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MillimeterHDV@Work
Aug 22, 1997 11:40 AM
What is Optimal Exposure? Part 2 of 4
Steve Mullen
In Part 1, we examined the issues involved in obtaining an optimally exposed image. Now we will look at video gain and shutter speed.
The purpose of video gain is to increase picture brightness without the hassle of increasing scene illumination. Unfortunately, at high gain values there is increased picture noise. Within a scene, we do not want shot-to-shot variations in video gain. That's because although varying the gain values will result in consistent shot-to-shot brightness, this might also yield shot-to-shot variations in video noise.
Therefore, video gain ideally should be set on a “location” basis. However, it is typical for lower-cost DV/HDV camcorders to automatically, and with no warning, add varying amounts of gain whenever the auto-exposure (AE) system demands it.
Though this certainly is not ideal, the low sensitivity of the very high-density CCD/CMOS chips used in HDV camcorders means that maximum gain is often required to obtain a reasonable picture. Thus in many situations with HDV camcorders, the AE gain setting will be identical to a manually set gain. The advantage to a gain level you set is that it can be constant throughout a series of shots.
While shutter speed certainly controls exposure, it has a more fundamental role of altering the temporal look of video. Nominal shutter speeds typically are set to be the reciprocal of the frame rate. Unless one has a Varicam, that means 1/48 second for 24fps, 1/50th second for 25fps, and 1/60 second for 30fps.
A perfect example of creative shutter speed is the Saving Private Ryan strobing look, achieved by increased shutter speed. A slow shutter speed, of course, adds motion blur. Shutter speed, therefore, should be seen as a creative tool, not an exposure-control tool.
Unfortunately, camera engineers often use shutter speed as the primary way of adjusting exposure. They design cameras to operate in “shutter-priority” mode. They do so because very accurate exposure can be achieved when shutter time is controlled by the AE system, on a frame-to-frame basis. Controlling the mechanical aperture via a servo motor is less responsive and may be less accurate.
The problem with shutter-priority operation is that as the camera controls exposure via shutter speed, it simultaneously alters the temporal look of the video. In short, there might well be visual side effects to exposure control via shutter speed.
In Part 3, we will look at the role of aperture control in obtaining an optimum exposure.
VASST Announces GearShift 1.5, HDV utility plug-in for Sony Vegas
VASST, a leading provider of training resources for multimedia professionals and hobbyists, today announced at WEVA, the release of GearShift 1.5. The new VASST offering is a fully featured HDV utility available for Sony Vegas software.
GearShift is currently the only HDV conversion utility for any NLE on the market, giving Sony Vegas users an unparalleled workflow for HDV editing.
GearShift 1.5 allows users to:
Convert .m2t files to 4:2:2 YUV;
Create a DV proxy for portable editing;
Create a 24p stream as a 4:2:2 YUV, 4:2:2 Cineform and/or DV proxy;
Create a 25p stream as a 4:2:2 YUV and/or DV proxy;
Convert a 1080i stream to a 720p stream;
Have batch-based access to all formats, including CineForm encoding;
Correctly replace proxies with HD media as either an .m2t or a 4:2:2 YUV file;
Extract media from various locations, and remember those locations when shifting gears;
Render regions and discard unused portions of video, saving disk space and render time;
Apply correct color processing for the HD color space to the DV proxies.
“We’re thrilled to see an HDV plug-in on the market that takes advantage of the expansive potential and adaptability of HDV,” said Leigh Herman, Sony pro AV product manager. “GearShift gives Vegas users a faster and more accessible approach to using HDV in an NLE environment.”
GearShift 1.5 is available via electronic download for $49.99. A one-week only special price of $39.99 is available by entering the discount code WEVA during the purchase process on vasst.com. Existing GearShift users may download the new version for free. Users who wish purchase GearShift or try it before buying may download the 14-day, full-functioning trial at http://www.vasst.com/search.aspx?text=gearshift. Visit VASST at www.vasst.com
Bonolabs introduces film-to-HDV telecine option
Arlington, Va.-based Bonolabs has added a film-to-HDV option to their tapeless high definition telecine services. This can be used to integrate clips that originated on film into HDV productions, and hi-def telecine transfers can be received on a mini FireWire /USB drive with 1080i or 720p files encoded and ready-to-edit. This service, like Bonolabs' tapeless options of 10-bit uncompressed and DVCPRO HD, is available for all film formats, whether 35mm, S16, 16mm, S8, or R8.
For those who are in the middle of a hi-def production and need a way to integrate essential material originally shot in NTSC, Bonolabs recently added upconverts from Digital Betacam to 720p or 1080i direct to drive or out to HD D5 tape.
Bonolabs is the technical initiatives division of Bono Film & Video, Inc., a family-owned and -run postproduction facility founded in 1961.
For more information on these or other high-definition services, go to www.bonolabs.com.
Continue the discussion on “Crosstalk” the Millimeter Forum.
