Phase-detection autofocus is a bit sloppy

May 13, 2023

Today I learned that phase-detection autofocus is fast and accurate, but not precise—the resulting focus when you hit that AF button is a tiny bit sloppy. When that AF beep is heard, your focus will be ever so slightly front or back of the actual focus point. I’ve even seen evidence on my lenses that that where you left your focusing ring the last time affects whether the next autofocus is front- or back-focused.

This means that you should never simply take the first number that Automatic AF Fine Tune gives you. Instead, take a dozen or more readings and average them out to get a feel for your lens’s focus bias, then manually adjust the fine-tune setting. Take a few photos of a sharp, stationary target, and pixel-peep until you get the setting about right. If you’re like me, you’ll get three super sharp shots out of four, with the last one just a bit soft. Here are my takeaways when using phase-detect autofocus:

  1. When shooting with wide aperture, take a bunch of shots, and hit that AF button each time. Remember, phase-detect AF is probabilistic in nature.

  2. DoF is your friend. Stop down that aperture a bit more to give yourself the best chance of getting your subject in sharp focus.

  3. Don’t bother with AF Fine Tune unless you notice that your shots are consistently back- or front-focused. Most lenses hover around 0 bias out the door unless they’ve been abused.

  4. When shooting a still subject with a tripod, use Live View to focus using contrast-detection. It’s much slower, but it will never misfocus. Alternatively, use manual focus and Focus Peaking.

  5. Don’t pixel-peep too much. As long as your photo looks pretty good on a 2 MP screen, it’s probably good enough. With cameras pushing 48 MP+, even the most insignificant misfocus will look awful at 100% magnification.