iPhone 4: LightMeter (app) & Theory

iPhone app: “Light Meter”:

  • Uses iPhone-4’s cameras (front or rear), displaying image with overlays reporting framerate (can specify fixed e.g. 1/60), f-stop, ISO.
  • Usage with my Sony XDCAM EX3 video camera:
    • In settings, I set Stops to Halves, as that’s what the camera uses.
    • I typically use an EX3 with a Tiffen T1 IR-blocking filter.  What’s the ISO for this arrangement?
    • What do the EX3’s ND filters do to the ISO?
      • EX3 has ND1=1/8, ND2=1/64
        • (From EX3 user manual, page 50)
      • I think ISO is linear, so if Camera is 320 ISO, they imply equivalent ISOs by simple division:
        • 1080p: Clear=>320, ND1=>40, ND2=>4.5
        • 720p: Clear=>400, ND1=>50, ND2=>6.25
        • 1080i: Clear=>640, ND1=>80, ND2=>10
      • Alternatively, for ND1 filter you can leave the app’s ISO setting as Clear (no filter) and instead adjust the app’s Correction Factor to -3 EV (though it’s maybe better reserved for simulating lighting variations e.g. due to weather, as in the Exposure Value Table further below).
        • I guess from this one off case that EV is logarithmic, since 2^-3=1/8 as per ND1.
        • That guess was later confirmed by further web research (further below), stating that EV is an “additive system”, i.e. operates in the logarithmic domain, base 2.
      • Caution: being an ISO/EV newbie, I can only hope this is is all correct!
      • Nevertheless, when I tried my naive settings they worked just fine – I was successfully able to use the iPhone Light Meter to obtain a sensible camera configuration for good exposure level and (given the ND filters) the kind of shot I want (e.g. degree of DOF).  When tested on the camera, they all worked out as expected.  Cool!
  • The Light Meter app optionally displays EV400, EV100, Lux, FootCandle.  Latter units are explained in great detail at at [johnlind…] link below.
  • The app can also “log” readings – in the form of jpg images of the screen and overlays including geographical location – to a DropBox account.  For example, when I clicked the [Log] button, a jpf file appeared on my MacBook in the folder [ /Users/davidesp/Dropbox/Photos/Pocket Light Meter].

Exposure Values & Exposure Theory:

  • http://johnlind.tripod.com/science/scienceexposure.html
    •  <<The full name for Exposure Value, or EV, is the Additive Photographic Exposure System.  Exposure Value has two equivalent definitions.  The first defines how much light will be admitted to the film by the combination of lens aperture and shutter speed.  The second defines how much exposure is required by the combination of subject luminance (e.g., how bright it is) and film speed.  Setting a combination of aperture and shutter speed on a camera with an EV that equals the EV for the subject luminance and film speed should result in a properly exposed photograph>>
    • (The article continues at length.  For example the “Additive” element reflects the fact that this system operates in the logarithmic domain. The article also distinguishes luminance from illumination, explains units such as point-source intensity in candelas, flux in lumens, light illuminating a surface in foot-candles,  light radiated from an area in foot-Lamberts, luminence in candelas per area (square foot or square metre)
    • An EV (Exposure Value) table is presented.  I guess (?) this is useful for the iPhone app, where EV can be shifted up/down by a control, to estimate what would be needed should the lighting conditions vary:
      • -1 EV:  light sand or snow
      •  0 EV:  bright or strong hazy sun (distinct, sharp shadows)
      • +1 EV:  weak hazy sun (soft shadows; distinct sun outline in clouds)
      • +2 EV:  cloudy bright (no shadows; sun creates bright area in clouds)
      • +3 EV:  heavy overcast, but not “black” (no shadows; sun location cannot be determined)
      • +3 EV:  open shade (in shadow but 60% sky not obscured)
      • +4 EV:  deep shade (in shadow with obscured sky; under forest canopy)
  • http://en.wikipedia.org/wiki/Exposure_value
    • Exposure value is a base-2 logarithmic scale
    • (This article has a more comprehensive table of EVs and weather conditions etc. than the above)


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