formulae don't work properly with lenses that have internal focussing.

    Oh, and administrator: could you please do something about all these empty posts? Like switching Peter C. Patton's email notification of new posts off? A "very active member" indeed, but he doesn't have much to say, has he?
     

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