Sunday, December 6, 2009

Basic Tech: HD Video And Autofocus

By William Sawalich Published in SLRs
Basic Tech: HD Video And Autofocus
If, the first time you picked up an HD video-enabled DSLR, you blasted full-speed ahead into figuring out how to shoot video without reading the owner’s manual (as I did), you were undoubtedly surprised to discover how poorly your camera focused (as I was). There’s a misconception that DSLRs can’t autofocus when capturing video. In fact, the cameras do have autofocus built in. It just sometimes pales in comparison to the fast and accurate autofocus some of these DSLRs utilize when capturing stills.

Even inexpensive camcorders autofocus, while many of the relatively expensive HD-capable DSLRs don’t. So what’s the deal? Well, let’s get one thing clear: DSLRs with video aren’t intended to replace camcorders. And while professional videographers tend to focus and adjust exposure manually, the ability to autofocus is crucial for many photographers.

When users got their hands on the first DSLRs with HD capabilities—the Canon EOS 5D Mark II and the Nikon D90—the less-than-exciting video AF capabilities were a letdown, especially compared to the awesome continuous autofocus these cameras have when shooting stills. Photographers had to give up highly evolved autofocus to focus manually or to use autofocus before recording began. But why?

A DSLR with an optical viewfinder also has a mirror inside the camera serving both the photographer’s eye and the camera’s autofocus. The mirror simultaneously reflects light into a very fast and accurate phase-detection autofocus system housed in the mirror box. When a still exposure is made, the viewing mirror flips up and out of the way, allowing light to travel through the shutter to the image sensor. Camcorders don’t have mirrors, however. They use contrast-detection autofocus that’s able to read right off the sensor. It works, but contrast detection can be slow and inaccurate compared to phase detection—particularly in low light.

In DSLRs, live-view technology paved the way for video recording and auto-focus. The ability to send a video signal from the sensor to the LCD also makes it possible to see what the sensor sees even while the mirror is up—in a still camera’s Live View mode or during HD video recording. In live-view still shooting, autofocus already relies on the more rudimentary contrast-detection approach. This camcorder-style autofocus also takes over during video recording.

Some cameras, such as some Micro Four Thirds models from Olympus and Panasonic, never had mirrors in the first place. These cameras have utilized increasingly fast and accurate contrast-detection autofocus to compete with larger viewfinder-sporting, phase-detection autofocus DSLRs. Consequently, these cameras added HD video capabilities and found themselves already well prepared to autofocus during recording.

Compared to the earliest HD video DSLRs (from about a year ago), autofocus during video recording is improving by leaps and bounds. Many cameras already incorporate specialized autofocus features into video recording.

There’s an additional focusing consideration for photographers capturing video with any digital camera, and that’s how it affects audio recording. Using the camera’s internal microphone, every focus noise is amplified, making the silence of a quiet autofocus motor even more important. Professional video lenses have long been quiet, and this trend is likely to continue as DSLRs become hybrid cameras. Until the lenses are absolutely silent, prefocusing and subtle manual focus adjustments still have a place.
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