Wednesday, April 30, 2014

Do You Use Any Aliases?

By Staff Published in Digital SLR Cameras
Do You Use Any Aliases?
Diagram A
The pixels on image sensors are arranged in a grid pattern. Anti-aliasing filters were designed to combat moiré, which occurs when patterns intersect like this. Today's highest-resolution sensors have extremely fine pixel pitch, which makes it unlikely to encounter such conflicts with real-world subjects.

Non-DSLRs with no low-pass filter include Sony's 24-megapixel full-frame a7 and RX1R, and Fujifilm models with their APS-C X-Trans sensor. The X-Trans features a unique RGB filter array that differs from conventional Bayer arrays by using a more random arrangement of the red, green and blue pixels in every horizontal and vertical row. This minimizes moiré and false colors without needing the blur-inducing anti-aliasing filter. And, of course, Sigma DSLRs and DP-series compact cameras use APS-C Foveon X3 sensors, which record all three colors at each pixel site and thus don't need low-pass filters (see the sidebar on Foveon sensors).

As medium format begins to shift from CCD to CMOS sensors, we're seeing the same avoidance of anti-aliasing filters in their CMOS models. Phase One's IQ250 and Hasselblad's H5D-50c, which both use 50-megapixel Sony CMOS sensors, don't have anti-aliasing filters. (Learn more about sensor technology in "Sensors Un-Sensored" in this issue of DPP.)

So what's the bottom line? If maximum resolution trumps all else, and you're willing to deal with possible moiré and artifacts in postprocessing, you'll enjoy the extra sharpness of a sensor without a low-pass filter.
So what's the bottom line? If maximum resolution trumps all else, and you're willing to deal with possible moiré and artifacts in postprocessing, you'll enjoy the extra sharpness of a sensor without a low-pass filter. (Note that moiré is most evident with subjects whose fine pattern conflicts with the sensor's pixel grid, so it often can be avoided simply by moving the camera a bit up or down, left or right, closer or farther away, or slightly rotating the camera or subject, if possible, or changing focal length or the focus point. You can use maximum magnification in Live View mode to check for moiré.) If you specialize in subjects with fine repeating patterns or shoot JPEGs rather than RAW (and, of course, JPEGs are a bad idea if maximum image quality is your goal!), then you might be better off with a camera that has an anti-aliasing filter. Of course, if you shoot medium-format digital, your choice is made: None of those cameras has an anti-aliasing filter.

Foveon Sensors

As explained in this article, conventional Bayer sensors record just one primary color at each pixel site; the missing colors for each pixel are produced by interpolating data from neighboring pixels. This compounds the problems of moiré and false-color artifacts, generally requiring the presence of a blurring anti-aliasing filter to minimize it.

Diagram B

Rather than using a Bayer array, 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 all three primary colors are recorded at every pixel site, no color moiré is produced, and no blurring low-pass filter is needed. As a result, a Foveon sensor of given horizontal-by-vertical pixel count delivers more resolution than a Bayer sensor of equivalent horizontal-by-vertical pixel count.

Foveon sensors have used this principle since the first Sigma DSLR in 2003. Now Sigma has introduced a new Foveon sensor in its dp Quattro cameras. Like previous Foveon X3 sensors, the new X3 Quattro features vertical color separation technology—using the fact that different light wavelengths penetrate silicon to different depths—rather than colored filters to derive color information. Like previous X3s, the new sensor stacks three pixel layers: the top one recording mainly blue, the middle one, green, and the bottom one, red wavelengths. Where previous X3 sensors had three layers of identical pixel count, the new Quattro features a 4:1:1 ratio: the top layer has four pixels for each pixel in the lower layers. This allowed Sigma to up the pixel count while reducing noise and speeding up processing and writing times, thanks to less total data per image file, all while retaining the essential Foveon assets: Each primary color recorded at every pixel site, no moiré and no need for a blurring optical low-pass filter. A new TRUE III processor designed for the Quattro sensor optimizes image quality and speeds performance. (For the record, the 14-bit RAW files output by the new X3 Quattro sensor measure 5424x3616 pixels, compared to 4704x3136 for the X3 Merrill sensor's 12-bit files.)

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