Friday, January 3, 2014

The Low-Pass Filter

What it does, and why ditching it can be a good thing
By Mike Stensvold Published in SLRs
The Low-Pass Filter
The optical low-pass filter (OLPF), also known as an anti-aliasing or blur filter, has been in the photo news a lot lately, mainly because several manufacturers have eliminated it from some of their DSLRs and mirrorless interchangeable-lens cameras. What's the deal?

The low-pass filter's purpose is to do away with the moiré and aliasing (color artifacts) inherent in the digital-imaging process. Moiré can result when a finely patterned subject's image at the focal plane conflicts with the pattern of the sensor's pixel grid. The conventional Bayer-array sensors used in most digital cameras suffer the most from these artifacts because the Bayer array requires data from multiple pixels to produce full color at each pixel site (see sidebar).

The low-pass filter blurs the image's high frequencies (fine detail) at the pixel level. This eliminates (or at least greatly reduces) moiré and artifacts—but it also reduces resolution. Medium-format photographers want maximum sharpness, and prefer to compensate for any moiré and artifacts on a per-image basis in postprocessing. So medium-format cameras don't have low-pass filters. Most users of smaller-format cameras, however, would rather not have to deal with moiré in postprocessing, so these cameras have utilized low-pass filters.

Bayer Vs. Foveon Sensors

The photodiodes used in most image sensors aren't color-sensitive. They just detect the amount of light that strikes them, not what color (wavelength) it is. To get color, the sensors used in most digital cameras are covered by an array of red, green and blue filters in what's known as a Bayer pattern (after the Kodak engineer who devised it). In Bayer sensors, each pixel receives red, green or blue light, while the filter over it blocks the other wavelengths. When the image is demosaiced (the name for the process of taking the digital data and turning it into a color image), the missing colors for each pixel are created by interpolating data from neighboring pixels. In the process, moiré and other artifacts are produced.

Rather than using a Bayer filter grid, the Foveon sensor stacks three layers of pixels, taking advantage of the fact that light penetrates silicon to different depths depending on wavelength. The top pixel layer records mainly blue (short) wavelengths, the middle layer, green (medium) wavelengths, and the bottom layer, red (long) wavelengths. So, no color moiré is produced and no blurring low-pass filter is needed. As a result, a Foveon sensor of a given horizontal-by-vertical pixel count will deliver more resolution than a Bayer sensor of equivalent horizontal-by-vertical pixel count.
Today, though, some cameras' megapixel counts are so high (and, thus, pixel pitch so small) that diffraction acts as a low-pass filter of sorts. Diffraction is the spreading of light as it passes through an opening, such as a lens aperture. The smaller the opening, the greater the diffraction. Essentially, the smaller pixels can "see" this blurring, even at wider apertures, while larger pixels (lower megapixel counts) can't. DSLRs without low-pass filters rely on the hope that diffraction will blur moiré and aliasing away with less effect on image resolution than a low-pass filter would.

Nikon began the trend with the 36.3-megapixel D800E early in 2012 (the D800E actually has half of a low-pass filter and cancels the effect; Nikon also offers the D800, with a weak low-pass filter). Other current DSLRs with no low-pass filter include Nikon's 24-megapixel D7100 and D5300, and Pentax's 16-megapixel K-5 IIs and 24-megapixel K-3 (the latter has a unique two-strength anti-aliasing function simulator, using a sensor-shift shake-reduction mechanism that you can activate when desired).

Non-DSLRs with no low-pass filter include Sony's 24-megapixel a7R and RX1R, and Fujifilm's models with their X-Trans sensor (which uses a different, more random pattern than the Bayer, making it less susceptible to artifacts). And, of course, Sigma's DSLRs and DP cameras use Foveon X3 sensors, which record all three colors at each pixel site and, thus, don't need low-pass filters.

Should you get a camera without a low-pass filter? If maximum resolution trumps all else, you work from a sturdy tripod, focus manually using live view, use very good lenses, and don't mind dealing with moiré and artifacts during postprocessing, then you'll enjoy the extra sharpness of a sensor without a low-pass filter. Often, just moving the camera a bit up or down or left or right, or closer, or changing focal length, can eliminate a specific moiré problem when you take the photo. If you shoot handheld, specialize in subjects with fine repeating patterns or shoot JPEGs rather than RAW (RAW is preferable if maximum image quality is your goal), then you'll be better off with a low-pass filter.
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