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Tuesday, October 23, 2012

Exposure For HDSLR Video

How to get your motion footage to look just right

When shooting motion with your DSLR, exposure suddenly becomes a bit more problematic than you may be used to. Looking at the main variables—shutter speed, aperture and ISO—you'll be making fine adjustments to all of them to achieve a good-looking image.

If there's one thing I haven't been sweating too much in the 11 or so years since I went from shooting slides to shooting digital, it's exposure. Don't get me wrong, I'm careful, and 30 or so years of shooting chrome forced me to be pretty good at figuring out my meter and the light. In fact, I shoot JPEG and RAW, and rarely have to resort to the latter due to an exposure error.

But shooting HDSLR video has rocked me from my exposure complacency for a couple of reasons.

First, we're totally back in JPEG territory. Yes, the 24 or 30 or 60 progressive frames that we shoot when we create video are, for all intents and purposes, highly compressed JPEGs. And highly compressed JPEGs, for all of their conveniences, aren't all that forgiving of exposure error.

In the video world, we talk about bit rate and not bit depth, and most of our DSLRs shoot in the pretty meager 25 Mb/s range (unless you have a hacked DSLR or a real video camera). Minimum broadcast requirements for outfits like the BBC are 50 Mb/s and up. What this means in real-life terms is that you can't really start messing with your footage in terms of changing exposure, color balance and filter effects too severely before it just starts falling apart and artifacting.

Secondly (and cue movie-trailer deep-voice guy), "Imagine a world where there is only one shutter speed." Yes, that's right, when you're shooting video, you have one ideal shutter speed (and it's twice the frame rate of your video), with maybe one speed above or one below in latitude before your video starts looking funky. So if you're shooting at 24 frames per second (the standard for "cinematic-looking" video), your target shutter speed is 1/50th of a second; 60 frames per second would be 1/125th of a second.

This "shutter speed as two times the frame rate" rule is called the "180-degree rule," and it developed because the amount of blur within each frame at those speeds looks natural to the eye as those 24, 30 or 60 frames go flickering by.

Go much slower than that, and your moving subjects appear to smear and look ghostly (a technique filmmakers use in dream scenes, flashbacks and hallucination sequences). Go much faster than that, and you get a very jumpy, staccato, hyper-real feeling (a technique put to great use by Steven Spielberg when he used a 1/1000th of a second shutter speed in the 24 frames per second opening of Saving Private Ryan).


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