Originally Posted by Greg Rogers WSR Ruby Review
The Auto Iris provides a dramatic improvement in virtually all dark scenes. When the droids are imprisoned in the Jawa transporter there is substantial bright image detail, but the background is hazy, which obscures image definition and depth in the Iris Off mode. In the Iris On mode the black level is lower and the haze is gone, but the detail is also darker, so the image is still partially veiled. But in the Auto mode the detail is brightened to its original levels and the image contrast is much improved.
The dynamic iris produces marvelous images in dark scenes, but there is a price to pay for this performance. An ideal Auto Iris would always maintain the same image brightness that is produced in the Iris Off mode, but that’s not possible. The iris aperture would be fully open whenever there is a peak brightness signal, and then reduce to minimum size to lower the black level in the darkest scenes. At an intermediate scene brightness the iris aperture would be partially reduced to improve the contrast in those scenes. However, image brightness is also reduced when the iris aperture is reduced, so the signal levels must be increased to maintain the original brightness levels. But it’s impossible to produce the original maximum brightness level when the aperture is reduced, and as the aperture is made smaller the peak brightness that can be created becomes lower. So, to maintain the original brightness levels in the darker areas, the brightest levels must be compressed to avoid clipping as the aperture size is reduced. In effect, the contrast in bright areas is reduced for improved contrast in dark areas. When the iris aperture is reduced in scenes that have very bright areas there is a substantial “brightness compression” artifact as the brighter levels are compressed together and bright detail is lost.
The trash compactor scene in Star Wars is an example of a moderately dark scene where the contrast is considerably improved without significant brightness compression. But, in the opening scene of Star Wars, after the star cruiser flies by, there is brightness compression in its engine nozzles. There is detail within the engine nozzles using the Iris On or Iris Off modes, but when the Auto Iris mode is enabled that detail disappears, and there is a white glare surrounding the engines. A more dramatic example of brightness compression occurs as the first battle is about to take place on the ship. The fixtures lining the white interior walls are visible through most of the scene, but just prior to the storm troopers bursting through the door the fixtures are almost completely obscured by brightness compression. If you watch this scene, you can’t miss it, because suddenly the fixtures on the wall practically disappear when the Auto Iris is enabled. These examples illustrate that, in some cases, you may not realize that the image has been degraded unless you have previously seen it without the Auto Iris mode enabled. In the last example, the brightness compression is unmistakably obvious.
The most common situations that produce severe brightness compression are brightly lit background windows, lamps, and light fixtures. In the opening scene of Back To The Future, the detail in the brightest windows in Doc’s lab is obscured, but the detail in the darker windows is mostly unaffected. Later, the bright white portion of the Twin Pines Mall sign blurs the edges of the green pine trees on the sign and narrows the lettering on the sign. On the other hand, the contrast in the wet parking lot is improved, as is the contrast in many other scenes, including when Marty plows the DeLorean into Peabody’s barn and the farmer stands at the doorway holding a bright lantern. The bridal suite scene at the beginning of Austin Powers: The Spy Who Shagged Me provides examples of how brightness compression affects lamps and light fixtures.
For every example of brightness compression there are more examples where the Auto Iris produces virtually CRT quality images. Dark City looks incredible, as I have never seen it before on a fixed-pixel projector. Manhattan also looks spectacular, with an exceptional black level and excellent contrast as Woody and Michael Murphy walk along the street after leaving Elaine’s. There are some occasional examples of brightness compression, but they are surpassed by the improved contrast and image depth throughout the film.