No Low-Pass Filter On Cameras?

Why the trend of cameras being made without low-pass, or anti-aliasing, filters is a good thing for nature photographers
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In 2012, Nikon introduced the D800 and the D800E, which were identical, but the D800E had the effects of its anti-aliasing (AA) filter removed. Since an AA filter slightly blurs the image at the pixel level, its absence (or negation) means potentially sharper images, albeit with a greater possibility of aliasing and moiré. Shortly thereafter, other DSLRs and mirrorless models began to appear with AA filters either negated or removed. As of this writing, Canon, Nikon, Pentax and Sigma make DSLRs and Fujifilm and Sony offer mirrorless interchangeable-lens cameras with no AA filter. There are also fixed-lens cameras with no AA filter from Fujifilm, Sigma and Sony.

So what’s the big deal? What does the AA filter do? Why is it in the camera in the first place, and why should landscape photographers, in particular, be paying attention?

First, there have been cameras without AA filters for a long time. Sigma’s DSLRs and fixed-lens DP Merrill and dp Quattro models use the unique Foveon X3 sensor that inherently minimizes color moiré and aliasing, and thus have never had AA filters. Medium-format digital cameras—whose users buy those big, expensive beasts because they value image quality, above all—don’t have AA filters because those users would rather deal with aliasing and moiré in postprocessing, when necessary, in return for those resolution benefits. So the absence of AA filters isn’t new, but their absence in mainstream DSLRs and mirrorless camera is.

What Is An Anti-Aliasing Filter?
An AA filter, aka OLPF (optical low-pass filter), sits over the image sensor and does away with aliasing—artifacts and moiré that occur when you sample the real world with a fine grid of pixels. Conventional Bayer image sensors add color moiré to the mix, since each pixel “sees” only red, green or blue. (The missing colors are provided to each pixel via interpolation using data from neighboring pixels and complex proprietary algorithms; this “demosaicing” process also creates color moiré.) Since Sigma’s Foveon sensors do record all three primary colors at every pixel site, they’re less susceptible to color moiré problems, and thus haven’t needed AA filters.

A typical AA filter consists of a top layer, which slightly displaces the image horizontally, an infrared filter to eliminate unwanted infrared radiation and a bottom layer that slightly displaces the image vertically. This blurs the image’s high frequencies (fine detail) at the pixel level, reducing moiré and artifacts. But it also reduces sharpness at the pixel level.

When the D800 and D800E came out, Nikon created quite a stir around the notion of anti-aliasing filters and why they might not be as necessary today. These diagrams of the Nikon D800 vs. the D800E show how the anti-aliasing filter, or optical low-pass filter, works (D800) and how its effect is negated (D800E).

Aliasing occurs when lines in the photo cross rows or columns of pixels on the sensor. Anti-aliasing blurs the sharply defined lines to smooth things out. As pixel densities increase, less and less aliasing occurs at the point of capture, which is why more high-resolution cameras are doing away with the anti-aliasing filter or negating the effect.

How Big A Problem Is Aliasing?
All Bayer sensors can produce color moiré, and all grid-pattern sensors can produce aliasing. But when the sensor’s sampling rate is high enough, i.e., when the pixel count is high enough, aliasing is less likely. That’s why medium-format cameras with up to 80 megapixels and today’s high-pixel-count DSLRs and mirrorless cameras can do away with the AA filter—the sampling rate is so high that moiré and aliasing are minimized. They mainly appear in subjects such as cloth textures and distant skyscrapers with lots of windows. You can reduce any aliasing and moiré that occur by stopping down the lens to increase diffraction. This will also reduce resolution, of course, so you have to find the ideal balance between artifacts and diffraction blur for your specific subject. For this reason, it’s not a bad idea to bracket apertures when using a camera without an AA filter. Sometimes, just changing the camera angle or distance can reduce or eliminate an aliasing/moiré problem. You can also deal with any moiré and aliasing that occur when you process each image, as medium-format shooters do. In its AA-filterless K-3 and K-S1 DSLRs, Pentax has an AA-filter simulator you can activate, if needed.

For landscape photographers, the techniques for reducing and eliminating moiré are academic. Because landscapes largely consist of such irregular patterns, you’re unlikely to encounter much aliasing at all. So, for the typical outdoor shooter, the AA-filterless cameras should be valuable tools, delivering fine definition and lots of landscape detail.

Canon’s new full-frame EOS 5DS R is the company’s first DSLR with the AA filter negated. The EOS 5DS R has a resolution of 50.6 megapixels, making it the current high-res king in the 35mm-style DSLR class. When announced, Canon made reference to nature and landscape photographers specifically as being a large part of the motivation for developing it. There’s also a standard EOS 5DS model, which has a low-pass filter. The 5DS R and 5DS don’t replace the EOS 5D Mark III. For the time being, they will coexist in the line. Also, the 5DS R and 5DS are said to be identical to the 5D Mark III on the outside. Look for a full review by OP Tech Tips columnist George Lepp.

Fujifilm doesn’t market DSLRs anymore, but their X-series mirrorless interchangeable-lens cameras include several that feature the company’s exclusive X-Trans CMOS and X-Trans CMOS II APS-C sensors, whose unique random RGB filter pattern is less susceptible to aliasing and color moiré than the more regular Bayer pattern used in conventional image sensors. These include the X-T1 and X-E2 (X-Trans II), and the X-Pro1, X-M1 and X-E1 (original X-Trans). Fujifilm’s X100T fixed-lens APS-C compact camera also features an APS-C X-Trans CMOS II sensor, and the company’s X30, XQ1 and XQ2 compacts feature 2⁄3-inch X-Trans CMOS II sensors, which don’t have an AA filter.

Nikon started the mainstream AA-filterless trend with the D800E and its AA-filtered D800 twin. Both have been replaced by the D810. In total, Nikon currently offers five DSLRs without AA filters: the 36.4-megapixel full-frame D810, and the 24-megapixel APS-C D7200, D5500, D5300 and D3300, the latter of which, at $499 (including kit zoom), is the lowest-cost AA-less DSLR at the moment.

The Pentax K-3 and K-S1 DSLRs feature APS-C CMOS sensors with no AA filter (24.3 and 20.1 megapixels, respectively), but you can activate the AA filter simulator, when desired. The simulator uses the camera’s sensor-shift Shake Reduction mechanism to rapidly move the sensor down one pixel, right one pixel, then up one pixel to slightly blur the image at the pixel level as an AA filter would. This lets you shoot with or without the AA effect. Pentax’s 20.1-megapixel K-S2 and 16.3-megapixel K-5 II S are also AA-filterless, but don’t have the simulator.

The “pixels” in image sensors just detect the amount of light that reaches them, not its wavelength (color). To produce color images, conventional image sensors feature a regular grid of red, green and blue filters over the pixels (called a Bayer array, after the Kodak scientist who developed it), so that each pixel receives just one primary color. Data for the missing colors at each pixel is obtained via interpolation using data from neighboring pixels and complex proprietary algorithms in a process known as “demosaicing.” This process results in less resolution from the sensor than a given pixel count should theoretically deliver, and the demosaicing produces color moiré. An AA filter reduces the moiré and aliasing, but further blurs the image at the pixel level.

Sigma’s Foveon X3 sensors take a different approach. Rather than use colored filters, the Foveon sensors use the fact that light penetrates silicon to different depths, depending on its wavelength (color). They stack three layers of pixels; in oversimplified terms, the top layer of pixels records blue, the middle layer, green, and the bottom layer, red. This means that every pixel records all three primary colors, so there’s no need for demosaicing, and thus no color moiré and no need for an AA filter. The result is that Foveon sensors deliver more resolution than Bayer sensors of equal horizontal-by-vertical pixel count. Sigma’s Merrill cameras (named for one of the creators of the Foveon sensor)—the SD1M DSLR, and DP1M, DP2M and DP3M fixed-lens compacts—all feature a Foveon X3 sensor with three stacked 4800×3200 layers and deliver images measuring 4704×3136 pixels. The newer dp0, dp1, dp2 and dp3 Quattro fixed-lens cameras feature the newer Foveon X3 Quattro sensor, which stacks a 5424×3616-pixel top layer over 2712×1808-pixel middle and bottom layers, and delivers images measuring 5424×3616 pixels.

Sony has been on a tear with their a7 mirrorless line of cameras. The high-resolution, 36.4-megapixel, full-frame a7R mirrorless camera doesn’t have an AA filter, nor does the 24.3-megapixel full-frame RX1R compact with built-in 35mm ƒ/2 Carl Zeiss wide-angle lens. Sony’s current DSLRs (SLTs, actually, with semitranslucent nonmoving mirrors and eye-level electronic viewfinders) have AA filters, but we’ve read rumors of new models coming out where the filter will be removed.