Why can flash freeze motion even with a slow shutter speed?
Asked 11/11/2015
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If I shoot with flash at a shutter speed like 1/10 second, why can the photo still look sharp or “frozen”? I would think the sensor is collecting light for the entire exposure, not just during the flash burst. How does flash actually freeze motion, and when would a slow shutter still show blur?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
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The key here is how much light will be hitting sensor during flash, and how much during the rest of exposure. The sensor does gather light during the whole exposure; all of it is blended into one static image.
In case the ambient light is much lower, it won't have nearly as much effect on the total light that reaches the sensor, and therefore only surfaces lit by flash will stand out.
On the other hand, if the sensor gathers illumination provided by ambient light in similar or even higher amount than illumination caused by flash, it will show in the resulting image.
Combining areas lit by flash and areas lit by continuous light is most notably used for slow-sync technique. Example by Robin, 1/10 seconds:
To capture ambient light in addition to flash, you'll need to change the ratio between them. You could reduce flash power and/or increase ambient light (stronger lighting or longer shutter time). Note that changes in aperture or ISO would affect both kinds of light similarly, i.e. they don't affect ratio.
Originally by user4390. Source · Licensed CC BY-SA 4.0
user4390
10y ago
0
Generated from our catalog & community — verify before relying on it.
A flash freezes motion when its light contributes most of the exposure. The sensor is open for the whole shutter time, but a flash burst is extremely brief—often around 1/1000 second or faster, and often even shorter at lower flash power. If the ambient light is weak, very little image is recorded during the rest of the 1/10 second, so the subject is mainly captured by that short flash burst and appears frozen.
If the ambient light is strong enough, it also records during the full shutter time and can add motion blur or ghosting. That’s why flash does not always freeze action: it depends on the balance between flash exposure and ambient exposure.
So:
- low ambient + strong flash = motion appears frozen
- significant ambient + slow shutter = blur can appear
This is also why a flash image can look similar at 1/10, 1/60, or 1/200 second, as long as you are at or below your camera’s flash sync speed and ambient light remains suppressed. If you intentionally allow more ambient light in, you get effects like slow-sync flash, where flash sharpness and ambient blur are combined.
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