Most digital imaging sensors are equipped with a set of filters, either adhered to the surface of the sensor itself, or, as is the case with some of Kodak’s digital SLR cameras, in front of the viewfinder mirror. Typically there are at least two filters, an infrared filter designed to block the long wavelengths that can pollute an image and show up as noise, and an anti-aliasing filter, also sometimes called a low-pass filter. The purpose of this filter is to suppress the interference caused by the mixing of two frequencies to form a third, unwanted one, which in photography is manifested as unwanted colors. Typical aliases in a digital images are opposite-color mixes, like red plus cyan, or blue plus yellow. When it’s really bad, it causes a glittery pattern nicknamed “Christmas tree lights.”
In the early 1990s, the first digital cameras didn’t have either of these filters, and the images were something of a mess. Moderns digital cameras have very sophisticated, very refined filtration. As imaging evolved in the past 15 years, however, these filters have undergone various iterations. Among other things, the anti-aliasing filter robs an image of sharpness, since it effectively blurs the image slightly to remove the aliasing. Over the years, low-pass filters of various strengths were employed. The Nikon D100, for example, had a fairly strong one. The D70, on the other hand, had a weaker one. In an effort to maximize sharpness, a couple of Kodak’s DSLRs, like the DCS-720x, the DCS-760, and the DCS-SLR/N didn’t have one at all, and relied entirely on software to handle the issue.
I thought of this the other day when I was photographing ice forming on my windshield to illustrate an approaching winter storm. I was using my Nikon D1H, and I noticed that the pattern on my dashboard speaker, which is a tight matrix of small holes, was rife with aliasing.
The worst example of aliasing I have ever managed was a few years ago at Oklahoma University football media day. I was photographing Dan Cody, an Ada standout who started for the Sooners. I posed him on the field, showing the seats of the 100,000-capacity stadium behind him, with my 14mm to help emphasize Cody’s size. Unknown to me, the repeating pattern of the stands in the background was exciting the aliases in the 720x sensor to a ridiculous extent, and later that day, I found there was nothing I could do for it in software.