Thursday, December 18, 2008
D-SLR State Of The Art, Part I
The line between still and video fades out as Live View evolves into HD motion video
|Cameras that feature Live View mode (L to R): Sony DSLR-A350, Pentax K20D, Olympus E-520|
In a film camera, light coming through the lens hits the film and creates a latent image, which then is turned into a visible image via chemical processing. In a digital camera, the film is replaced by an image sensor—CCD or CMOS—which captures the information and sends it to the camera’s image processor and A/D converter, which turn it into an image and pass it in digital form onto a memory card for storage. You then can display the image on the camera’s LCD monitor after shooting it. In Live View operation, the image processor sends the image to the LCD monitor, so you can see it before shooting.
With Olympus’ pioneering E-330, the camera contained two image sensors, the main one at the image plane to record images and a smaller one in the viewfinder to provide the live image to the LCD monitor. Most of the light entering the camera from the lens was directed to the main sensor, but a portion was directed to the second Live View sensor. Because the SLR mirror didn’t have to flip up and out of the way to provide live viewing, the camera’s standard TTL metering and AF system could be used. There was a second Live View mode, in which the mirror did flip up out of the way; focusing in this mode was manual only.
Sony’s DSLR-A300 and A350 use a similar system, in that there are two CCD sensors, a main one at the image plane and a Live View one in the viewfinder. When you move the camera’s OVF/Live View switch to the OVF (optical viewfinder) position, light is directed to the viewfinder eyepiece, as with any SLR. When you move the switch to the Live View position, the pentamirror tilts to redirect the light onto the Live View CCD, which provides the image for the LCD monitor.
The advantages of this Quick AF Live View system are simple operation (just flick the switch, no menu hunting needed); all camera features are available during Live View; camera performance (shutter response, AF speed) is the same as in standard non-Live View shooting; and there’s no viewfinder blackout during autofocusing. The disadvantages are that the optical finder image is a bit dimmer; the live image is lower resolution and shows slightly less than 100% of the actual image; and you can’t check depth of field or zoom the live image for fine manual focusing.
|More cameras that feature Live View technology (L to R): Leica Digilux 3, Samsung GX-20, Panasonic Lumix DMC-L10|
Some single-sensor Live View D-SLRs offer two Live View modes. In one, the SLR mirror momentarily flips down, so the camera’s standard and fast TTL phase-detection AF can be used (Canon’s Quick mode, Nikon’s Hand-held mode and Olympus’ AF Sensor mode are examples). The advantage here is that phase-detection AF is much quicker than contrast-detection AF; the downside is that the Live View is blanked while the SLR mirror is down.
In the other mode, the mirror stays up so the Live View isn’t interrupted, and AF is provided by a slower contrast-based system off the sensor, as in a compact digital camera. (Canon’s Live mode, Nikon’s Tripod mode and Olympus’ Imager AF mode are examples. Note: Imager AF isn’t available with some Four Thirds System lenses.) Some newer D-SLRs also offer a face-detection AF mode, in which contrast-based AF detects and focuses on a human face in a scene (you can manually select another face if you prefer focus to be on that one).
With Live View D-SLRs, you can tether the camera to your computer, send the live image to the computer monitor and, using the provided software, operate the camera from the computer—handy for remote and studio shooting.
Sony’s new 24.6-megapixel, full-frame DSLR-A900 doesn’t have a true Live View mode (Sony figured this pro model’s users understand and appreciate the big, super-bright 100% optical viewfinder), but provides a feature called Intelligent Preview. With this feature, accessed by pressing the depth-of-field preview button, you can see on the preview the results of changes you make to such things as exposure value, shutter speed, aperture, Dynamic Range Optimizer and white balance, without filling your memory card with test shots.
FROM LIVE VIEW TO HD VIDEO
In fall 2008, Nikon announced its D90 with the ability to shoot 1280x720-pixel, HD-quality movies, and shortly thereafter, Canon announced its long-anticipated EOS 5D Mark II, with 1920x1080 HD capability. In both cases, Movie mode is a sub-feature of Live View mode. Both use CMOS sensors because a single CCD sensor can’t read out fast enough (there are many CCD-based HD camcorders, but they’re all costly three-chip devices; a three-chip D-SLR would be prohibitively expensive).
While some might look upon movie capability in a D-SLR as a gimmick, it’s actually quite practical. Canon Explorer of Light Vincent Laforet’s now famous video, “Reverie,” was shot over a weekend with an early EOS 5D Mark II (see the video and Laforet’s comments at his website, www.vincentlaforet.com). As Laforet explained to our editor at a recent Canon event, you can put a D-SLR in places you’d never consider placing a pro video camera (hanging off the side of a car, for example). That, coupled with the creative flexibility (and relative affordability compared to high-end video) of an interchangeable-lens system, opens a whole new world of options for photographers, both the pro and the enthusiast.
D-SLR MOVIE ADVANTAGES
The image sensors in digital SLRs are much larger than those in HD camcorders (or compact digital still cameras). The resulting bigger pixels mean image quality is much better, especially at higher ISO settings, while the resulting reduced depth of field due to the bigger sensors produces a professional “look” unobtainable with a camcorder. D-SLRs also accept a wide range of lenses, including fisheye, super-wide, super-telephoto, macro and even tilt-shift, many with built-in image stabilization. And with the D-SLR, you can record a high-resolution still image at any time during movie recording simply by pressing the shutter button (there will be a brief gap in the video each time you do this, of course).
D-SLR users who want HD video capability currently have two good choices in the 21.1-megapixel Canon EOS 5D Mark II ($2,699) and the 12.3-megapixel Nikon D90 ($999)—one aimed at the professional photographer and the other at the consumer, but both very capable cameras.
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