A headshot has one primary objective: showing your character through facial expression. The purpose is to show a casting director what your face looks like, because a lot can be told through the face.
Headshots are not body shots! The proper framing of a headshot is fairly close in, maybe only showing a small portion of the actor's shoulders. But leave room for cropping! Shoot wide enough so you can crop 8x10 format (most cameras shoot 4x6, which is equivalent to 8x12, meaning you need to chop 2 inches off the sides (or bottom and top if you shoot portrait).
Also, remember that you don't want to shoot so far away that the quality deteriorates after cropping. Just try to frame the headshot so that the head and a bit of shoulders is visible. Also, know whether your actor needs portrait or landscape pictures, meaning if the camera is vertical or horizontal. This helps to determine your framing as well.
Sometimes though, headshots might be framed wider than just the head. But really, the best way to decide what framing you need besides the standard that I explained above is to think about what roles the actor best plays, and then determine if a certain framing is going to help your actor fit in the role better. For example, an overbearing mother role might be really tight in with a little bit of distortion around the sides... Maybe use a slight wide-angle lens. On the other hand, playing a sad, lonely character might entail shooting a little bit farther back, using a little bit of space to emphasize the loneliness. But these techniques must be extremely subtle!!!! The primary purpose of the headshot is to show the face. You don't want an artistic picture that detracts from the headshot's primary goal, otherwise your actor won't get any calls, and you wont have a returning client. Not to mention referrals.
Good luck becoming a headshot photographer!
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