Digital's sensitivity to color also impacted the cinematographers' work with another effects standard: bluescreen. "The HD cameras didn't record as much of the information in the blues as some of the other channels, which created little halos around objects," Miller recalls. "Those obviously had to be eliminated, so we added a little bit of white light to our bluescreens. We also found it difficult to get an even exposure throughout the whole bluescreen. It was actually easier to shoot many models with backlight/frontlight, meaning we had a white background behind the model with the beauty lights switched off so the model would go black, and we then adjusted the gamma to give less detail to the whites. That flattened out the screen's exposure without requiring a lot of additional light manipulation, which saved us some time."
Hi-def video's greater sensitivity to overexposure was another consideration. "Hi-def is said to have 10 stops of latitude, but we found that we were only getting around seven stops," Sweeney says. "You have to pay attention to your highs and lows. When I was shooting the miniature desert for the sequence where the good guys chase Count Dooku, I was trying to get a hot look on the key side, but the angle of light reflecting off the dunes changed as the camera moved and created some hot spots on the model. It only took 1/6 of a stop, which was frustrating, because film has quite a bit of range before it completely blows out. So I played the key light as bright as I could, then netted down those little specular highlights that would kick up in the desert so those areas would blend with the desert around it."
The ILM team quickly discovered that HD's inherent sharpness required the creation of models that were more finely detailed. Miller shot some early test footage of the Episode II miniatures and says they initially looked too much like models. "We quickly discovered that we had to add more detail to the models than we normally would, because film grain helps lend detail and realism to miniature shots," he remarks.
Shooting miniatures requires perfect depth of field from the lens to the rear wall of the set, and a typical miniature shot requires an exposure of 1 fps in order to achieve that. The Sony cameras the ILM team was using, however, cannot shoot below 24 fps. "We had about five times less [effective exposure] because we were shooting at 24 fps," Rosenberg explains. Adds Sweeney, "The cameras were definitely fast, the equivalent of 320 ASA, but we were stopped down in the T8 to T16 range most of the time, so we still had to pump light in to get as much focus as we could. Many of the models were larger, which allowed us to use bigger lights."
In addition to the typical lighting instruments used in photographing models Inkies, Tweenies and 1Ks the ILM crew was rigging 20Ks, 10Ks and 5Ks in the rafters of ILM's stages. "We usually hide little lights all over our sets," says Miller. "Sometimes we found that a smaller light wasn't generating enough candlepower, but a bigger light wouldn't fit; in those cases, we'd position a mirror on the wall and bounce a large light into it."
Naturally, the larger lights used to light the larger miniatures generated a lot of heat on set. Sweeney recalls that on one occasion, the heat created a problem with the camera: "I was using Par lights for the shot in the arena where the monsters are coming out the door. I needed a lot of light to make the smoke between the lens and the model show up. The Pars were about five feet away and shining directly on the camera. The camera got so hot we had to shut down for about 15 minutes and let it cool off, because I was getting some interesting electronic noise.
"It's all related to the scale of the model the arena was 35 to 40 feet across, so I was using a 20K up in the grid and pounding light into the model set. The trick with miniatures is to get the right angle of light on them, and it's harder to manipulate a 20K than a 1K or 2K," he adds. "Sometimes you end up with something that isn't quite what you want. But in the case of the arena, the larger light source acted more like the sun, which gave us a better look."
The ILM cinematographers say they look forward to the development of HD lenses with deeper stops and HD cameras with adjustable frame rates. "In order to simulate reality when we shoot miniatures, we've got to extend our depth of field, so our normal working stop is T16 to T32," Sweeney notes. "The HD lenses used on Episode II only go to T16, and like most lenses, they're not necessarily at their best at that point. And because the cameras only shoot 24 fps, we can't do long exposures."
"The amount of miniature effects work filmed at sound speed is minimal," Rosenberg adds. "The key to shooting a miniature so that audiences are convinced it was photographed in its real environment at its real size is shooting fewer frames per second. In order to simulate motion-control speeds with a 24p camera, we needed a creative workaround, which was a form of frame-blending in order to drop frames."
Longer exposures also enable effects cinematographers to match motion-control moves across models with live-action camera moves. In effects, it's often impossible to match-move the camera, the rig or the model in real time when the plate cameras are moving many times faster over real distances. But by shooting fewer frames per second, the live-action move can be "cheated" via motion control on film, that is. It's another story with digital. "Normally, we run our motion-control rigs at 1 fps because of the long exposure times needed," Miller explains. "But the digital camera is running at 24 fps, which means that if the rig is traveling at 1 fps, you suddenly have 24 times more frames than you need. We had too much footage, but we experimented and eventually figured out a way to make the format work for motion control."