External Flash Meter

Last night after dinner, Anon from Ymous fame, knocked on my door to ask for help. He’s the “senior photographer” at his photo club, so he can’t ask for help there. His wife bought him an external flash meter for his birthday. I haven’t used one of these, a Gossen, from the late 70s, that’s the late 1970s.

Anon tried the Sekonic flash meter with his Nikon D700 and his SB800. It was giving readings that were at least 5 f/stops under exposed. But the Sekonic was giving the correct reading when he tried it with his small point-and-shoot camera. What’s wrong?

I tried the Sekonic with my Canon 7D, 580EXII flash. It didn’t work either. The metering was way, way off.

Because of the old age, too much liquor in the days of my youth and too much smoke, it takes longer for the brain cells to connect. Then I realized and remembered how the eTTL flash system works on the Canon. Nikon is almost identical. All the eTTL flashes produce 2 flashes.

1. The first flash is used to send a test light, that will be read and analyzed by the camera’s reading system. The first flash is a much weaker flash, by around 5 f/stops. It’s so close to the “real” flash firing, that we, humans, can’t see it as 2 separate flashes.
2. The actual flash with the proper output as determined by the test light.

It the same for Nikon as for Canon. Then I realized, that it makes no sense to use the flash meter when the flash is set to auto. It makes no sense to measure variable flash output that can’t be repeated. To use the flash meter, you must set the flash to manual. You can set the flash output to full, ½ or ¼… power. Then it’s constant and with the flash meter, you can adjust your exposure with either the speed (up to the sync speed) or the aperture.

So why did it work with the point-and-shoot when the flash is also an automatic flash. The short answer: incompetent operator. The longer answer, since the flash is so small and the test were at 3 to 4 meters / 10 to 14 feet, it was almost always using the full power and they do not produce the 2 flashes. The metering with point-and-shoot cameras is done through the regular light meter.