In order to calibrate my cheap and cheerful new monolights, I took a series of pictures with my LX7 at a variety of different settings on the (un-calibrated) power dial. I used GIMP to measure the intensity of the same spot in the scene at different power levels, and plotted these against the angle of the power knob. I then fitted a quadratic model to get the angle as a function of the GIMP intensity and intensity squared. Next I took a series of shots of the same scene with different apertures to find the GIMP 'intensity difference' associated with a one stop change in exposure. Using this values I plotted the angle either side of the mid-power-point (knob set vertically) at 1/3 stop intervals; that's how this image was created. It shows the angle I need to set the power knob to on the flash unit to get more or less power in 1/3 stop increments. Note that it's not linear!
Why do all this? Well, my LX7 only stops down to f8 so I can't test the exposure below f8; and my Schneider 150m is f5.6 full open so I'd like to be able to use more stops than just f5.6 and f8. Which means I need to up the flash intensity and stop down the lens from an exposure I've tested with the LX7.
Armed with this handy cheat sheet, I set about testing my theory...
...and this was the result. The negative appears a little underexposed. Shadow deatil has been lost, the mid tones are not where one might like at around zones 4 to 6, and the highlights lack clarity. In fact the only thing that is visible (just) is 'Kodak 100TMX' at the top.
My first thoguhts was 'did I load the holders green side up'. If so how many holders would I ned to check?
Then I remembered something about flash synchronization. And the answer was I had not set the shutter to 'X' so it was triggering the flash before it opened (which was what you needed to do with flash bulbs and presumably powder before them). I went back and took four more using the my flash power cheat sheet at different exposures (with the shutter now set to X!) - and they were indistinguishable in terms of exposure! And since I was doing a portrait for a friend the day after, that was a worthwhile experiment.
