How can I blur the background with motion while keeping the subject sharp and properly exposed?
Asked 5/10/2015
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2 answers
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I want to create an image where the background shows motion blur, but the subject stays sharp, in focus, and well exposed. What shooting technique and lighting setup can achieve this effect?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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Use a flash with rear curtain sync and a long shutter speed.
The long shutter blurs the background and the subject.
However, at the end of the exposure, the rear curtain synced flash fires, which essentially adds a second exposure on top. The flash is significantly brighter compared to the ambient light, but due to its limited reach, it only hits the subject, not the background. (mostly) The so far accumulated blurry exposure of the subject is overpowered by the flash exposure, which is clear, because it is so short.
The flash is high above to the left of the camera, which you can tell from two things:
- The shadow under her chin
- Her head is very clear. Her leg appears to be transparent. This means the light is stronger above, near her head, overpowering the blur of the long exposure entirely. Due to the inverse square law the light falls off rapidly and is not powerful enough to do the same with the legs - the background shines through. The light must be near/above her head.
This is a nice creative technique. I have mixed feelings about using it for a vogue shot. Sure, it all looks artsy and creative, but being a product shot (I'm talking about the dress!) it doesn't work so well for me. The dress mixes with the background, it is blurry, it is not entirely visible... Imagine a BMW ad that only shows the rear lights of their new car disappearing in a dust cloud that the car leaves behind. Sure, that all could be (and very likely is) all intentional, but I'd rather see the dress more clearly, more from the front. What I'm trying to say is that you should not overdo this.
Originally by user35348. Source · Licensed CC BY-SA 4.0
user35348
11y ago
0
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A common way to do this is to combine a long shutter speed with flash, especially rear-curtain (second-curtain) sync.
How it works:
- The long shutter records ambient light over time, which creates motion blur in the background.
- During that same long exposure, any subject movement may also blur.
- At the end of the exposure, the rear-curtain flash fires. Because flash duration is very short, it creates a sharp “frozen” exposure of the subject.
- If the flash is much brighter than the ambient light on the subject, that sharp flash exposure will dominate, while the background remains blurred from the longer shutter.
Why the effect looks natural:
- Rear-curtain sync places the sharp flash exposure at the end of the motion trail, which usually looks more believable than front-curtain sync.
- Keeping the flash mainly on the subject, not the background, helps separate a sharp subject from a blurred scene.
In short: use a slow shutter for ambient motion blur, rear-curtain sync flash to freeze the subject, and position/adjust the flash so it lights the subject much more than the background.
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