How can I consistent achieve tack sharp images, when wanting to focus on eyes in a portrait? I am always using single focus and moving the focus point to the eye and recomposing if necessary. What else would you recommend or what factors can get in the way of a crisp portrait?
This is in the situation of a natural light portrait, no flash, and auto focus settings. I want to achieve a tight shot with the focus precisely on the eyes. Thank you all for your help!
In addition to damned truth's answer, using live-view and zooming in on the eyes will allow you to ensure your focus point is exactly where you want it to be. Focal lengths between 80mm and 105mm offer a flattering perspective for portraiture, a 50mm prime on an APS-C crop-sensor body will give you the equivalent of an 80mm lens. Primes are nearly always sharper than zooms, which again will help in your quest for that "tack-sharp" image.
Ensure your subject is well lit and try to use a fast enough shutter speed that you have no chance of motion blur (i.e. mush faster than the usually suggested 1/focal length). Or use a tripod, remote shutter release or and the mirror lock up function of your camera and avoid touching your camera immediately before the shot is taken.
Ask your subject to stay as still as possible when you are taking the photograph
Stop your aperture down to somewhere close to your lenses sweet spot (I think this is usually around f/8 - f/11).
I approach this problem by thinking of what can lead to blurry images, then eliminating those factors.
Given your parameters, you will be sometimes shooting in low light with a long lens. That means opening up the aperture (reducing depth of field) or reducing shutter speed (increasing the chance of camera shake or subject movement), both of which can make it harder to get sharp portraits.
A high-quality stabilized lens, pro-level camera with many focus points and good high-ISO performance, good handholding technique (ideally a tripod but that won't work for active subjects) and knowledge of the DOF at any aperture/focal length/distance combination are all useful.
Practice your focusing and handholding technique. Since you want to use natural light, learn how to find the most and best illumination (open shade outdoors, window light with reflector fill indoors). Avoid low-light situations or use artificial light if it's dark.
Focus and recompose is fine IF you know that your initial focus point is in the same plane as the eyes. Remember that lens DOF is not flat, it's curved.
Finally, don't just shoot a couple of frames. I personally shoot a lot of images so we have a lot to choose from. It's easier to get a hero pick from fifty exposures than five.
