Why does a flash freeze a picture even if I use a shutter speed like 1/10 sec?
I don't understand it, because the light will hit the sensor the whole time, not just when the flash is fired.
Answer
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.
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