Focus, Focus, Focus - The Eyes Have It (Usually)
Good focus may not be the most important factor in an image, but it and proper exposure are my top two, and exposure arguably offers more leeway in producing acceptable images by post processing. A light or dark image can be worked somewhat, but all the sharpening in the world won't fix an out of focus shot (or one where the point of focus is in the wrong spot).
If you're framing a head or head and shoulders type shot, the eyes are generally where you want to concentrate the focus. More and more compact digitals are offering face detection technology, and this may be a viable focus option, but the possibility exists that with the face filling most of the frame, face detection may not recognize it as such. An alternate method would be to select a focus option from the camera menu that uses a small, central focus point and set the camera AF mode to single rather than continuous. You acquire focus with a half push of the shutter using the eye(s) as the focus point, and then are free to recompose the actual shot, if needed, by holding the half push while you compose the shot you want and then capture. Here's a couple of shots that, while certainly not candid, illustrate the importance of careful focus when you're in tight on a face. (Both shots are 35mm film transparencies (slides) that have been scanned and digitally reproduced).
The image on the left has good focus on both eyes despite the shallow depth of field; the image on the right looks good small, but when you look at it enlarged the focus point appears to have been the model's right eye, and the left eye is noticeably soft. The softness is a byproduct of the shallow depth of field, but mostly due to the initial focus point, which should have been a bit more toward the left eye to bring both within the depth of field.
If you have doubt as to the ability of the camera to bring both eyes into focus in an image such as the righthand image, where the subject is turned and the eyes aren't in the same focal plane, it's probably best to use the closer eye as the focus point and let the rest of the face fend for itself. On shots where you're some distance off the subject and/or the eyes or face might present a relatively difficult point of focus, you can always try for focus on some part of the subject or scene that's in roughly the same focal plane as the important features.
Both images below are examples of this - the focus point on the lefthand image was in the waist area and the in the image to the right, the focus point was about where the shirt and the shorts overlapped.
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