Understanding Low Contrast FiltersAugust 28, 2013
A low contrast or “LowCon” filter is a technique used by some cameras and cinematographers to reduce the appearance of subject contrast and soften highlights. In this article, we examine the mechanism and relative trade-offs behind these filters.
A low contrast filter is typically placed in front of a lens, but can also be an extra fixed layer in front of the camera sensor. It works by inducing veiling flare, usually similar to but much subtler than adding a thin layer of petroleum jelly to the front of a lens. LowCon filters are therefore often grouped with “mist” or “fog” filters.
Veiling flare spreads light from the brighter regions throughout the scene, thereby lifting the blacks and softening the highlights. It can also make highlight clipping appear more gradual, particularly if these regions glow, halo or “bloom” into adjacent tones. This may increase the recordable dynamic range by a fraction of a stop, but the dynamic range of the sensor itself does not increase.
However, veiling flare means the lifted blacks and softened highlights are achieved by cross-contaminated light, not necessarily by light from the subjects themselves. Veiling flare therefore adversely affects lens clarity and MTF, which is ordinarily a key benefit of high-end camera lenses. If the style does not need to be modified later though, then this potential disadvantage may be acceptable.
TYPES & ALTERNATIVES
Low contrast filters have a variety of names, strengths and influences. More generally, the family of filters which soften contrast and detail are typically called low contrast, ultra contrast, fog, mist, softening, frost or diffusion filters. Some act more on the highlights or shadows, others act more uniformly on fine details — but all act to scatter light. Consult with your specific filter manufacturer’s documentation for more on their particular implementation.
Regardless of implementation, these styles can often instead be more flexibly controlled in post-production. If the goal is to soften detail, such as for smoother skin or a dream-like sequence, then this can be achieved by blending the original frame with a second blurred version. Alternatively, if having a glow around highlights is important to smooth the transition to white, then this can be achieved by overlaying just the blurred white regions onto the original frame and adjusting the opacity and blend mode to taste. Typically “screen mode” blending is used for this effect.
In REDCINE-X PRO®, softening can be accomplished using a slider that also preserves image resolution. This appears as a “Clarity” slider within the “Post : Look : Effects” right-side panel. The example above was created using -0.5 for the slider setting:
Alternatively, if lower contrast is the goal as opposed to also having a softer image, the curves tool in REDCINE-X is another common option. The curves tool works by using a curved line to map input tones into output tones. To brighten shadows and darken highlights, use a subtle inverted S-curve. See the Intro to Grading with REDCINE-X for more on how that tool works. Another option would be to color grade using REDlogFilm as opposed to REDgamma, then to only add a subtle contrast curve if necessary.
However, if the goal is solely additional dynamic range, then other filters and techniques are often better suited. Graduated ND filters extend the recordable dynamic range, for example, but without similarly compromising clarity and MTF. These have been an effective and proven tool with landscape capture for around a century. However, these filters also require a uniform light to dark transition across the frame, and can make objects that protrude from the dark to light region appear unusually dark, such as tree or mountain tops in a landscape.
With RED® cameras, HDRx® is an additional way to increase dynamic range, and substantially more so than is possible with the previously discussed tools. It works by recording two exposures within the interval that a standard motion camera would record only one. Then, in post-production, these can be automatically merged to extend dynamic range anywhere from 2-6 stops:
Low contrast filtration is a useful creative tool for affecting the look and feel of footage. However, any particular style should ideally be fully controllable and not fixed in-camera, such as with a specific film stock or sensor-level filtration. Ultimately, the goal of any camera should be to capture the cleanest, highest fidelity images possible. Then, in post-production or with the use of interchangeable filters, that footage can be degraded if desired for a particular style. If a more “filmic” look is needed with glowing highlights, then this can be added. Alternatively, if the shadows need to be brightened, then this can be done as well. Otherwise any degradation by the camera itself would be irreversible.