I am a regular Reddit user and am constantly browsing the subreddit /r/photography. Yesterday an awesome question was asked that got some great replies and I knew I had to share it. The user Lemonfighter asked "Is there anything wrong with just always using the center Auto Focus point?" At first I thought "Not really, just do what feels right." I went into the comment section and was surprised with some awesome replies instead.
"I recently got a 55mm f1.8 prime lens and I'm enjoying shooting with it wide open. As for focus, I'm finding it easiest to set the AF focus area to "centre", place whatever I want to focus on in centre of the frame, back-button focus on it, and then move the camera to wherever to frame the shot. This works pretty well for picking interesting looking people out of crowds, etc. Is there anything wrong with doing this? Does anyone else? It feels like you're not supposed to do it this way, what with all the other focus area settings (wide, zone, flexible spot). This article states that the problem with doing this is that if you change the composition too much from when you focused then the exposure will get out of whack, but that generally isn't an issue, I find." - /u/Lemonfighter
Again, my first thought was to think it really wasn't that big of a deal to use just your center point. Though, depending on your aperture, it can be hard to get the perfect focus on something such as the eyes on a portrait photo. When I shoot portraits I almost ALWAYS make sure to focus in on the eyes. That means to compose my shots on a thirds line or in a nice position, I need to change my focus point from the center. It just feels natural for me to have to do that because keeping the center focus point just seems like to much movement for me. Having to focus, then move my entire body/camera to compose the shot, then take the shot, leaves a lot of time for a small adjustment to have happened which might make my focus point I wanted to not be as sharp.
Reddit user Cdrdj posted one of the best responses I saw in which he actually put together an example (seen below) explaining why focusing and recomposing is not always the best. /u/Cdrdj said the "easiest way to demonstrate what is happening is to reach out and touch the very middle of the computer screen in front of you with your index finger. Make sure you are sitting straight up and that your arm is fully extended in front of you. Lock your body and shoulders and arms in this position as you touch the middle of your screen. You are now IN FOCUS."
At this point, this is what your camera would be set to if you were in your center focus point. You haven't began to recompose the shot yet, but you have began the initial focus. The next step listed was the now "recompose" your shot. So you need to now angle your body by rotating it at the waist so that your index finger is pointing to the edge of your screen. Right now, your finger should no longer be touching your monitor. Do not forget, the end of your finger was the focus point. This is an example of how recomposing can actually take that point and move it off of where you initially thought it was, in terms of distance.
After that comment and example from /u/cdrdj, a response from user Kingofjaffacakes was posted which was quite detailed. "Over anything but the nearest of focal distances, the change in subject distance relative to the field of view and hence focus error, is negligible." To see what he means, he pointed out that you should repeat the test with your arms the length of the focus distance. Keeping in mind that you must understand the limitation of "focus-and-recompose" and that for some cases, it is definitely ok to use, but for others, it can be hard to avoid switching your focus point.
When choosing another focus point, sometimes you still may have to recompose. Make it a goal to try and have that amount of movement after the focus as small as possible. Below are some images where I highlighted which focus point I would have used in the shot, considering I only had 9 available to me (which most beginner DSLR's have). I'd grab my point closest to the eyes, then recompose my shot as little as possible.
A lot of people do just use the center focus point, which is not necessarily a bad thing. This post and the responses that were seen just make a great perspective on how using different focus points will decrease any opportunity of losing focus on that perfect spot on your subject.
What is your style of going about focusing on the subject? Center focus point, multiple focus points, manual?