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December 4, 2009

DSLR aliasing: not seeing the forest for the trees

I unpacked my brand new Canon 5d Mark II a few months ago and excitedly clambered onto my Manhattan rooftop to put it through some tests. Within minutes I discovered the nasty aliasing artifact seen above, wherein the uniformly brown brick building down the block seemed to have a pattern of gray concentric circles overlaid across its surface. I knew about, and expected, the DSLR aliasing problems, and even wrote that I had some ideas about how to defeat them (on the rooftop that day, I tested several contrast and softening filters in an attempt to defeat the oversharpened look, to little avail). But I knew what I was getting into, and for the price, I've learned to (more than) love the one I'm with.

Barry Green over at DVXuser has written an in-depth explanation of what aliasing is, and how it affects DSLRs. If you're shooting movies (or thinking about shooting movies) with a DSLR, it should be required reading, along with Stu Maschwitz's follow-up. Barry writes:

The HDSLR is tasked with two jobs: taking still pictures, and taking video. If the Optical Low Pass Filters (or anti-aliasing filter) were tuned to deliver great video, you'd find that your still pictures were just awful! An OLPF is, basically, a blur filter – any detail too fine for the video system to resolve gets turned to blur. But video is, at most, a 2.2-megapixel frame. HDSLRs are designed to handle 14-megapixel stills, or 18-megapixel, or 21-megapixel or beyond!

Basically what I (and thousands of others) are doing is using the secondary purpose of our DSLR as its primary function. That cameras with such clear shortcomings in the video department are selling like hotcakes -- because of their video mode -- must infuriate Jim Jannard et. al over at RED, who had some of their DSMC (Direct Simulation Monte Carlo -- no, wait, "Digital Still and Motion Camera") thunder stolen by Canon and Nikon. But RED's forthcoming offerings have a one clear advantage over the DSLR: on RED, both still and motion images are the same resolution, so they can develop an Optical Low Pass Filter perfectly tuned for both. And considering RED cameras are built first for moving images and then for stills, instead of the other way around as with the current crop of DSLRs, filmmakers should be in for a treat in the coming year.

If the aliasing is so bad, though, why not just use a "real" video camera (you know, the kind that doesn't shoot 21MP stills)? Because while DSLR aliasing is a problem -- and a sometimes-significant one -- to focus on it (get it?) would be to miss the bigger picture (get it?!?). It's the ultra-sensitive, shallow depth-of-field (in HD) that we love, and worrying about the aliasing issue is akin to not seeing the forest for the trees. Or maybe it's more like seeing the forest as a bunch of jagged trees.

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