Avoiding Motion Blur
In response to some questions in the comments section about power settings and freezing motion with a speed light I added a new post to the blog today.
Yesterday I got a question from a friend on Facebook asking how to get their flash to sync with their camera at a shutter speed higher than 1/125 so they could freeze the motion of their subject (children in this case). This question also comes up in almost every CreativeLive strobe lighting class that I've worked on. I hope that this post can help clear up the big misconceptions about shutter speed and flash photography.
Instead of using children, I decided on something that moves even faster--a box fan.
In the first series of images below we see 6 images of the fan. The first one (1) was taken without a flash. The rest were taken with different brands and models of studio strobes. And the most important thing to recognize here is that all 6 images were taken at 1/15 of a second, and the fan was running at the same speed in all the images. What I hope to show you here is that in the studio with strobes the shutter speed is pretty much irrelevant as long as it isn't so slow that ambient light infiltrates the exposure or isn't too fast that the camera's shutter cannot synchronize with the flash, which will leave a black band across one side of the image (see my previous post on sync speed for more information on that).
As noted above, photo 1 was taken at 1/15th second with no flash
#2 was with a Speedotron 805 pack and a model 102 head (1/670 second flash duration)
#3 was with the same Speedotron 805 pack, but with a model 202 head (1/900 second flash duration)
#4 was taken with an SPS Systems Excalibur 1600 monolight (unknown flash duration)
#5 was taken with a Speedotron Force 5 monolight (1/1500 second flash duration)
#6 was taken with a Paul C. Buff Einstein monolight in Action mode (1/4826 second flash duration, and yes, the fan was spinning)
In the above series of images, the shutter speeds in both rows go from 1/15 to 1/30 to 1/60, and finally 1/125 (all at f/7.1). In the top row the studio strobe was not turned on, so you can see the base exposure. There is a slight rendering of the subject in the 1/15 and even the 1/30 second shots, but not enough to affect the exposure when the strobe is turned on (second row). At 1/60 and 1/125 there is no ambient light at all in the exposure. All of the images in the bottom row show the same exposure, controlled by the ISO, flash power, and aperture. The shutter speed is inconsequential to the overall exposure in a reasonably dark studio setting.
What about speedlights? What about high-speed sync and speed lights? Let's look at another set of images. As above, #1 is the same reference image as used above at 1/15 second with no flash.
#2 was taken with a Canon 430EXii flash at 1/15 second
#3 was taken with a Canon 430EXii flash in high-speed sync mode at 1/500 second
Here you can see that engaging high-speed sync does not help stop subject motion. It actually led to more subject motion, even though the shutter speed was at 1/500 second. Additionally, high-speed sync robs the flash of power, leading to an underexposed image. This is because high-speed sync works by pulsing the flash multiple times at a lower power during the exposure. This makes the speed light less bright, helps burn through batteries, and does not help stop motion blur.
I hope that this helps clarify why shutter speed is usually not an important factor in determining exposure or in stopping motion when working in a studio with speedlights or strobes. The important factor here is the flash duration of your strobes or speed lights.
Taking it outside
Shutter speed with flash does play an important role in outdoor flash photography with a lot of ambient light. There your ISO and aperture control the exposure on the flash-lit subject and the shutter speed affects the brightness of the background. By slowing down your shutter speed in this situation you will make the background brighter. By speeding it up, you darken the background.
This situation (outdoor flash photography) is where high-speed sync (HSS) comes into play. Without HSS you are limited to shutter speeds at or below the camera's sync speed. Depending on the camera, this could typically be anywhere from 1/60 to 1/250. You will need to consult the owner's manual for your particular camera to find the sync speed.
The usual situation here is that you want to take a flash photo outdoors on a sunny day and want to use a large aperture (maybe 2.8 or faster) to blur out the background. The base exposure, without flash in this situation might be something like 1/5000 second at f/2.0. If you add a flash that doesn't have HSS your shutter speed is limited to, in this example, 1/160 second. This means that you cannot shoot at f/2.0, you need to stop down to f/11. But this gives you too much depth of field (the background objects are too in focus and distracting). By engaging HSS on your speed light you can now sync the camera at 1/2000, but your flash will need to be in close to your subject because of the reduction in output power that enables HSS.
The image on the left above is at 1/160 second at f/11 with no flash
The middle image is 1/160 at f/11 with flash
In both images, the background is too distracting. To fix that, we want to shoot at f/2.0
The image on the right is 1/5000 at f/2.0 with no flash to give shallow depth of field. But the eyes are in shadow. We would like to use a flash to fill in the shadows.
Here we start with the last image from the last series. 1/5000 at f/2.0 with no flash
In the middle image I turned on the flash without high-speed sync which dropped the shutter speed to 1/200 second (maximum sync speed on the Canon 5D mkIII). Because I am at f/2.0 for the background, the image is way overexposed.
On the right I turned on high-speed sync which let the camera and speedlight sync at 1/5000 second at f/2.0. Now there is some light in the eyes, shadows are opened up a bit, and the background is nicely blurred to help make the subject stand out.
Controlling the ambient exposure
Earlier I mentioned controlling the ambient exposure via shutter speed. Let's take a look at that. Here I have combined high speed sync, manual exposure, and manual flash settings.
The base setting for the sky exposure is 1/2000 at f/4. My on-camera flash is set to full power so that it has enough reach for high-speed sync. The images above are shot at 1/640, 1/1000, 1/2000, and 1/4000 with flash. Notice that the background is lighter in the first two images, normal in the third, and darker in the last image. The last image, at 1/4000 shows a darker subject because the flash didn't have enough power to handle high-speed sync even at full power at this distance and at such a high shutter speed.
Questions? Comments? Thanks for following along!