Getting a sharp portrait is important because no matter how good your exposure, composition, lighting is, if your portrait is not sharp, it is not a good portrait. To make matter worse, there is very little you can do to correct blurred image due to camera shake during the post-process stage.
There are two main factors affecting the sharpness of the portrait, namely, the shutter speed and the focus point.
If you are handholding the camera and the shutter speed that you use is too slow, there is a high chance of getting a blurred or soft image due to camera shake. One of the best ways to avoid this is to use a tripod when you have to use slow shutter speed.
However, if you do not have a tripod with you and have to handhold the camera while shooting, a rule of thumb to use is that the shutter speed that you use cannot be slower than the focal length of the lens. For example, if you are using a 100mm lens, you should not use any shutter speed slower than 1/100s.
Many camera manufacturers have come up with technologies to allow photographers to use slow shutter speed and correct the image blur in the lens. These technologies are called Vibration Reduction (VR) for Nikon and Image Stabilization (IS) for Canon. According to these camera manufacturers, using these in-the-lens correction technologies will provide the photographer blur-corrected images of approximately 3 – 4 stops worth of camera-shake compensation.
In the situation where you don’t have a tripod and have to reduce the shutter speed below the focal length of the lens in order to get the correct exposure, try to increase the ISO instead of reducing the shutter speed to get the correct exposure. This is because there is software like noise ninja that can reduce noise in the post-process stage but there is little you can do in the post-process stage if the image is soft or blurred.
In portraits, the eyes of the subject must be sharp and in order for the eyes of the subject to be sharp, the focus point of the portrait should be placed in-between the eyes on the nose bridge of the subject.
First and foremost, the photographer must have auto focus point selection turn OFF in the camera because if he or she allows the camera to automatically select the focus point, the camera might select the wrong focus point and it will result in an out-of-focus portrait.
Some photographers use the “focus and recompose” technique and it may lead to soft images especially when they are shooting with wide aperture like f/2.8 and below. This is because the depth of field will be very shallow and any large movement after focus is locked will lead to soft images. Therefore, try to shoot with smaller aperture like f/5.6 and above and select the AF point that is closest to the eye although the centre AF point tends to be more sensitive if you want to use the “focus and recompose” technique.