Sq-a and setting iso

Discussion in 'Medium Format' started by stephenfarrell, Jun 7, 2009.

  1. I'm a little ashamed to ask this question. But i have a Brinca Sq-a loaded with Ilford fp4 @ 125iso.
    The Sq-a 120mm back has 1/3 increment markings between the 100 and 200 iso speed settings. Should a 125 iso film as i'm using just be set to the first increment past 100?
    Sry about this terrible question btw!
  2. Hi Stephen
    Yes, indeed. 125ASA/ISO is 1/3 stop more than 100ASA/ISO, so one notch up does it.
  3. Thank you very much Ian.
  4. The "easy" way to remember what 1/3rd stop shutter speed or ISO increments are, is that the multiplier is the cube root of 2. Windows calculator tells me this is 1.2599, or 1.26 near enough.
    So the sequence runs:
    50 ISO x 1.26 = 63 (~64 ISO)
    64 x 1.26 = 80.64 (~80 ISO)
    80 x 1.26 = 100.8 (~100 ISO)
    100 x 1.26 = 126 (~125 ISO)
    126 x 1.26 = 159 (~160 ISO)
    160 x 1.26 = 200
    200 x 1.26 = 252 (~250 ISO)
    250 x 1.26 = 315 (~320 ISO)
    etc. And you just need to double these figures to carry on the sequence from here.
  5. Great Rodeo Joe. That will come in really useful.
    Thanks a lot.
  6. The dial on the back is just a reminder, it doesn't do anything to the camera itself. But I guess you already knew this.
  7. Didn't realise that but thanks for your contribution.
  8. The ASA/ISO film speed dial on the film back can automatically couple to a finder which has a built in exposure meter. ...such as a metered prism finder or metered magnifying hood. With a flip up hood, the dial is simply used as a reminder of what speed is in the film holder. This is useful if you have more than one back, and have different film speeds in them.
  9. Cheers Jack. I was wondering how this decorative element fitted in with a metered prism. Thanks for you reply.

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