Headroom is the amount of space between the top of your subject's head and the top of the frame. It might seem very insignificant, but it's crucial to get this distance just right.
This is typically a problem with the starters as they use the middle focusing point for focusing and then do not recompose the photograph. As they focus on the eyes with the middle focusing point they generally end up leaving the remaining space on the top, which adds up too much space above the head.
Its a good idea to use the side focusing points overlapping the eyes while holding the camera vertically or to recompose the shot as you release the shutter.
Failing to do so and you'll end up with a photo that has lots of space above the face.
On the other hand if you go tighter, either go quite close or leave a little bit of space above the head. When edges of the head start to curve, do not cut off there. Cut off before they start to curve or let the complete head show.
There is just the right amount of head room that you should leave in your photos and one learns that with experience. If you are shooting up close or are zoomed in on a subject, leave less space above. Although I do not recommend this as a regular practice, if you are doubtful about the space, leave a bit more and then you can crop it later to the right amount. once again practice to get it right while shooting rather than doing so in the computer later on.
Do remember, that it does not mean that you can never leave more space above the head. sometimes when you are doing an environmental portrait or for any other justified reasons, it may actually be fine to leave more headroom. In other words, if you are aware and and feel that it will add to your photograph do so but not by mistake or because you are simply focusing on the eyes and then not recomposing.
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