Here’s why you get Aliasing when shooting 2K with a 4K sensor

Alister Chapman has a lot of experience shooting with Sony cameras, especially the FS700, PMW-F5, PMW-F55 and others.

In this article he explains why there is a confusion and misunderstanding about aliasing and moiré, what is really going on under the hood and how you can fix it.

Sony’s FS700, F5 and F55 cameras all have 4K sensors. They also have the ability to shoot 4K raw as well as 2K raw when using Sony’s R5 raw recorder. The FS700 will also be able to shoot 2K raw to the Convergent Design Odyssey. At 4K these cameras have near zero aliasing, at 2K there is the risk of seeing noticeable amounts of aliasing.

Well what’s done in the majority of professional level cameras is to add a special filter in front of the sensor called an optical low pass filter (OPLF). This filter works by significantly reducing the contrast of the image falling on the sensor at resolutions approaching the pixel resolution so that the scenario above cannot occur. Basically the image falling on the sensor is blurred a little so that the pixels can never see only black or only white. This way we won’t get flickering between black & white and then grey if there is any movement. The downside to this is that it does mean that some contrast and resolution will be lost, but this is better than having flickery jaggies and rainbow moiré. In effect the OLPF is a type of de-focusing filter (for the techies out there it is usually something called a birefringent filtre). The design of the OLPF is a trade off between how much aliasing is acceptable and how much resolution loss is acceptable. The OLPF cut-off isn’t instant, it’s a sharp but gradual cut-off that starts somewhere lower than the sensor resolution, so there is some undesirable but unavoidable contrast loss on fine details. The OLPF will be optimised for a specific pixel size and thus image resolution, but it’s a compromise. In a 4K camera the OLPF will start reducing the resolution/contrast before it gets to 4K.

Read the full article and detailed explanation here. It might just save you a headache when shooting 2K with a 4K sensor!