Light Meter Conversion

Discussion in 'Film Discussion and Q & A' started by saltface, Nov 20, 2008.

  1. saltface

    saltface TPF Noob!

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    I've come across this old 35mm film camera that has a light meter in the body (but not through the lens). Since the meter is set for 50mm, I've had to do some guesswork with other focal lengths. I've found a formula of:

    New_F-Stop = Old_F-Stop x ((50mm ± Other_Lens_Length)/50mm)

    but this really limits what I can use. Does anybody know of software or a better formula? Thanks.
     
  2. Helen B

    Helen B TPF Noob!

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    Where did you find that formula? There should be no need to change the f-number when you change lenses. That's the whole idea of using the f-number (a ratio of focal length/aperture diameter, in simple terms) instead of the actual aperture diameter.

    Best,
    Helen
     
  3. saltface

    saltface TPF Noob!

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    I guess that didn't occur to me.
    It was on a paper that came with some extension tubes. (I see how it would definitely apply there).
    I guess the meter is just off, then. Everything is underexposed.
     
  4. Helen B

    Helen B TPF Noob!

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    How are you metering? What sort of a meter/camera is it? With external or handheld meters it is fairly important to ensure that you aren't metering too much from the sky, for example.

    Best,
    Helen
     
  5. saltface

    saltface TPF Noob!

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    It's a Mamiya/Sekor with no model number.
    I've just been doing averaging. I'm not sure of the angle it reads. The sensor is a little blue rectangle above the timer.
     
  6. compur

    compur No longer a newbie, moving up!

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    It's not unusual for the meters on old cameras to be off.

    Test it against a camera/meter with known accuracy and compensate
    accordingly.

    You can compensate by adjusting the camera's ASA dial.

    Or, you can put a piece of wratten filter of appropriate density over the
    non-TTL meter window to compensate for underexposure (or a filter on
    the lens to compensate for overexposure).
     

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