Or, more importantly, the duration of the light really. I’m gearing up to do some motion freezing shots using flash. Now, why do this with flash? We could just use a really high shutter speed and yes, the shutter on my Nikon D800E will go to 1/8000th of a second, which is fast enough to freeze most action in the “medium sized world”. However, there are a couple of reasons why this is not a good idea.
First, for shots where we need to the camera to react to a noise, vibration or something breaking a detector beam, there may not be time for all of the mechanical gubbins inside the camera to lumber into action before the event is over: the mirror has to raise, the whole shutter has to start it’s operation etc. All of this can add as much as 50ms between the trigger event, and the exposure starting. The second reason has to do with the amount of light we have to work with: at 1/8000th of second, not a lot of light is going to get in.
Now, one of the best things about speedlights is that you can’t alter the power. Yep read that again 🙂 Speedlights do not work in the same way as conventional studio lights. Whereas studio lights charge up their capacitors with just the right amount of energy for the light output you want, speedlights are always fully charged. You control the amount of light by altering the duration of the light. They have a tap inside – these days this is normally an Insulated Gate Bipolar Transistor or IGBT. So rather than a trigger voltage discharging the whole cap across the flash-tube, your speedlight can turn the flow of electrical energy to the tube on and off. The power of your speedlight btw, may amaze you. Power is the rate or flow of energy (the rate of “doing work” in generic terms). It is not the amount of energy delivered. Your average speedlight holds around 80 joules of energy in its capacitors. To empty them fully takes about 1/900th of a second. 1 watt is 1 joule per second, so dividing 80 by 1/900th gives us 72 kilowatts. That’s a pretty bright light – for just over a millisecond…. Continue reading →
How much do you need? I had no idea. I’d seen a few flour (or “dust”) shots around on flickr, and Pinterest and this seemed like an ideal job for my new high-speed IGBT Lencarta studio lights – movement, flour backlit, hair flying etc. Well, I bought 4 bags of Sainsbury’s most basic plain flour. I now have 3 and a half bags of plain flour – which, as it turns out is not much use for baking anything…..
