Conversations in Light: Focusing in Low Light
You’re just walking along with your camera, and then your heart starts racing as you see it…you instantly recognize that what’s in front of you will make a fantastic photograph! You whip the camera to your eye, carefully compose your image, depress the shutter-release button, and then—nothing. The lens tracks back and forth, hunting for focus. The annoying whir of the lens fills your ears and you’re left not with a great photograph, but with a gnawing frustration that this advanced and sophisticated camera couldn’t focus when you needed it.
Focus is a critical aspect of creating a stellar photograph, but there are times when the autofocus system of today’s DSLRs is challenged. Though they’re precise and sophisticated and work most of the time, modern cameras have their limitations. It’s important to understand what these limitations are in order to ensure that you capture that next great image.
Most modern DSLRs use a phase-detection system for focus detection. This design works by dividing the light coming through the lens into two separate images, which are then compared. Here’s how: A beam splitter is used to direct the light through a semitransparent area in the main reflex mirror and to a secondary mirror, which directs the light to the autofocus detector (often found at the base of the mirror box). The resulting two images are compared for similarities and a separation error is determined, which helps
to establish the direction the lens needs to be turned to achieve focus.
Contrast-based focus is more commonly found in compact cameras and on DSLRs when using the camera’s Live View function. Focus is achieved by analyzing differences in adjacent pixels. It can be a slower and less precise system than phase detection. Though different manufacturers will boast about their autofocus systems and how many autofocus sensors they’ve built into their camera bodies, they all share something in common: They’re dependent on light and contrast in order to achieve focus. Without enough light and contrast in the subject or the scene, the camera system will find it difficult, if not impossible, to detect focus.
The Real World
If you have an abundance of light and contrast, your camera will often have very little difficulty achieving focus. Whether you’re using multiple sensors or a single point, having a wealth of light and detail will likely result in a tack-sharp image. The focus problem commonly occurs, however, when light levels drop. The loss of light and reduction in contrast and fine detail quickly result in the camera laboring to lock focus.
One way to improve your camera’s ability to focus in low light is to use faster glass. A zoom lens with a maximum aperture of f/2.8 or a fixed focal length with an f-stop of f/1.8 provides better light-gathering capability than a variable-aperture zoom, especially a standard kit lens that may have a maximum aperture range of f/3.5–5.6. A faster lens, which provides several stops more of light gathering, will markedly improve your camera’s ability to accurately detect focus.
Some cameras feature an autofocus-assist light that, when enabled, emits a light on the subject. This brightens the subject, increases contrast, and provides the camera with the needed information for focus detection. Some advanced and professional DSLRs don’t have this feature and are instead dependent on the autofocus-assist light that’s emitted by a camera-mounted flash. Though sometimes distracting to the subject, these systems do help the camera focus under challenging lighting conditions.
Old School Remedies
When all else fails, there’s always manual focus. If light levels are particularly bad, and if one’s eyesight isn’t what it used to be, this can still be difficult. This is where the use of hyperfocal-distance focusing can be of help. By simply estimating the distance of your subject from the camera and using an aperture with a good degree of depth of field, you can still achieve acceptable focus. This is especially true when using focal lengths of 50mm and wider which, because of their optical qualities, already provide generous depth of field, even at moderate apertures, such as f/8 or even f/5.6.
Before TTL or automatic flash metering, photographers would determine the optimal distance for a flash exposure and try to maintain that distance for an accurate exposure. Though not ideal, this solution can certainly serve in a pinch. Another alternative is to point the camera at another object that’s approximately the same distance to the camera as your subject, but is brighter or possesses more contrast. You can then lock focus, recompose on your subject, and shoot. As long as the camera-to-subject distance is maintained, you should be able to achieve acceptable focus.
Although there’s no way to achieve 100% performance in low light, you can increase your chances of achieving good results. By understanding the limitations of your camera and remembering that you have alternative solutions, you should be able to keep the frustration, aggravation, and out-of-focus pictures to a minimum.
If you prefer to use flash off-camera, the autofocus assist is no longer an effective tool. Though the flash output will still be illuminating your subject, the autofocus-assist light may no longer be targeting the same area where the camera is attempting to detect focus. In this case, camera manufacturers offer some accessories that may help.
Nikon has the option of using a tethered TTL flash with an SC-29 cord. The module that connects to the camera’s hot shoe has the benefit of a built-in autofocus-assist light, while the other end has a hot shoe on which the flash is attached. This provides the creative control of positioning the flash off-axis while still aiding the camera’s ability to focus under low light. The big plus here is the ability to maintain TTL flash metering.
Canon doesn’t have such a cable; instead, it offers the ST-E2 wireless flash transmitter, which is designed to trigger a Canon Speedlite using a line-of-sight infrared trigger. Though some of the latest Canon cameras provide the ability to control wireless Speedlites in-camera, the ST-E2 also includes the autofocus-assist light, which helps improve focus detection.
Photographers who use radio triggers to activate their flashes or strobes don’t have the benefit of any autofocus assist, unless this feature is already built into the camera. For such a setup, you might want to consider DeluxGear’s PinPoint Focus Assist. The PinPoint Focus Assist attaches to your camera via its tripod socket, and it emits a narrow green light beam, which provides the increased brightness and point of contrast to help the camera detect focus.
Now that you have a better handle on your low light shots, here are some tips from RC Concepcion for Lightroom. “5 Smart ways to use Smart Collections“