Sometimes, you have a great lighting setup that gives you the exact look you want, but you need to fill in the shadows ever so slightly. This great video shows you how a ring flash can allow you to fill in shadows without changing the overall look of your setup.
Coming to you from David Bergman of Adorama TV, this helpful tutorial shows you how a bit of on-axis light (in this case, a ring flash) can be used to fill in shadows just a bit on a subject. If you're looking for a darker or moodier look to a portrait, it can often be the case that the shadows on your subject might be a bit too much, particularly around the eyes. Depending on the pose, increasing the key light's power might not help and could change the overall look of the portrait. Meanwhile, you have to be careful where you add a fill light and at how much power, as this too can change the look of the portrait. It turns out that a ring flash is a perfect way to open up the shadows without changing the overall look. Check out the video above for the full breakdown.
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I've been using the Quadra Eco for several years now for this purpose, extremely handy for a lot of situations and especially run and gun lighting.