Does this mean my U2 is twice as bright as my L4

The U2 that I received has a somewhat greenish cast to it (to my eyes) at least when compared to my L4 which looks somewhat blue now. As an aside how do yo tell which is more white? Is there a scientific way to do this?
Anyway, I don't have a lumens or lux meter so I took my SLR and stopped it down to f22. When filling the frame with the lit L4 it read 1/200th of a second. When filling the frame with the U2 on the highest setting it read 1/400th of a second. Does this mean then that my U2 is twice as bright as my L4? It doesn't look like it from my eyes, but I'm sure many of you know better than me.
Thanks!

Re: Does this mean my U2 is twice as bright as my

A rule of thumb is that you won't really notice much difference in normal use between two lights unless one is twice as bright as the other. If you put them side-by-side you can pick out differences more precisely, but doubling the light output isn't as dramatic as it might sound. Twice as much is really just the next notch up on the scale.

I don't know much about the U2 (or enough about your test) so I can't answer your question directly, but I can say that it's quite possible from what you've said. I know I was surprised how little difference there was the first time I compared one light source with another twice as bright. The easiest way to do it is to take two identical lights and shine them together at one spot, then turn one away and see how much difference there is. Perhaps you can find out the relationship between different brightness levels on the U2 to give you another point of reference for your eyeballs.

- Russ

Re: Does this mean my U2 is twice as bright as my

It's so hard to say, but.....

As you zoom in/out to 'fill the frame' you are changing the speed of the lens itself (though the f22 setting should fix that). I'd think you want to measure using the same zoom as well as F stop.

As I recall, it will catch 4 times the light at 1/200 as it does at 1/400. Or was that between F stops???? Darn, now I have to look it up.

Daniel

Re: Does this mean my U2 is twice as bright as my

Each f-stop is a doubling of light but it may be that your camera was on the cusp from one stop to the other when comparing these two light sources. The color correction may also have some bearing. /ubbthreads/images/graemlins/icon3.gif