Optical low-pass filter using lithium niobate crystal positioned before the sensor; prevents moiré by splitting light into two images offset by 4–6 micrometers.
Technical Details
The OLPF consists of one to three layers of lithium niobate crystal with a thickness of 0.2-0.8mm, positioned 0.5-2mm in front of the sensor. The separation distance of the double images for full-frame sensors is 4-6 micrometers horizontally and vertically. At 4K resolution, this corresponds to an offset of approximately 0.7 pixels. Modern variants also use infrared filters and anti-reflection coatings. Some manufacturers like RED offer switchable OLPS with piezoelectric actuators that can move the filter by 10-20 micrometers.
History & Development
Sony introduced the first OLPF in the Mavica camera in 1981. Panavision integrated OLPS into their Genesis series digital film cameras in 1999. Starting in 2007, manufacturers like Leica (M9) and later Nikon (D800E) eliminated the OLPF for higher sharpness. RED implemented the first switchable OLPF with the Epic Dragon in 2013. Since 2018, cameras like the ARRI Alexa LF have used variable OLPS with three settings: off, medium (0.5 pixels), and strong (1.2 pixels).
Practical Use in Film
Emmanuel Lubezki consciously opted against OLPS for "The Revenant" (2015) to achieve maximum detail sharpness in nature shots. Hoyte van Hoytema used strong OLPS for IMAX sequences in "Dunkirk" (2017) to avoid moiré in aircraft structures. For architectural shots, DoPs activate the OLPF when dealing with geometric patterns like tile roofs or metal grids. Fashion shoots often use OLPF-free setups for crisp fabric textures, while skin shots benefit from the gentle filtering.
Comparison & Alternatives
Unlike traditional soft-focus filters, the OLPF only reduces high-frequency details without shifting contrast or color. Digital post-processing can remove moiré effects, but it costs rendering time and detail. Variable neutral density filters only affect the amount of light, not the sharpness. Modern AI-powered demosaicing algorithms in cameras like the Canon C70 partially replace mechanical OLPS with software solutions with 2-4ms latency.