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Light measuring tips and techniques

Discussion in 'Racer Physics and Technical' started by Mr Whippy, Aug 20, 2010.

  1. Mr Whippy

    Mr Whippy

    I downloaded an app for my iPhone (free) called "Light Meter"

    It will expose like a point meter where you tap the screen, and then give camera settings it would use to meet that exposure.

    Using the following calculation which makes some rough assumptions, you can calculate Lux from the values given.

    lux = 50 x f.squared / shutter time (s) * ISO

    The great thing about this application is that you can lock the ISO, shutter speed and f stop to anything you like.

    Another great thing is that if you lock f stop and shutter speed, then the ISO will scale to any value, unlike the f stop and shutter speeds which are locked to the conventional camera scales.

    I thus locked the camera settings at F5 and 1/2000th, giving

    lux = 2,500,000 / ISO

    This allows us to use a rough table as follows:

    ISO >> lux

    10 >> 250,000
    20 >> 125,000
    25 >> 100,000
    33 >> 75,000
    50 >> 50,000
    100 >> 25,000
    125 >> 20,000
    250 >> 10,000
    500 >> 5,000
    1000 >> 2,500
    1250 >> 2,000
    2000 >> 1,250
    4000 >> 625
    5000 >> 500
    10,000 >> 250

    I've just used it here at work on as white objects as I can find.

    The dark ish overcast sky outside is ISO 430 ~ 5,800 lux

    My desk with a white piece of paper on it is ISO 2750 ~ 900 lux (a little bright, they are new lights and we often turn them off as they are too bright!)

    So far it seems fairly accurate, and as a free app on the iPhone, with an ISO that scales to any integer right up to silly big numbers, it seems really good as a rough gauge for people wanting to collect real intensity values on the move!

    Also great because once you have grabbed the ISO/lux, you can then take an image of the same scene too, great for skies or sunsets or whatever you are capturing!

    Feel free to post values you have found, or other techniques on here, to make it a useful resource for anyone wanting easy ways to get realistic values for their environments.

  2. Mr Whippy

    Mr Whippy

    To add more to this thread.

    I have been playing with some fixed output lights using my iPhone and my digital SLR (D70s Nikon with 1.8 50mm portrait lens)

    So far I seem to be getting around 25-30% higher lux values with the Nikon.

    I am not sure if this is to do with the metering accuracy, the grey point the Nikon may be exposing to hit may be different, or it may well be that the iPhone doesn't have a good point for it's metering, so the Nikon is giving more accurate results for the points I choose, but perhaps the iPhone is giving a good figure too, but for a wider area.

    I'll have to test more here!

    If anyone else has an SLR and iPhone with camera, it'd be interesting to see results.

    I'll try post more soon on this. I'm very tempted to buy a light meter too, just so I can see if there is a fixed error difference for these devices so we can just correct for it in the calculation itself!


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