Italian model Giulia joined me in studio recently for some experimental portrait photography using various lights, strobes, gels and modifiers.
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When using a strobe/flash, the photograph  is exposed in a split second; anything from around 1/500th of a second to around 1/20,000th of a second, depending on the gear and settings. Most studio photographers light their portrait shots this way, and even if the camera shutter is open for longer than the flash duration, the latent, ambient light, doesn't contribute to the exposure. But combining a long shutter speed with a bright continuous light source can yield interesting results, as you can see here. 

https://s16.postimg.org/5aptfqsn9/DMU_9720_Edit.jpg

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Nikon D750. Sigma 85mm 1.4 @ F9. 
ISO 100. 
Shutter = 0.6 seconds. 
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You effectively have a "double exposure" of sorts - the near instant flash exposure which freezes time beautifully, along with the continuous light source, which contributes to the exposure for as long as the shutter is open. I tried various speeds, from around half a second up to around 2 seconds. As you can see below, the results were very varied and a lot of trial and error was involved. 

https://s16.postimg.org/49u64d11h/survey.png

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Of course, shutter drag and "ghosting" is usually something photographers want to avoid, but a good understanding of how strobes and ambient can contribute to your photographs allows you to take control and make the pictures you want. 

If you'd like to know more on the subject, Adorama on YouTube recently posted a very useful video about "ghosts" in photography. 

https://www.youtube.com/watch?v=ZiQbUdVK3WE

More from my shoot with Giulia soon! 
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