Tutorial: Anti-Aliasing Low Pass Filters on Digital Camera Sensors

April 22, 2016  •  Leave a Comment

Prologue: In December 2013, readers on a dpreview.com forum were having a discussion about whether all digital cameras had AA (anti-aliasing) filters or not. One participant knew for a fact after reading about digital sensors for 15 years that 99.9% of cameras have AA filters and that AA filters are a normal part of all cameras with Bayer-type sensors except some high-end models. Another participant disagreed. From this conversation and others, I realized many people did not understand how the light path is designed between the back element of the lens and a camera’s actual sensor surface. In the following edited tutorial, I attempted to shed some light on that design:
 

Let's try to clarify this a little further in simple terms, since to a degree you are both right. Re:"I know for a fact that 99.9% of cameras have AA filters", it would be more proper to say that 99.9% of cameras have Low Pass filters. When you are looking through the camera lens flange and thinking you are seeing the sensor, you're not, you're looking at a Low Pass filter in front of the sensor. This is what is being shaken when cameras with anti-dust mechanisms start up, and this is what anti-static materials are applied to. Now a Low Pass filter can be configured in two ways (in simple terms) in the light path from the lens to the sensor. The typical way is a horizontal split of the light beams in the first element, Infrared removal in the second element, and a vertical split of each horizontal beam in the third to a four point source. The effect is anti-aliasing (AA) filtering. Newer designs include a vertical split in the first element, Infrared removal in the second, and recombination of the split beam back to a single point source in the third. The effect is NO AA filtering. So "99.9%" of cameras do have low-pass filters, but not necessarily AA filters. Again, this is in simple terms and what's in the secret sauce of each OEM's sensors is not always made public.
 

The first participant replied about previously thinking that the terms "anti-aliasing filter" and "low-pass filter" meant the same thing and along with others appreciated being educated. But a further question was posed that even when a Low Pass filter does not include AA, isn’t there still a blurring effect on the image since anything that manipulates the photons before they reach the sensor must lower image quality. I replied with the following:

Yes, I would generally agree, but consider this. The AA filter works because the grating in the first element splits the light horizontally to two beams, eliminates infrared in the second element, and then splits each horizontal split beam vertically in the third element to now create four beams as a source on the sensor. A beam of light exiting the lens that would have hit only its own real estate on the sensor (again, in simple terms) now shares that real estate with the beams around it that have also been split. This sharing of the same real estate by two or more beams that exited the lens create anti-aliasing because there is now little chance that two beams of opposing frequency will hit the same pixel, a basic cause of aliasing. The splitting and blending do introduce a loss of acuity, as we are all well aware.

When the low pass filter is not designed to include AA, the first element does what the third element of the AA type filter does, splits the beam vertically before having infrared removed in the second element. The third element then is the exact mirror of the first element and vertically recombines the beam to its original state (now ex-infrared). Basically it covers the same real estate that it would have if it didn't pass the filter at all. What now becomes important is the size of the real estate it is illuminating. The smaller the pixel relative to that light, the less chance that two beams of opposing frequency will hit the same pixel. That's why designers can get away with not including AA filtering on very high megapixel sensors. Of course, it does not always work that way in all situations (the quality of the lens as well as photographic technique come into play here), but at least acuity is generally not lost due to the Low Pass filter.
 


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