Low light situations are notorious for wrecking havoc on your camera's ability focus properly. As light levels (and contrast) drop, focus errors can become more and more common. This is especially true when using fast zooms and prime lenses wide open, where the shallower depth of field will reveal poor focus with brutal clarity. Believe me, as a music photographer, shooting in very low light is basically my job!
If you've struggled with achieving dead-on focus with fast lenses when shooting at wide apertures, here are 5 tips for making a better focus lock.
Whether you're shooting live music photography in a dimly lit venue or simply want to get the best AF performance from your camera and lenses, these tips will help you nail focus more reliably and with better precision.
One note on using fast f/1.4 or f/1.8 primes. One thing I've heard over and over from newer photographers is that their prime lenses don't “focus well” wide open at their max aperture. The real truth is that prime lenses simply require better AF technique. A lens will focus as well or as poorly wide open as it does at any other aperture. Focusing errors (both due to uncalibrated equipment or operator error) are simply made much more apparent with the razor thin depth of field produced at wide apertures. If you're new to shooting with primes, these tips will definitely help you.
Calibrate your DSLR
This first tip is pretty self explanatory, but it bears notice. All lenses should be calibrated to fine tune the AF performance, and fast primes are no exception. This can be done with focusing aids like the Datacolor SpyderLensCal, but you can use more informal methods to get your cameras and lenses dialed in. Use the AF micro-adjustments settings to compensate for any tendency to front or back-focus and you're in the best position for more reliable and accurate AF performance.
A common source of focusing errors in low light is competition of things that are higher contrast than your subject. This could be a light behind the subject, or an object that's slightly in front of them (for live music photography, microphones are notorious for fooling AF sensors and producing poor focus). Because AF sensors are actually much larger than what's indicated in the viewfinder, it's very easy for them to lock onto a false target if the contrast is high enough. Giving potential competition a wide berth or changing your angle slightly to give the clearest view to your intended point of focus will help ensure the best results.
Target High Contrast Areas
Contrast is everything with accurate autofocus. To ensure the best results, use areas of high contrast to focus on your subject — defined edges and hard lines of contrast will allow AF sensors to best achieve precise focus. If your subject has lower contrast, look for look for areas of high contrast that are in the same plane as what you want in focus. This might mean autofocusing on a pattern on a subject's shirt instead of their face, or a sharp shadow on their ear instead of the eyes. When using alternate AF targets, make sure they as close to the same distance away from the lens as your intended point of focus to make the best use of this tip.
Use the Center AF point
Not all AF points are created equal. In virtually all DSLRs, the center AF point is the most sensitive and accurate from the factory, due to calibration and the type of sensors used for the central point. Use this to your advantage by prioritizing the center AF point over other points for more precise and reliable AF performance. To get an even better understanding of your gear, dig into the manual of your DSLR and find out which AF sensors are the more sensitive cross-type AF points (two dimensional sensors, across vertical and horizontal axises) so you know which points will be most accurate.
Don't Focus and Recompose (UPDATED)
Updated because my friend Danny North made me.
The method of focusing and recomposing is something I personally use extensively. That is, achieving AF lock on a subject and then recomposing for the preferred composition. That said, if you are experiencing trouble with precise focus, the focus and recompose method may be to blame. Using the focus-recompose method will introduce focusing errors — it's simply a matter of geometry.
It's best to compose the frame as closely as possible when achieving focus, without dramatically recomposing the scene when you press the shutter release. The best practice is to pick an AF point that is closest to the point of focus you want, and which will create the smallest change in focus when you recompose. Any recomposition after achieving AF lock will result in focusing errors — with this technique, it's about understanding the limitations of precise focus.
Distance and depth of field play a critical part of whether focusing shift will be perceptible when focusing and recomposing. The shallower the depth of field, the more the dramatic the shift in focus resulting from recomposing will be.
If you're struggling with nailing AF, I hope these suggestions help. The last two pieces of advice — to use the center AF point and to understand the limitations of the focus-recompose method — are slightly more editorial (and even contradictory). Rarely does a great photo have its subject so conveniently in the middle of the frame that one can use the center AF point without recomposing.
Ultimately, timing, composition, and a compelling subject all trump technical perfection. I'll take a slightly out of focus with a killer composition over a bland framing with tack sharp focus any day of the week.
For more reading on understanding autofocus, definitely check out this great article by Cambridge In Colour.