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While many "looks" can be achieved in post-production, certain aspects of the image are better controlled before the image is recorded. Optical filters modify the light before it enters the lens. The benefit of using optical filters instead of digital filters is that there is no added time in post production and less degradation of the image quality, especially in HDSLRs, which already have limited color space and a highly compressed image. On the other hand, using optical filters commits the image to a specific look, making the image less flexible in post. There are, however, certain effects that simply cannot be done in post, making these filters essential to most filmmaking applications.
Any time additional optics are placed in front of the camera, the image quality can degrade, even if only slightly. It follows, then, that the quality of any optics should be at their finest to minimize this effect. Filters can vary in their optical quality. Typically, the price will reflect this, so for simple low-budget shoots, compromises like color shifts can be acceptable (and fixed in post). Most professionals, however, will want to invest in higher-quality filters so as not to be a weak link in the image chain. Look for multi-coated filters, which help reduce glare from reflections hitting the filter. Glare can reduce the level of light transmitted through the lens and degrade the image quality.
There are two main ways of mounting filters to the lens that need to be considered: you can use screw-on filters or drop-in filters.
Screw-on filters mount directly to the front of a lens, but need to match the lens's front diameter screw threads. This can be limiting because it means either having to buy duplicate filters of different sizes for each lens, or using adapters. It also means more time when swapping lenses in the field because the filters first need to be unscrewed from the first lens and then screwed onto the second lens. Additionally, some filters, such as graduated ND, are not used with screw mounts. Conversely, a variable ND filter is available only as a screw-on type.
Drop-in filters are placed within square (or rectangular) frames that are then dropped into a simple plastic holder or a professional matte box. The main advantages are speed and simplicity, and the need to carry just one set of filters for all your lenses. Swapping a lens is quick and simple, and can be done without having to un-screw and re-screw filters. This system also offers the additional benefit of shading that a matte box provides (more on this below). On the other hand, matte boxes can add bulk and weight to the rig, so they may not be suitable for certain "run-and-gun" applications.
Screw-on UV or clear filters, in popular fashion, are usually mounted onto the lens as soon as it's purchased to protect the front glass element from being scratched. This is even more critical with modern multi-coated lenses, which can be more sensitive. UV filters limit the amount of ultraviolet light that strikes the lens, to reduce UV contamination in the image. This is more of a concern at higher altitudes where the Earth's atmosphere is thinner and filters out less of the sun's ultraviolet light.
Most professional video cameras have several levels of ND filter built into the camera, because unlike in still photography, the shutter speed is typically not changed. The only viable option is stopping the lens aperture down, although this will produce deeper depth of field, which may not be desirable. In order to keep shallow depth of field in bright conditions, ND filters are used to cut down light transmission through the lens so the aperture doesn’t need to be stopped down. ND filters come in a variety of strengths. The most common one used is the 0.6 ND because it's a middle point. When a stronger effect is required to match the brightness of a sky and foreground at sunset, a 0.9 ND may work better. The higher density filters are for stronger effects.
Graduated ND filters cut down light at the top of the frame and then gradually become clear toward the center. There is generally a soft transition from the dark to the clear part of the filter. These filters are used to control the range of the dark and bright areas (typically a sky) in a scene so that it falls within the acceptable dynamic range of the camera. In order to align the transition of the filter to the horizon in the scene, the filter needs to be moved up and down. For this reason, graduated filters are not mounted to the lens directly, but need to be used in conjunction with a filter holder or matte box.
Variable ND filters are essentially a whole set of ND filters in one. They are made up of two polarizer filters; as one is rotated, it blocks light from being transmitted through the lens. Variable ND filters can also be used while recording to cut down the light, instead of changing the aperture (still lenses do not transition smoothly from one aperture setting to the next). Variable ND filters are screw-in only and cannot be used in a matte box.
One important thing to be cautious of when using ND filters is infrared (IR) light pollution. ND filters cut down visible light, but don't cut down light in the infrared spectrum. With each stage of ND filter, IR light transmission is increased, causing desaturated colors and low contrast images. This issue is most noticeable in a scene with a lot of green, like grass and trees—the green colors shift toward the reds and become more of a muddy brown. IR pollution complicates things in post production and limits the color-grading process for HDSLRs. In order to combat this issue, filters that cut down on IR need to be utilized when heavy ND filters are used.
The main options for cutting down IR pollution are the use of a separate IR filter in front of the other filters (only one should be used), or the use of ND filters that include an IR blocking layer. The latter is ideal because it results in less glass in front of the lens.
Polarizer filters diminish glare and reflections in surfaces like water and glass. They can also be used to increase contrast in parts of a scene, like a sky. If a matte box is used, it should have a rotating filter stage in order for the polarization effect to be controllable.
Diffusion, or soft focus, filters can be used to soften the edges in an image or to diffuse bright lights, potentially producing a soft, glowing effect. They are useful for creating a “dreamy” look. For example, many users prefer the look of the Canon 5D over the 7D because of its softer renditions of fine details. Using a diffusion filter can help address this concern, making the video look less “video-like.”
Diffusion filters may also be helpful in addressing the issue of moiré. However, there will be a loss of resolution in the scene when diffusion is used, so these filters should be tested first in the desired scenes.