Until recently virtually all digital cameras had an anti-aliasing or low-pass filter over the sensor. This filter had the effect of softening the image very slightly to reduce the likelihood of moiré patterning happening in parts of the image with a very fine repeating pattern that is close to the resolution limit of the sensor.
Moiré patterning is the strange patterning that used to be seen quite often on TV when presenters selected the wrong tie. It occurs when two repeating patterns of similar frequency over-lie or cross each other.
In digital photography moiré patterning can occur when the lens projects a pattern of fine lines onto the sensor and the lines have a frequency that is close to that of the sensor grid.
Leave it out
Since the introduction of the Nikon D800E we have started to see more cameras being introduced without an anti-aliasing, or low-pass, filter over the sensor. This is because the pixel density of sensors has become so high that there are fewer and fewer patterns that we are likely to photograph that have a high enough frequency to cause moiré patterning.
The benefit of leaving off the filter is that the camera is able to record a little more detail and produce slightly sharper images, with less need for post-capture sharpening.
Rather than simply omitting the anti-aliasing filter when building a digital camera, manufacturers have to use a filter that doesn't have the normal anti-aliasing properties, but still retains the ability to block infrared light, for example. So a new filter design is required.
Naturally, creating a new filter that is only used in a few cameras is more expensive than using an existing one that is used in many cameras, and to some extent this helps explain the increased cost of cameras such as the Nikon D800E and the Pentax K-5 IIs over their standard counterparts, the Nikon D800 and Pentax K-5 II.
Using a different filter
Now that a non-low-pass filter has been created and moiré patterning has not been found to be an issue for most photographers and most scenes, the new filter design is becoming increasingly popular. As more and more cameras are made with the new filter it will come down in price, making it more likely to be used.
We have already seen two compact cameras, the Nikon Coolpix A and Ricoh GR, that don't have an anti-aliasing filter. These are premium compact cameras with 16.2 million effective pixel APS-C sized sensors, and both are capable of capturing an impressive level of detail.
It's relatively early days with the Nikon Coolpix A and Ricoh GR, but we have shot hundreds of images with each and haven't encountered an issue with moiré patterning. Neither has there been any outcry about issues with moiré patterning from the older, and popular, Nikon D800E.
As we become increasingly comfortable with the idea of cameras not having an anti-aliasing filter, and possibly becoming more adept at correcting moiré patterning post-capture, we can expect more cameras to be made without the filter. As the non-AA filter price comes down further, we anticipate it being used in more everyday compact cameras.