Have DSLR? Want Ambient Lumen Meter?

Then just consider the answer to the following questions.

If you were to manually meter the ambient light of 100 lumens to be correctly exposed at 1/100ths of a second, then, all else being equal, what speeds would provide the correct exposures for:

- 400 lumens ?
- 30 lumens ?
- 0.20 lumens ?


Granularity... 1/3rd stop. Accuracy... +-10% (IMHO).

Merry Xmas folks!

Interesting questions, reppans. As far as I can understand, your method is intended for relative comparison of flashlights, since there's no way to measure lux with a DSLR (at least none that I know of).

Will play with my Nikon P300 - not DSLR, but has plenty of manual controls and quite precise measurement, that should suffice.

Cheers

reppans said:

Then just consider the answer to the following questions.

If you were to manually meter the ambient light of 100 lumens to be correctly exposed at 1/100ths of a second, then, all else being equal, what speeds would provide the correct exposures for:

- 400 lumens ?
- 30 lumens ?
- 0.20 lumens ?


Granularity... 1/3rd stop. Accuracy... +-10% (IMHO).

Merry Xmas folks!

Click to expand...

Well ,here's me being a complete dumbass and not knowing anything about DSLR's and exposures, but how about:

400 lumens - 1/400ths of a second
30 lumens - 1/30ths of a second
0.20 - erm, 5 seconds?

Like I said, I know nothing about photography, but figured I'd throw it out there

You can't use those units.

If you had lux instead of lumens, you could do something...but, lumens alone won't help unless you know the distribution of the light on your target.

For example, I could have a floody beam that puts out 750 lumens...and a target several meters away that might have a certain brightness that requires a certain exposure.

If I substitute a light with a tighter focused beam, but only 100 lumens, that same target might be BRIGHTER, and require a shorter exposure or wider aperture, etc....even though I have fewer LUMENS.

Ironically, your camera probably ONLY measures LUX, and NOT lumens. We can't SEE lumens...lumens represents the light going from the flashlight to the target....and missing the target too, etc.

Lux represents the light bouncing back to us FROM the target...which is ALL a meter CAN see.



You could try using the camera's light meter to take a reading, and then set the setting to the same thing manually...and then just shoot everything at that setting and see what it looks like.

Tam - yes, originally intended to be just a relative comparison. But pick the right light and mode with which to calibrate with, and you'll see how well fitted it can be with a variety of manufacturers by matching most or all of their lumen specs. Then it becomes plainly obvious who's specs are conservative, who's are liberal, and who's are full of it.

FPS - yes, correct answers. Short summary is that you merely "show" the camera what some given lumen level looks like (calibrate it), and then you meter away any light, any mode, and the metered shutter speed, in the inverse fraction, is the measured lumens (1/400=400 lms, 1/30=30 lms, 5 sec=1/5th or 0.2lms).

On lux vs lumens - I'm measuring the ambient light, bounced off a wall to equalize flood/throw beam differences - same idea as the ceiling bounce method we all use to compare lights. Doesn't, matter that my sample area is only the size of a golfball (using center-weighted). After calibrating @100 lms, for example, all the camera knows is that the same golfball-sized spot should be 4x as bright for 400 lms, 30% as bright for 30 lms, and 1/500th's times as bright for 0.2 lms. And it does that exceptionally well.

Here's the set-up.... CLICKY

Oh yeah, sorry if it wasn't obvious, but the way you calibrate the camera is to "show" the camera a flashlight with a 100 lumen mode, for example, and then vary the ISO and aperture to force a correct metering at 1/100ths of a second. Then you lock that ISO and aperture and meter solely with the shutter speed.