What is a low-pass filter?

- When you're shopping for a camera and looking at spec sheets, you might come across the term low-pass filter or anti-aliasing filter. These are two terms for the same thing. It's a filter that sits in front of the image sensor inside the camera. It's not a part of the camera that you'll ever interact with. It's nothing you have any control over. It's also not something that all cameras have and that's why it's a term you may come across when camera shopping. The lack of low-pass filter can be considered a feature, so it's something that some camera vendors hype.

A digital image is composed of a group of pixels. Each one of those pixels is a little sample of the light that is bounced off of the scene that you pointing your camera at. Because those pixels are arranged in a regular grid pattern, if you point your camera at something in the world that also has a regular repeating geometric pattern, it's possible that the interference between those two regular geometric patterns, the one out in the world and the one on your sensor, will create a visual effect called moire. Over the years, I've needed to teach about moire and so I'm always on the lookout for examples of it and honestly, it's hard to find if you normally use a good camera.

After years and years, I have precisely one example, this one. Note the colored patterns on the motorcycle seat. Moire won't always be color. Sometimes you'll see diagonal lines. But anything with a regularly repeating pattern can produce moire, so textures in cloth, chain link fences, tiles on a distant rooftop. One reason that moire is so rare is because the low pass filter on a camera does a great job of keeping moire at bay. The downside is that a low pass filter works by slightly blurring your image.

So while the presence of a low pass filter on a camera makes you less likely to see moire, it also means that the camera is not capturing images that are as sharp as they could be. This is why images from most digital cameras require sharpening. If you're shooting JPEGs, then that sharpening is performed in camera. If you're shooting RAW, you have to do the sharpening yourself in your RAW converter. However, even with a low pass filter, moire is still possible and it's very difficult to remove from an image. That's why it's worth suffering a little bit of softening on the sensor.

But there situations where moire is extremely unlikely, such as landscape shooting. The natural world doesn't typically order itself in fine-grained repeating patterns so landscape shooters don't really have to worry about moire and landscape shooters are often very finicky about sharpness. To accommodate people who would rather risk some moire for improved sharpness, some vendors now sell versions of their higher-end cameras that lack low pass filters. For example, Canon sells the EOS 5DS, which has a low pass filter and the 5DSR which does not.

That's R, which stands for no-low-pass filt-R. Can't wait to see how that ends up in the little transcription down below the movie here. Anyway, Fuji's X-series of cameras, all lack low-pass filters because of the way their sensors work. Rather than using a regular repeating pattern of color receptors, the Fuji X-Trans sensor uses a jumbled pattern of receptors and that prevents moire. This means that images straight out of the camera are noticeably sharper than images from cameras that have a low-pass filter.

Do you need to worry about this? That all depends on how picky you are about sharpness and how picky you are about sharpness might be dependent on how large you print your images if you print them at all. I'm not someone who is super-picky about sharpness. I want my images in focus but that's different. I shoot with a Fuji X camera and I do appreciate that the images are sharper straight out of, straight off the card, but I don't think my final prints are noticeably sharper than when I was shooting with a Canon 5D and sharpening in my RAW converter. Recently I was looking at a bunch of Henri Cartier-Bresson images.

Afterwards, I opened some recent street shots of my own, shot with a Fuji and I had this, kind of momentary jarring experience because my images were so much sharper than the Cartier-Bresson prints that I had been looking at. My images looked harsh. They were hard-edged. So, sharpness is really a matter of personal preference and taste for it varies from person to person and I'm finding it's varying, kind of for me, from time to time or subject matter to subject matter. If sharpness is critical for you and you don't mind risking some moire, then opting for the low-pass-free version of an SLR might be a good choice for you.

If you'd like to know more about lenses and these types of issues, check out my Foundations of Photography: Lenses course.