Most broad-area AF systems I've encountered are biased toward focusing on whatever is closest to the camera. In this case it's the guy with the binoculars.
During my brief vacation Massachusetts I decided to experiment with the way I normally focus. Instead of using only the center AF point and then recomposing I decided to try using all nine of the AF points in my EOS 30D so I wouldn't have to recompose.
In theory this should be faster and more accurate. In practice it was neither. Because it's difficult to gauge the visual sharpness of the focusing screen, I found I had to pay close attention to which focusing points lit up. They stay lit for less than a second and aren't always easy to see in bright light, so confirming focus is often easier said than done.
And then there's the tendency of automatic systems to focus on whatever is closest to the camera or offers the highest edge-contrast. This isn't much of a problem when you're focusing on a flat surface. When you're focusing on three-dimensional objects though, you'll often find that the camera focuses on the foreground when you really wanted to focus on the background--or vice-versa. The accompanying photos illustrate this nicely.
In case you're wondering whether it might have been better to focus on the guy in the background, here's the result. The question is, which do you prefer? Or would you have stopped down enough that they were both in focus?
If you shoot with a small aperture, say f/11 or so, you'll often have enough depth-of-field that you can focus somewhere in-between the foreground and background so that both will look reasonably sharp on screen or in print. But once again, targeting a specific area isn't easy when you're using nine or more focus points.
Finally, as mentioned in a previous post, it's not easy to assess depth-of-field through the viewfinder. Things look more in focus than they really are, especially when you're using medium or short focal length lenses at medium-to-far distances and shooting at wider apertures such as f/4 or f/2.8. There's not enough depth-of-field for everything to look in focus, but not so little that you're obviously using a selective focus technique.
The bottom line here is that as wonderful as automation can be, there's still no substitute for keeping your brain fully and actively engaged while photographing. Put some thought into what you're trying to achieve and the most effective way of achieving it, then make sure your camera is doing what you want it to.
As for me, I'm going back to focusing in the center, then recomposing (except when it doesn't make sense to do so, of course). I'm also going to be a lot more careful about how I choose apertures. (No more Program mode, thank you.) I'm going for either obvious selective focus or generous depth-of-field, without much in-between. It's time to show these damned cameras who's the boss. Wish me luck.
I've just discovered your blog Gordon. Quite interesting material! As to focus...well don't get me started. In the last few months I purchased my first DSLR. What held me back so long was the *expletive deleted* focusing screens on these plastic blobs. I valued small and light and adaptability for my old lenses so I ended up with an E-410. I can manually focus with my older lens, but with spotty results. It's not the small size of the viewfinder but the toothless screen. There is the option of putting in an after market screen but I just hate split-wedge screens. I would really like a matte with grid lines designed for manual focusing but don't hold out much hope for such a low demand item.
Posted by: john robison | June 13, 2009 at 11:24 PM
One word: rangefinder.
Posted by: Andreas | June 14, 2009 at 10:03 AM
I enjoyed your insight on this. Left me with the feeling that I've been there before. With my 350D, I only used the center-point, then recomposed, since it was the only high accuracy point. In my 50D, I use all 9 AF points, and select them on-the-fly by means of the joystick, photo by photo. Is focus accuracy compromised? I read that the focus-recompose technique may lead to severe focusing errors, particularly when the subject being focused is near the edge of the frame (aspherical designs?).
Can you shed some light on this matter, according to your own experiences? Thanks!
Posted by: Bruno Monteiro | June 15, 2009 at 01:09 PM
Just a quick question for you.
You say that you only use the center point for focusing, but much of what I have read online suggests that such a method will give less than optimum focus. I'm wondering if you've had any issue with this in your shooting and, if you have, if you've developed any means for dealing with it.
Posted by: James B | June 15, 2009 at 01:23 PM
Using the center-focus point and then recomposing does introduce focusing errors in proportion to how close you are to the subject and what f-stop you're using. The closer you are and the wider your f-stop, the more problems you'll have. Focal length is also a factor: The shorter the focal length, the closer you can be and/or the wider the aperture you can get away with.
I can't get much more specific without writing a whole new post. Suffice it to say that when I'm shooting three-dimensional subjects at less than 5 feet or in the macro range I switch to whatever focus point requires the least amount of re-composition. I also try to place the focus point over something that has high-contrast texture or an edge for the AF system to grab onto. When I'm shooting a subject that's basically flat and parallel to the image plane I simply stick with the center point and stop down enough that the corners will be reasonably sharp. I hope this helps.
Posted by: Gordon Lewis | June 15, 2009 at 01:38 PM
That is pretty much what I'm used to. Any DoF calculator will show you how accurate you need to be (generally, could be around 1-5 cm) in close-range so focus gets nailed right.
Thanks for your shared thoughts!
Posted by: Bruno Monteiro | June 18, 2009 at 10:53 AM