Nikon has patented a technology that can electronically adjust a camera's low-pass (AA) filter based on the situation. By using a liquid crystal panel, the AA filter can either be turned on and off, or set to 'normal' or 'high' intensity. The first design could allow for a digital SLR to have its AA filter turned off at the press of the button. The second design would have a mild anti-aliasing effect for stills, and a stronger effect to eliminate moiré in movies. The company suggests still more could be achieved by mounting two such filters back-to-back.

The patent claims the entirely electronic design means a two-mode low-pass filter can be implemented in a way that doesn't decrease the reliability or durability of the camera (rather than a mechanical system that switched different filters into place).

How it works:

The designs are essentially a development of the filters used in the D800 and D800E. The standard D800 has a low-pass filter that utilizes a property called birefringence - where a substance splits light depending on the light's polarization. The thickness of the filter defines how much the light is split, so the thickness of the filter can be tuned to match the sensor it's placed in front of (Blurring the light across the width of one pixel cuts out the high frequencies that would cause moiré).

The D800E features a second birefringent plate of the same thickness, set so that it re-combines the light split by the first plate - cancelling the effect.

The latest design can be an extension of the system used in the D800E, but instead of having two birefringent plates that cancel each other out, there's an extra layer that can selectively combine or subtract their effects.

The Nikon patent inserts a liquid crystal layer between the two plates. A liquid crystal layer can be made to rotate the polarization of light or just let it pass straight through, depending on whether an electrical charge is applied to the layer. This ability to change the polarization of the light as it passes between two polarization-sensitive plates lets you decide whether the effects of the two plates is additive or subtractive.

In the simplest system described in the patent, the first layer separates the light before it hits the liquid crystal. With the liquid crystal turned off, the polarization of this split light is rotated through 90 degrees, causing the light to be recombined by a second birefringent plate. However, if the liquid crystal is turned on, it doesn't rotate the light, and the second filter exaggerates the splitting effect and increases the blur.

The result is a low-pass filter with two modes. In one instance you could have a filter that can either be turned on or off (offering a high-res mode if you're shooting landscapes or situations without repetitive patterns).

Design 1: AA filter on/off

Here, the liquid crystal is turned off meaning the light reaching the second plate is the opposite polarization from when it hit the first one - cancelling the two plates' effects.

This allows a single point of light to be focused on a single point on the sensor. This is the equivalent to having no low-pass filter.

[Photo credit: Japan Patent Office]

By turning on the liquid crystal, the polarization of the light is unchanged and the effect of the two plates is combined.

As a result, light lands on two different points on the sensor, blurring the image and acting as a low-pass filter.

[Photo credit: Japan Patent Office]

Alternatively, you could design the filter with a stills mode (blurring light by the width of one pixel to avoid moire in full-res images), and a video mode that splits the light over a greater area (to prevent moiré at the much lower sampling frequencies used by video).

Design 2: AA filter normal/high

Here, the front birefringent element is thicker which, when the liquid crystal is off, has the same effect as a standard AA filter.

[Photo credit: Japan Patent Office]

With the liquid crystal on, light is split even further, providing a stronger AA effect ideal for video recording.

[Photo credit: Japan Patent Office]

Putting two such assemblies back-to-back would further expand the capabilities of such a system. These could include a camera with off, stills and video modes, or allow the horizontal and vertical filtering to be separately adjusted to take into account how the sensor is sub-sampled when shooting video.