One of the first “rules” many photographers learn is the rule of thirds – images are generally more visually appealing if the subject is not smack dab in the center of the frame. It can be fun to push that rule and to have subjects that are even more off center and come up with more visually interesting compositions.
Sometimes starting to experiment with new and exciting compositions with the subject off center can lead to images that are less sharp, and the camera’s side autofocus points are often blamed for the error. If you haven’t ever noticed or thought about autofocus points, they are indicated by the small red dots that light up inside your camera’s viewfinder when you take a shot, signaling that your lens has locked focus on that spot. Your camera by default should be set to choose the focus point automatically, however, once you start taking images with the subject off center, sometimes the camera will mistakenly choose a focus point on a different object other than your intended subject. The most popular remedy for this problem is focus and recompose.
What is focus and recompose and how can I do it?
Focus and recompose simply means choosing your camera’s focus by depressing the shutter half-way with your subject centered in the frame, and then “recomposing” your image by moving your camera while keeping the shutter depressed. On most cameras this will maintain the focus you have selected even though you have not yet taken the picture, and you can keep a “lock” on your subject even if it is well outside the limits of your camera’s autofocus points. The center autofocus point is often touted as being the most sensitive or accurate, so many photographers will set their camera to manually select focus points and select the center focus, seeing that as the most accurate way to focus and recompose. Now when you take a picture, your subject should be directly under that center focus point every time you press the shutter and that will lock your focus before you recompose. If you are taking pictures of a person, their eyes should be used as your point of focus.
Sounds simple. Are there any catches?
Yes, I’m glad you asked. Depending on the lens and camera combination you are using, as well as your aperture and distance to your subject, you might not ever notice any problems with this method. Focus and recompose is, for the most part, a very valid and useful method for capturing sharp images. The problems, however, start when you start experimenting with wider apertures and closer distances to your subject. If you are not familiar with depth of field, it is (to put it simply) the distance in front of and behind your focal plane that will be in focus in your image. If you’ve ever taken an image with a macro lens you may have noticed a diagonal line of focus running through the image – this is your “focal plane” being illustrated by your shallow depth of field. The focal plane is an imaginary line, drawn parallel to the face of your camera’s lens. The wider your aperture the smaller your depth of field, meaning that your focus needs to be more accurate for your image to stay sharp. If you are using a lens with an aperture of 1.8 and shooting wide open, depending on your distance and focal length you may only have inches (or fractions of an inch) in focus in front of or behind your subject.
If you are standing directly in front of and facing a subject and recompose by turning your camera to move that subject off center, the distance to your camera’s focal plane will remain constant (because you have locked focus) but that distance will no longer be the same as the distance between your camera and the subject.
Even though the distance between yourself and your subject hasn’t changed, the fact that the camera is using a focal plane parallel to the face of the lens means that when you angle your camera you have essentially moved that focal plane behind your subject. If you are standing fairly close to your subject and using a wide aperture while shooting, that movement may even be enough to throw your image slightly out of focus. This is in fact a very common reason that photographers might feel that their lens has a backfocusing error – when in fact they have effectively been moving the focal plane behind their subject just by virtue of recomposing the shot. Confused? I thought you might be so I’ve drawn a diagram. I think this is best explained in pictures.
And a few real life examples to further illustrate the point. The following images were all shot with a 28mm lens at f1.8 with the exact same distance to subject. In the first image I focused using the center focal point and took the image.
In this image I used the same center focal point but shifted the subject to the bottom of the frame (keeping the same position horizontally but changing my angle in the vertical direction).
And finally I used the same center focal point but shifted both horizontally and vertically, moving my subject to the bottom right hand corner of the image.
It can be hard to see the difference in a web-sized image so I have cropped each image to 100% on the eye (which I used to focus) so that you can see the loss in sharpness on the second and third images.
While it’s impossible to know exactly how much any given image will be affected by this issue because there are simply too many variables (lens choice, distance to subject, f-stop, etc), here are a few rules of thumb.
- The wider the lens you are using the more opportunity there is to dramatically recompose. The more you move your subject in the frame the more you’ll notice a focus shift.
- The lower your f-stop the shallower your depth of field which means you will notice the effect more.
- The closer you are to your subject the shallower your depth of field which also means you will notice the effect more.
- While moving your subject in one direction or the other (horizontally or vertically) will have an effect, moving it to any corner (diagonally) will have the greatest effect.
All of this is not to say that you shouldn’t focus and recompose or that focus and recompose is not a valid tool. It’s just important to always know the mechanics of how your camera works and how the variable at play will affect your final image. Once you know how things work you can make an informed decision about how you want to shoot each image, and your photography (especially focus!) will improve as a result.