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Aliasing and Moire

Another issue to be aware of before diving headfirst into the DSLR world is the problem of aliasing (moire is an offshoot of the same problem). Aliasing is a problem with DSLRs because the cameras were designed primarily to shoot still photos at much higher resolution than their video mode allows. So, in order to drop from 5616 x 3744 in still mode to 1920 x 1080, the camera skips lines. Ideally the camera would sample all of the pixels and average them out, giving you a smooth image (think about resizing an image in Photoshop and choosing Bilinear or Bicubic); instead, because DSLRs lack the processing power required to do this in real time, they just toss every other line or so (think about resizing an image in Photoshop and choosing “Nearest Neighbor”). What you’re left with are unseemly jaggies. The end result is most commonly seen on thin lines and patterns and ranges from overt to invisible. For an example, watch the gray rooftop here:

The first thing you should do when switching your DSLR from still to movie mode is to make sure the Sharpness setting is turned all the way down. This won’t come close to eliminating aliasing under all conditions but it should reduce your camera’s problems. As the chips inside these cameras get more powerful, we might see a HDSLR that does some in-camera image sampling; but to eliminate aliasing problems, the best solution might be to actually lower the camera’s resolution (this is one of the advantages of a 5-megapixel RED camera over a 21-megapixel DSLR).

For more on the technical reasons behind aliasing and moire, see Barry Green’s post at DVXuser and Stu Maschwitz’s follow-up (I commented on both as well). For an eyeful of ugliness you can also click on the image at top right, which was one of the first things I shot from my NYC rooftop on a 5D; that brick building is not ensconced by gray concentric circles.