Tuesday, February 17, 2009
Straight To Video
The newest evolution in D-SLR technology has finally brought us high-definition video and stills in one camera. Here’s a look behind the scenes.
While these two D-SLRs aren’t meant to compete with the robust features of a dedicated camcorder, they offer a number of advantages. From a design perspective, there isn’t much difference between designing a sensor for a D-SLR and a camcorder, but D-SLRs use larger image sensors than camcorders, and for that reason individual pixels are much larger. Bigger pixels on bigger sensors provide better image quality, as well as a higher ISO range, lower noise, better dynamic range and better performance in low-light situations. In effect, you’re getting video with bigger and better image sensors.
The image sensor for the Nikon D90 is a 12.3 -megapixel CMOS type in Nikon’s DX format. The image
comes off the sensor into Nikon’s EXPEED image-processing pipeline, then to the memory card.
For larger, more responsive sensors, quality optics are a requirement. D-SLR lenses are heavier when compared relative to camcorder lenses, but they’re more precise. Larger sensors provide shallower depth of field that’s unobtainable without expensive specialty add-ons in a camcorder. This allows for effective selective-focus photography, keeping the viewer’s attention on the subject, not to mention the creative options provided by a wide range of excellently designed lenses, including fish-eye, wide, telephoto, macro and even tilt-shift/perspective control, often found with built-in image stabilization.
“Relatively large imaging formats, such as the 24x36mm image sensor used by the EOS 5D Mark II,” explains Ranga, “require lenses that can project an image circle at least 43.2mm in diameter, whereas smaller imaging formats, such as the types used in today’s camcorders, typically require an image circle of 7mm or less. As a result, there are huge differences in lens design related to this specification. Smaller imaging formats also require shorter lens focal lengths to match the angle of view of larger imaging formats; for example, a 9mm lens on some camcorders has approximately the same angle of view as a 50mm lens on the EOS 5D Mark II. If photos are taken with both cameras from the same distance and at the same aperture, then depth of field will be shallower on the EOS 5D Mark II because of the longer focal length. Therefore, at any given combination of angle of view and aperture, the EOS 5D Mark II’s 24x36mm imaging format creates a shallower depth of field that smaller imaging formats can’t duplicate.”Camcorders Vs. D-SLRs
Camcorders offer a host of features made specifically with videographers in mind. For instance, camcorders often provide a variety of frame rates for quickening motion, slow motion and other effects—the Nikon D90 offers 24 fps and the Canon EOS 5D Mark II uses 30 fps. Perhaps the greatest limitation with video on current D-SLRs is the length of time that they’re able to record video. The D90 offers up to five minutes of 1280x720 (at the 16:9 aspect ratio) and up to 20 minutes at standard 640x424 or 320x216 (both at the 3:2 aspect ratio). The EOS 5D Mark II captures to CF cards, whose file format limits the size of a video clip to 4 GB, about 12 minutes of full HD.
There are other limitations. While taking video, the D90 and EOS 5D Mark II both use contrast-based autofocus, which is much slower than the autofocusing systems designed for still photography. In the D90’s video mode, autofocus ceases entirely once the focus has been set and recording begins, though manual focus is still possible. There are limited sound capabilities, as well. Both cameras offer a built-in monaural microphone, though the EOS 5D Mark II also offers a 3.5mm stereo miniplug jack.
Which isn’t to say that these D-SLRs don’t offer photographers an opportunity for recording excellent video that can be used for supplemental material. Even professional videographers are getting a much more portable solution that also provides great image quality and effective still images.