Weston Film Speed Formula

Discussion in 'Exposure Discussion' started by zemzem, Sep 10, 2006.

  1. zemzem

    zemzem Member

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    I ran across this film speed (value) for a Weston Master lll:

    ISO / 5 x 8 = Weston Setting. Anyone heard of this? I have a working Master lll and I'd love to use it. Any other ideas for matching meter to modern ISO? Thanks
     
  2. Nick Zentena

    Nick Zentena Member

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    That sounds wrong to me. It's either 100 ISO equals 80 Weston or 100 Weston equals 80 ISO. The first one sounds right.

    Take it outside and try a sunny 16 test.

    Or just do a film speed test.
     
  3. zemzem

    zemzem Member

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    I've also heard the 100 ISO equals 80. Thanks, I'll give it a try.
     
  4. DBP

    DBP Member

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    One of my older books around here has a table with Weston and ASA. Weston is 0.8xASA.
     
  5. zemzem

    zemzem Member

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    Thanks. It's looking like 100 = 80 is the right formula. Thanks for the math. No I just need a day off to shoot!
     
  6. BrianShaw

    BrianShaw Member

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    I'm sure you guys have the arithmetic worked out, but my Master III's (all of them) has ASA in addition to Weston numbers. I know Master II and earlier are in Weston but never saw a III like that.

    p.s. No matter what, it's a GREAT METER!
     
  7. Jim Noel

    Jim Noel Member

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    Here is the list.
    Jim

    Weston ASA
    2 2.5
    2.5 3
    3 4
    4 5
    5 6
    6 8
    8 10
    10 12
    12 16
    16 20
    20 25
    24 32
    32 40
    40 50
    50 64
    64 80
    80 100
    100 125
    125 160
    160 200
    200 250
    250 320
    320 400
    400 500
    500 640
    640 800
    800 1000
    1000 1280
    1280 1600
     
  8. Nick Zentena

    Nick Zentena Member

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    I thought the III was still Weston and the IV was ASA?
     
  9. BrianShaw

    BrianShaw Member

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    No, Weston III meter has ASA.

    EDIT: Ah... I need to correct myself... my Weston III has ASA. Not all do. According to http://www.westonmeter.org.uk/ there was a European version with Weston Numbers and an American version with ASA. Mine (all of them) are the American version.
     
    Last edited by a moderator: Sep 12, 2006
  10. Roger Hicks

    Roger Hicks Member

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    Add 1/3 stop to Weston to get ASA.

    I'm not sure that the conversion is always accurate because I don't know the equations for Weston speeds, i.e. the criteria might be different, but it seems likely they used the 1940 Kodak criteria and the speeds are therefore strictly interconvertible, unlike (for example) old DIN, Scheiner or H&D.

    Cheers,

    Roger
     
  11. bob01721

    bob01721 Member

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    That's how I use my Weston II. Much easier than trying to remember formulae and charts which are only restatements of "+ 1/3 stop" anyway.
     
  12. zemzem

    zemzem Member

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    thanks for the numbers