What's A Catchlight?
A catchlight is simply a light's highlight reflected off the surface of your subject's eyes. They subtly breathe light into portraits by adding a little more depth to the eyes and are something that painters were using in portraits long before photography was a popular pastime.
How Do I Create Them?
What light source you're using, how strong it is and how far it is from your subject will change the shape and size of the catchlight. The larger the light source, the bigger the catchlight will be and if you have multiple light sources, you can end up with more than one catchlight appearing on the eyes.
You can use artificial or natural light to create catchlights, just remember that direct flash will produce a much smaller catchlight than flash that's reflected off a brolly and if you're working outside, the catchlights can have a slight blue tint to them. When working indoors with fill-in light as well as a key / main light source, your additional, fill-in light may create an additional set of catch lights in your subject's eyes. Some people like the effect so if you're one of these, just make sure one is dimmer than the other so they're not too distracting. You can always try removing one set in your editing software too, but it's always easier to get it right in-camera first rather than relying on Photoshop.
Where To Position The Catchlights?
If you look at the eye as if it was a clock you should try and get the catchlight to sit, ideally, at 10 or 12 o'clock but anywhere between quarter to and quarter past should work just as well. Of course, this isn't set in stone and you may prefer to use a lower position. Placing your light source above your subject's head will also help you get the positioning of the catchlights right.
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