(If you find yourself wondering what the modes under discussion are, refer to the 1DX manual page 69, available for download here. These exact same features are also available on the 5d mk III.)
I love the single point spot AF modes in Canon’s 5D mk III and 1D X cameras. If you’re trying to get focus lock on a small point such as a burning candle, and the camera keeps focusing on the background instead of the flame, the spot AF mode can quickly solve this problem. Another great use is if you’re trying to photograph a face framed by bushes or tree foliage. In this case the camera often grabs focus on the surrounding leaves, rather than seeing through them to the face behind. Spot AF can solve your troubles and keep you moving quickly to the next shot.
I liked them so much, in fact, that I started to leave the camera set to this mode all the time.
Then I began noticing a little less consistency when shooting in low light than I’d like, so I gave CPS a ring. I’ll be going back to the normal single-point (non-spot) AF mode, thankyouverymuch.
The spot mode is great for the specific situations outlined above, but according to the CPS rep I spoke with, I’m also forcing the camera to attempt focus based on one quarter of the information available in the non-spot single-point mode. This causes more hunting and less overall accuracy. According to the rep, the spot AF mode was originally intended for macro photographers who routinely work at incredibly close distances. Yes, it works great under the conditions I outlined above, but other than that it’s best to give the camera as much info as possible.