One of the things we learn when we start out with photography is that shutter speed can be used to help freeze fast action. Unfortunately, the way a camera’s shutter works makes it a poor candidate for stopping fast movement. Not only are most shutters too slow only reaching a max effective speed of 1/4000 or 1/8000 of a second, but the way the shutter works with the sensor can induce an effect called “rolling shutter”.
Camera’s use a trick to reach the high shutter speeds you see on your camera dial. When you dial in a 1/4000 of second exposure the shutter doesn’t expose the entire sensor for 1/4000 of a second. What it does is expose each row of pixels for 1/4000 of a second in sequence. The shutter narrows down to a slit and passes over the sensor at a rate that exposes each row of pixels for the desired amount of time. What this means is that there is a gap in time between when the first row is exposed, and the last row is exposed. If your subject is moving quickly this can cause the object’s position to visibly change between when the first row of pixels is exposed, and the last row of pixels is exposed.
If you have a ceiling fan you can easily test this phenomenon.
In the first photo below I’m going to take a picture of the ceiling fan on high using a constant light with my camera at 1/4000 of a second shutter speed.
The fan blades should be perfectly straight. Notice, that not only are they not straight, they’re blurred at the very ends where the movement is the fastest.
The next photo was taken using a flash to freeze the action, I set the flash to 1/128 power and used a shutter speed of only 1/160 of a second.
You’ll notice that the fan blade is now perfectly straight. This is because the flash duration at 1/128 power is much, much faster than the shutter in the camera. The flash I used for this example, the Godox AD200, has a flash duration of only 1/13300 of a second at 1/128 power. Thanks to the flash not only is the blade shaped correctly, it’s also perfectly sharp even at the very end where it is moving fastest.
One important thing to realize is that when critical pixel level sharpness is required, you can use flash to remove even the slightest blurring effects of camera shake or subject movement.
Do you have any examples of when using flash was preferable to using the shutter to stop fast action? Let us know about them in the comments below!