What I don't know, and why
I use Speedlights a lot, although I prefer studio flash, but as Speedlights weigh 1lb and studio flash 10, I tend to carry Speedlights. So much, you’d think that I really understand how they work to automatically set the exposure. I don’t. Here are my observations. The Nikon (and most likely, the others brands) have an electronic brain that detects the light bouncing off of the subject and then attempts to calculate how much flash to provide to properly light a scene. Unfortunately, from my experience they don’t work half the time and for no specific reason that I’ve been able to observe.
Let’s start at the beginning. Way back when, you’d but a Speedlight and note its full power GN or guide number, based on the ISO or ASA speed of the film you use. For example, you would use Tri-X which is rated at 400 ISO or ASA. The guide number of the Speedlight would be (fore example) 125 which required some simple calculations to get the correct exposure. If your subject in this example was 40 feet away you had to shoot at about f/2.8 to get the correct exposure. If 20 feet, the f/5.6. You just divide the guide number by the subject distance to get the basic aperture setting. In other words, divide the GN by the distance to find the approximate aperture setting.
I’m assuming that the flash is the only light source, so if you wanted lighten the shadows in daylight, you’d stop down (reduce the aperture) by a few stops to get the correct lighting balance. In the above illustration, to f/5.6 and f/11 respectively. Of course, you would need to modify the exposure based on experience.
Things got more complicated when the flash manufacturers developed methods to change the light output on demand so the GN would vary, and complicate your life. Eventually the GN concept went away in favor of automatic flashes… the only problem being that they don’t work very well.
Broadly speaking, just as you press the shutter release the flash fires a pre-flash (with a sensor) to gauge the subject distance and the surroundings to determine the correct flash output depending on the lens aperture. Sometimes it works well, and sometimes it doesn’t.
What I don’t know is why the flash tries to read the subject distance from the light bounce when it could read it from the focusing mechanism which should tell it how to set the GN. In this example, you’d point the camera at the subject 11 feet away and press the shutter release, the camera should tell the flash that the subject is 11 feet away, the ISO rating is X, the main subject is Y and the aperture is Z and set the corresponding amount of light output. Instead it uses its pre-flash to judge the distance. I just don’t know why.
In any instance, the Nikon Speedlights tend to overexposure the main subject, completely washing them out. Some time ago, I went back to manual flash exposure using either 1/2 or full power and having memorized the GN for each setting. It works fine almost every time.
Related posts:
- The f-stops here
- The strobe experiment
- Wireless flash triggers
- I do something that no other photographer does (that I know of), and why
- Circles of confusion
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