DWU2 Loc: Phoenix Arizona area

I looked at several photography LED lights on Amazon. There was no mention of guide numbers. Wouldn't the guide number concept still be applicable, both for measuring the lighting power of the device and for exposure calculations?

BebuLamar

No! The flash GN is a product of both the flash intensity and duration.

Searcher Loc: Kent, England

Guide numbers may be calculated by multiplying the f stop x distance. (shutter speed and duration are not part of the calculation).

Some LED lights are sold with GN as part of the specification (B&H quote GN on some of their lists) http://www.bhphotovideo.com/c/buy/Macro-Ringlights/ci/649/N/4168864821

Most LED lamp manufacturers do not quote GN for the simple reason they do not remain constant on battery power. I have four 128 x bulb arrays, all made by the same manufacturer, all the same model, but all with differing power output. These are battery powered without close tolerance voltage regulators and as the voltage from the battery packs go down, so does the light output and ipso facto, so does the guide number.

When the batteries are fresh, my LED lamps have a GN of about 10 (using feet as the distance measure and at ISO 200) and after an hour of continuous use this GN diminishes to about 5 or 6. I would add that these units are of the cheap and cheerful variety as opposed to those higher quality products used in professional studios.

A really good (understandable) article which explains GN and includes an exposure calculator can be found at http://www.scantips.com/lights/flashbasics1c.html

Edit: Two more reasons for not quoting GN on LED panels:
The panels are dimmable (everchanging GN)
The colour temperature on some units is adjustable and therefore the GN is not constant.

michealj Loc: West Virginia

Searcher, your the Man!! :-) :thumbup: :thumbup:

zigipha Loc: north nj

The same argument could be said about real flashes - if you tilt the head or zoom the flash, or adjust the manual power setting (if applicable), then the gn would change. but they are tested with known settings and no reason the led's cant be tested the same (full power, not color shift). Battery charge is a bit different, but could quote "full charge". At least it gives something to work off of.