I recommend buying a small lighting kit. Remember that the smaller the subject, the cheaper it will be to light them because you don't need big, bright lights anymore. You just need something small and simple.
The basic and most effective way to light a head shot client is by setting them against a black backdrop and having them stand about 5 to 10 feet away. Then, set up 3 main lights, one pointing at their face from a slightly high angle off to the right hand side, one pointing at their hair from behind, and one pointing at their face from below, along the lower left hand side. Using this tehniques will make it easy to light just the person's face for head shots. The closer you move the lights, the brighter they become and the less you need to use the ISO function on the camera.
Remember: lighting just a head and face is much easier than lighting the whole body. A cheap way to light the body is to buy a few work lights and place some good diffusion in front of them. This will give you a cheap softbox effect for the headshot, which allows for an amazing look.
Hopefully these tips have been helpful and will allow you more control over your indoor headshot sessions!
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