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This post was written as a follow up to my last article on using Single Point AF. In the comment section of that article there was a bit of debate over which focus point to use and when. One camp was stating that there was really no point in using anything other than the center focus point. A lot of people thought it was silly to use anything else because the other focus points aren’t as accurate. Some examples of comments for this argument include…
So, is there any truth or accuracy in these statement? Well for one, any time your subject is moving that could be considered an action shot. So I shoot action all the time. As far as the focus-recompose debate goes, I strongly disagree with the focus-recompose method and in this article I’m going to present my case against it. This debate may not start a flame war or anything like that (well, maybe), but I do think it’s a good idea to dive deeper into this topic and find out which technique is better. Center point focus and recompose….or full access to all your camera’s focus points? Let’s go.
I think you’ll agree that I’m pretty awesome at drawing stick figures with my Wacom Tablet, so I thought it might be easier to explain why focus-recompose often fails by creating a little diagram and then explaining this diagram in steps. So here we go…
In the diagram above, ‘A‘ represents pointing the camera up towards the subjects face and placing the center AF point over the subjects eye to achieve focus. Most photographers using the focus-recompose method know that nobody wants to see an eye or face right smack dab in the middle of the frame, so they then recompose the scene by moving the camera down to get the entire body in the frame or to simply move the subjects face off center. This new camera angle is represented by ‘B‘ in the diagram.
I’m certainly not a mathematician but one of the few things I remember from geometry is the Pythagorean Theorem and the common sense that the length of A is longer than the length of B, and that if you were to lay the A line down on top of the B line, you would see the difference in length between the two.
If you stand 4 feet from your subject and point the camera up at the subjects face, then you are no longer 4 feet away from what you’re focusing on. If the length from your camera to your subjects chest is 4 feet and the length from your subjects chest to their eye is 2 feet, then the length from your subjects eye to your camera is 4.5 feet. Are you getting this!? That means that if you focus on your subjects eye, move the camera down to their chest to recompose, then your focal plane is now half a foot behind your subject! The difference between the two lengths is show in figure ‘E‘ in the diagram, with ‘C‘ being the actual focal length when recomposed and ‘D‘ being actual distance to the subject.
Some readers out there might just be wondering what the big deal is about 6 inches. So here’s a screenshot from an app I’ve got on my iPhone called Depth of Field Calculator. This app will calculate your depth of field based on the settings and information you provide. If you can’t quite see the screenshot just click on it to see a bigger version.
The app lets us know that if we are 4 feet from our subject with a 50mm prime wide open at f/1.4, then our depth of field is only 0.16 inches. In other words, our focal plane will begin at 3.92 feet away from the subject and focal will end at 4.06 feet. Now if you remember from the diagram above, if we focus on the eye and recompose to where the center of the camera is pointed at the subjects chest with their head in the top third, then our 0.16 ft plane of focus is actually 4.5 feet behind our subject. Therefore, we have an out of focus image. Want proof? I thought you’d never ask!
The demonstrate my point, I slapped a 50mm prime onto my 5DMII and went over to the dart board in my office. With my camera on a tripod I placed the center focus point over the ’20’ on the dartboard, achieved perfect focus and then took one picture. Without changing anything I recomposed the scene by moving the camera down until the center focus point was over the center of the dartboard and took one more shot.
Camera info: Canon 5DMII with Canon 50mm lens, f/1.4, Shutter Speed 1/125th, ISO 250
And here are both images zoomed in to 100% to see if they are sharp….
Focus-Recompose is easy, intuitive, quick and self-defeating. While this method will work in some situations, there’s no way to know how well it will work without calculating your depth of field before each shot to see if you have any wiggle room with your depth of field. So don’t be afraid to use your other focus points to avoid focusing and recomposing. I won’t disagree that the center point is the most accurate but the other focus points are hands down a better option than recomposing without refocusing.
Agree or Disagree? Let me know in the comments, but do your best to keep it civil. There are people trying to learn here and we don’t need another article turning into a Canon/Nikon debate!
Are You On Google+?
I know this is off topic, but man have I fallen for Google+. If you’re still in need of an invite code, use this link to get in. If I run out of invites from that link I may still be able to get you in by email. Once you’re in be sure to add my profile to one of your circles and drop me a line. Maybe we can get some people together for a G+ hangout/discussion over this topic. Cheers!
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