Speedlighter.ca. Speedlighter. Speedlighter!
So yeah, let me talk about speed for a moment. Speed as in “fast exposure speed, in order to freeze movement”. Fast exposure speed = short exposure time. 1/2 second is a long exposure time, i.e. a slow exposure. 1/1000 second, on the other hand, is a short exposure time, i.e. a fast exposure.
So how so you get a fast exposure time? One of two ways, it turns out. Either one of:
- A short shutter time, or
- A short light flash.
You see, what matters is the duration during which the light reaches the sensor. Whether that is short because the shutter only opens for a short time or because the light itself only flashes for a short time makes no difference at all. It is the same thing. A short exposure.
So let’s say I’m taking a fresh picture of a rapidly spinning spinning top. And let’s say further that I want to freeze the motion, to see the spinning top detail. Since I’m using a flash, I cannot use a fast flash shutter speed; The fastest I can go with my 5D camera is 1/200 of a second. So I’m going to have to achieve a fast exposure by using a short flash of light.
Fortunately, that is exactly what a flash fires. At full power it fires a flash of about 1000th of a second, or 1/4000 second at 1/4 power. Nice. Assuming that ambient light plays no role, your effective shutter speed is now nice and fast: 1/4000 second.
But not fast enough:
(1/200 sec, 400 ISO, f/32, 1/4 power flash)
OK, it’s still blurry, because it is spinning rather fast, so even 1/4000 second cannot freeze that motion. Now what?
The solution is in the sentence above: “At full power it fires a flash of about 1000th of a second, or 1/4000 second at 1/4 power”.
Because how does a flash set its power? Simply by shortening the time that it is on. Full power means 1/1000 second on a typical flash (small or large). Any longer and it overheats and burns out. So:
- Half power means 1/2000 second, half the time.
- Quarter power means a quarter of the original time, so 1/4000 second.
Oh wait. So “lower power flash” means “shorter duration flash”?
Yes! So if I set the flash to 1/128 power, I get an effective exposure time of 1/128,000 second. That’s like a really, really fast shutter:
(1/200 sec, 400 ISO, f/5.6, 1/128 power flash)
Now, as you see, with an effective exposure time of about 1/128,000 second, the top’s motion is completely frozen. So while my shutter speed is unchanged, it does not matter. The light is only on for 1/128,000 second. So that is my effective shutter speed.
The lesson? To freeze motion, use low power flash. The lower the better.
Very well explained. Thank you.
Thank you, Alan. Much appreciated.