Adam Wilt gives the Optical Low-Pass Filters (OLPF) in the Sony F5 and Sony F55 a comprehensive look:
OLPFs suppress high frequency detail that can cause aliasing and moiré, and most cameras have an OLPF carefully chosen to match the characteristics of their sensor’s native resolution (Nikon has a good writeup on OLPFs here). But when a sensor is scanned out in a non-native way, or at a different resolution, the OLPF’s characteristics may no longer be suitable: if it’s too strong, you’ll get an overly soft image; if it’s too weak, you’ll get excessive detail, aliasing, and moiré. For example, the Canon 5D Mk II’s OLPF is optimized for 21 Megapixel stills capture, but it’s woefully inadequate when that camera is used for 2 Megapixel HD capture. Folks like Mosaic Engineering make add-on OLPFs for the 5D Mk II and other cameras to better filter the image for HD capture, but these add-on filters add bulk and change the optical path length of the light behind the lens, so focusing marks shift.
The F5 and F55 cameras presently offer three different shooting modes:
4K native capture, with pixel-for-pixel 4K recording options.
4K native capture, with HD or 2K recording downsampled from the 4K image
2K native capture in high-frame-rate mode, using the full-frame sensor but with a less elegant hardware readout – binning or pixel-skipping or the like – similar to the 240fps mode on the FS700.
The camera ships with an OLPF optimized for 4K capture and 4K recording (4096 x 2160). Downsampled HD (1920 x 1080) and 2K (2048 x 1080) look very good, too, but are very slightly prone to high-frequency-detail aliasing (though in truth it’s no worse than most HD cameras, which use weak OLPFs to maximize the perception of sharpness in their images). In all these cases the F5 and F55 make very crisp images; some might even feel they are too crisp.
In high-frame-rate mode, aliasing is more pronounced: yes, there’s plenty of detail, but there’s also a greater risk of nasty aliases on fine patterns and sharp edges.
What if you could swap out the camera’s OLPF as needed for different situations? What, indeed…
Read the full report and his findings and conclusions here.