Roll and Judder: Keeping Widescreen on the Level
Shooting widescreen video with a handheld camcorder is made more difficult by their side-of-the-camera grips. Worse, the heavier the camcorder, the more a side grip contributes to hand strain as one tries to keep the widescreen shots level. Folks try to solve this problem by using the swing-out LCD as a second “handle” even though the panel was never designed for this function. However, this problem need not exist.
In the old days, before a handheld video camera could even be imagined, all but the cheapest film cameras had some type of “pistol grip.” The virtue of this type of grip, compared to the side grip on today''s handheld video camcorders, becomes obvious in the pictures at right.
Because the grip is positioned at a point centered under the camera, the tendency for the camera to “roll” (side to side) is minimized. Of course, with a Bolex 8mm H8 and 16mm Rex, the camera is top-heavy, which presented a different problem.
With cameras such as the Bolex Regular 8 camera (shown above), both “roll” and “pitch” (front to back) were well controlled. (I''ve owned all the cameras pictured, and the Bolex K2 was my favorite.)
Naturally, the pistol grip caused problems when a camera needed to be stowed in a bag. The Beaulieu Super 8 Model 4008 ZM Super 8 (shown above) tried, unsuccessfully, to solve this problem with a short grip.
The Minolta XL-Sound 84, a Super 8 camera (shown below), used a more successful angled folding grip that was kinder to my wrist.
At NAB 2007, I was approached by Bob Franco, who showed me the Camhandle he has developed and is selling at his website, www.camhandle.com. Below, you can see how the Camhandle stabilizes a camcorder.
Because the Sony HVR-V1U functions so well in automatic mode, it works well with the Camhandle. However, when you do need to use your left hand to make an adjustment, the handle can no longer stabilize your camcorder. It''s great to see folks working on aftermarket solutions; however, it seems to me that as with film cameras, the ultimate solution must be designed and implemented by camcorder manufacturers.
Lightweight camcorders that rest on your shoulder are one way to solve the problem—assuming the shoulder pad is shaped to hold the camera level. (Unfortunately, most shoulder cameras have a thin, flat pad that requires one to hunch a shoulder to keep the camera level.) Sony may soon announce the 3-CMOS HDV camcorder shown below—perhaps at IBC 2007.
Video Judder
Ever since Panasonic introduced its AG-DVX100—the first low-cost camcorder that shoots 24fps—folks have posted on listservs that “24p video has more judder than does film.” These posts are answered in a consistent manner:
- “You must set your shutter speed to 1/48 second, not 1/60 second.” When the person responds that it was set at 1/48, the response becomes:
- “You must shoot following all the rules in the Cinematographer''s Handbook.” When the person responds that he or she has shot film and knows how to shoot 24fps, the response turns dogmatic:
- “If you set the shutter speed at 1/48 and shoot correctly, there simply can''t be more judder with video than with film.”
Privately, over the years, many of us have acknowledged to each other that although 24fps footage from the CineAlta, XDCAM HD, and Varicam do look like film, some (but not all) low-cost 24p/25p camcorders do record a “stroby” picture.
The reason for our perception is found in a BBC document . The document states, “Judder is inevitable in any display system that shows source images more than once"—as is necessary with low-temporal-rate video to prevent flicker—"but the perception of it depends on the sharpness of the images.” And, “The sharpness of object edges is controlled by the system MTF.” (Italics mine.)
The document goes on to say, “By comparison [with film], video cameras maintain their frequency response ... in the middle frequency range ...” And, “It is this difference, the middle-frequency image content, that dominates the perception of image judder, ... and is the reason why video-sourced images are often considered to judder more than does film.”
Lens MTF
Many shooters buy or rent expensive lenses that have an extended MTF that transmit images with lots of fine detail.
The presence of fine detail creates a balance to the presence of mid-frequency detail that represents object edges. Inexpensive lenses pass only the mid-frequency —edge—detail.
CCD MTF
There is a fundamental difference between film and video cameras. Film has a fairly flat frequency response, as shown at right.
Digital video cameras, however, sample the image. The number of horizontal samples provided by the imager defines its sampling “rate.” According to the Nyquist theorem, there must be two samples for each cycle—vertical line pair—in the signal being digitized.
Therefore, the sampling rate—called the Nyquist sampling frequency—must be twice the detail frequency. If higher frequencies—finer detail—are digitized, they will be recorded as low frequencies called aliasing.
To prevent aliasing, a low-pass “anti-aliasing” filter is placed in the analog path between the sensor(s) and the analog-to-digital converter(s). Because no filter can truncate a signal exactly at the Nyquist frequency, the filter cutoff frequency must be set at a slightly lower point, thereby causing an unwanted loss of fine detail. The more sophisticated (read: expensive), the steeper the filter''s slope and the less fine detail is removed and, therefore, the higher the system MTF.
Sensor resolution also plays a role in system MTF. Specifically, the ratio of capture resolution and recording resolution. A 1:1 ratio, or higher, is typically the case with camcorders that record 720p. A different situation exists with 1080i camcorders. Expensive camcorders typically have a 1.33:1 ratio because they employ three CCDs with 1920 pixels and record at 1440 pixels. Other camcorders, however, have only a 0.67:1 ratio because they use sensors with only 960 pixels. Camcorders with 1920x1080 CCDs, therefore, have a large ratio. The greater the ratio, the higher the Nyquist filter cut-off frequency—and the higher the system MTF.
The chart below schematically shows the theoretical bandwidth of film (blue), the frequency response of an expensive HD camera (green) that has a high MTF, and the response of an low-cost HD camera (red) with a low MTF.
As shown above, a low MTF camcorder captures little fine detail. The solution, shown below, is to equalize the camera's frequency response by boosting the mid frequencies.
This is called "aperture correction" and can be adjusted to some degree by the camera's Sharpness control.
While some fine detail is restored by aperture correction, edge sharpness is also significantly increased. Because moving edges carry judder, judder is also amplified. (I also suspect hard edges, when objects are moving very rapidly, stress the MPEG-2 encoder, thereby causing macroblocking. This may be why macroblocking is so rare on film-sourced HDTV.)
Additional judder—background judder—is due to the huge depth of field from low-cost camcorders that employ tiny image sensors.
It is not possible to reduce judder—as many try—by simply cutting sharpness to the minimum, because this results in a severe loss of fine detail.
Another option is to shoot 1080i60/p30, where the higher frame rate inherently slightly reduces judder. Another advantages of 30p is that no pulldown removal is necessary. You can edit in either a 30p or 60i timeline. Therefore, unless you plan to convert to film, a great option is shooting 1080i60/p30 with its low temporal rate.
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