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Converting Lux to Guide no's.

Discussion in 'Help Team' started by Footloose, Mar 24, 2012.

  1. Footloose

    Footloose Well-Known Member

    I'm seriously thinking of buying a LED light panel, because I wish to try the video capabilities of a camera I have. The problem is I've been trying to track down a simple chart that will allow me to compare the continuous light output from these panels rated in Lux, to that of the Guide nos used on flashguns. I've tried searching with 'Lux to Guide no converters' but to date, all I've found is a lot of rambling on about all the different standards used for measuring light output, but have yet to encounter a Guide no to Lux comparison chart. Has anyone else found a chart which does this job?
  2. daft_biker

    daft_biker Action Man!

    Pick a number fairly close to zero....it'll probably be close enough!

    I've not seen one that's anywhere near as bright as a flashgun and one of my flashguns is only GN14. I've not seen that many though.....maybe the high end ones are worth comparing with the kind of output you can get out a flashgun?
  3. P_Stoddart

    P_Stoddart Well-Known Member

  4. beejaybee

    beejaybee Marvin

    Apples and oranges ... the point about light panels - or any other form of continuous illumination - is that, if you need more light, you can just leave the shutter open longer.

    The power output during the active phase of an electronic flash gun is in the kilowatt or even megawatt range. One thing is for sure, if you needed illumination as strong as that with light panels, you wouldn't be carrying the power source around with you.
  5. Bob Maddison

    Bob Maddison Well-Known Member

    A company called International Light Technologies have both an online converter and a downloadable version EVLux_Convert.exe which works very well to do just that. It converts a measured EV (or LUX measurement at the subject) to a LUX (or EV). However, converting the brightness of any light source to and EV or LUX value is quite different. Everuything depends on the area of the light source and how narrow (or wide) it beam is, as well as the light to subject distance. A single tiny LED can be very bright but cast a negligible light on the subject.
  6. Footloose

    Footloose Well-Known Member

    Thanks for the links ... I'd already seen the Photo Net page, and whilst it might show a lot of info (and do my head in at the same time!) it still didn't end up answering the original question! ... Unless of course, I'm far to thick to see it!
  7. GeoffR

    GeoffR Well-Known Member

    If we are being pedantic, the output of a flash is measured in "Joules" or "Watt Seconds" though rarely quoted in those units because a Guide Number is more useful. The output of an LED panel is continuous and usually measured in Watts so leave a 10W panel on for 1 minute and you get 600 Joules, the average flash duration is around 1/10,000 second so even if the output were 600 Joules it's brightness equates to a continuous light source of much greater power. As beejaybee points out, a power supply for a 6MJ continuous light source isn't going to be portable.

    For an average of 12 hours each day you have access to a considerably more powerful light source, I tend to prefer using that where ever possible. It isn't portable either.
  8. john_g

    john_g Well-Known Member

    I looked into this a while back and put together a spreadsheet to show what exposure would be needed for different set-ups.

    If you take a Litepanels Micro ( http://www.litepanels.com/language/pages/micro.php ), which quotes a value of 70 Lux at six feet (1.8 metres), I calculate that, for an aperture of f4 at ISO 200, the exposure time at this distance would need to be approximately 0.38 seconds. This assumes a viewing angle of 30 degrees i.e. the illumination will have halved (-1 stop) by the time you are 30 degrees off axis.

    If you are considering an alternative product, post a link to it's specification and I'll see whether I can plug the relevant values into my spreadsheet.

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