Until a few years ago, almost all DSLR cameras shipped with an anti-aliasing (AA) filter on the sensor, also called Optical Low Pass Filter (OLPF). However, a number of recent flagships or premium mirrorless cameras, such as those from Nikon and Sony (the Nikon Z7 and the recently released A7R IV), don’t come with the anti-aliasing filter. For an ordinary consumer, this can make things confusing. Should they look for an AA filter on their next camera, or are they better off without one?
Aliasing and moiré
To answer this question, it is important to understand the job of an optical low pass filter on a camera. You might have heard of the term anti-aliasing if you’ve played games on PC. That’s nowhere close to what an AA filter does on a camera. However, it gives a good starting point. Aliasing refers to distortions that occur when different signals that are sampled together become indistinguishable (or aliases of one another). In photography (or videography) aliasing occurs when the frequency of repeating patterns appears to be the same when seen on the imaging sensor in the camera. The sensor has a hard time separating these frequencies and blends them together instead, creating a phenomenon called moiré.
Moiré usually happens in situations where the object in the frame has ‘high-frequencies’ of repetitive patterns like multiple lines. Common examples include fabrics and animal feathers. The camera creates wavy patterns that are not seen on the actual subject being shot in such scenarios. Consider the photo below. Given there’s a repetitive pattern on the fabric which is beyond the sensor’s ability to resolve, we see resulting moiré in the image.
Solving for moiré using an Anti-aliasing filter
Of course, nobody likes moiré. While there are ways to remove it in post-processing, pitching that to the customers won’t win camera makers a lot of customers. Instead, they deploy anti-aliasing or optical low pass filter on the sensor. As the name suggests, the sensor blocks high-frequency information getting to the sensor that is responsible for moiré. In simple words, it softens the image to some extent so that the sensor doesn’t have to resolve for these conniving high-frequency repetitive patterns.
It is what it sounds like and the result is that you lose some level of detail and sharpness. Not a bad trade-off for photos you can actually appreciate.
why do new cameras skip on the anti-aliasing filter then?
Improvement in sensor technology as well the higher resolution of these sensors allows them to capture and resolve much more details than what was possible until a few years back. This higher resolution has eliminated moiré to a great extent. So much so, that an AA filter is no longer worth the loss in sharpness that it brings.
Of course, you may still find some moiré in some situations, especially when working with video. But those instances are few and far in between. Simply saying, it is worth taking off the AA filter in these high-resolution cameras.
However, not everyone has given up on these filters yet. Canon, for one, continues to use an optical low pass filter in its latest R6 and R5 mirrorless cameras. It is rather interesting that they decided to use one on the R5, given the high-resolution sensor. However, as per Canon, this a new low-pass filter design that features a quad-layer, 16-point subsampling. It combines this with a Gaussian distribution technique to deliver sharpness. In layman’s terms, Canon claims it reduces moiré without reducing the sharpness.
Conclusion
Optical low pass filters aren’t necessarily dead or bad. You will see them on most of the premium sub-25 megapixel shooters from leading manufacturers since it is still a requirement at this resolution. However, due to the sensor’s ability to resolve detail, most high-resolution cameras (40MP+) don’t require an AA filter. That said, having an AA filter in such a camera isn’t necessarily a bad thing. We recommend checking detailed reviews of such cameras to ensure they don’t lose out on the sharpness.
