Optical filter mounted on lens — corrects color temperature or creates effects (ND, polarizer, UV). Cheaper, faster than grading fixes.
Lens filter / Correction filter
You need a consistent color temperature across multiple takes quickly, and the grading suite isn't available yet – this is where the lens filter comes into play. It's the first line of defense between the sensor and the subject, and anyone who is careless here will pay dearly in the edit later.
The lens filter sits directly on the lens and optically corrects what the lighting or natural light is tricking you with. The classic tools for this are wide-ranging: ND filters (Neutral Density) dim the light without shifting color – essential if you want to shoot in daylight with an open aperture and need to keep the ISO low. Polarizing filters reduce reflections on water or glass and saturate the sky. UV filters have long since become a cheap protective measure, but the color correction filters – 81A, 82A, CTO, CTB – these are the real workhorses. An 81A filter converts daylight to tungsten temperature when your lights are too warm and you don't want a color cast in the grading.
On set, this works elegantly because you can immediately see how the correction is working – no reliance on LUTs, no waiting in line for the colorist. If you're shooting between artificial light and a window, a poly-filter combination ensures that your main light doesn't fight the exterior lighting. The costs are minimal; a good ND filter costs less than an hour of grading time. Important: Quality counts here. Cheap plastic filters create color fringing and vignetting – you'll notice this immediately in 4K footage.
Practically, this means: Before shooting, determine the color temperatures (use a Kelvin meter, don't guess), assemble the correct filter combination, and test it. A 37mm set (ND gradations, warm and cool filters) fits in your bag and saves you hours in post-production. Anyone who thinks they can do all of this later in grading underestimates both the rendering time and the limits of software – optical correction on set is always poorer in noise and color deviation than digital post-processing. The lens filter is your first quality control.