Understanding Lens Vignetting
Vignetting is an imaging phenomenon that happens with virtually every optical system. It can even also be added intentionally in post-production. In this article, we investigate the types and causes of vignetting so it can be better anticipated and controlled.
OVERVIEW OF TYPES
Vignetting appears as a radial darkening toward the corners of a frame, and can actually arise for several fundamentally different reasons:
All types of vignetting appear more pronounced at far focusing distances and with lower crop factors, or when high contrast tonal curves are added in post-production. Mechanical vignetting is the only type that can be fully eliminated by camera technique, but only optical and natural vignetting can be addressed in post-production.
CAUSE OF NATURAL VIGNETTING
Light projected toward the center of a camera sensor strikes at a right angle, but further from the center, the angle of incidence decreases. This is unavoidable anytime you have a standard lens and a rectangular sensor:
This variation in angles causes radial dimming for three reasons. First, light hitting the edges of a sensor has to travel a longer distance than light hitting the center, which in turn causes the light to become progressively dimmer, similar to what happens when moving further from a studio light. Second, just as how the sun’s UV index is highest mid-day when the sun is overhead, the flux of light striking the camera sensor decreases further from the center of the image. Third, the lens pupil as seen by the light rays appears more elliptical and thus narrower when viewed at an angle (compared to the circle that it looks like straight-on).
With RED® cameras, reducing the resolution increases the effective sensor crop factor, which in turn can be used to fully control the appearance of vignetting. With the RED DRAGON®; sensor, a resolution setting of 5K matches the lens coverage of Super 35 mm film, and 5.5K matches the coverage of MYSTERIUM-X®.
Lens choice is also important. Lenses designed for larger film formats are typically much less susceptible to vignetting—regardless of the aperture setting or focal length. A full frame 35 mm lens is unlikely to vignette on a Super 35 mm sensor, for example. Design compromises with zoom lenses also make these more susceptible to vignetting than prime lenses, and particularly at the wide end of the zoom range.
Vignetting can also be reduced substantially using a radial graduated neutral density (GND) filter, where the minimum transparency occurs in the center with a transition to clear by the corners. However, these are typically designed for situations where the darkening is primarily from natural vignetting; otherwise changes in lens aperture would also necessitate a different ND filter.
Perhaps the simplest and most versatile way to address vignetting is in post-production. Files from cameras that record raw pixel information are capable of substantial changes in exposure after capture. A customized radial GND filter can therefore be applied which compensates for the lower exposure toward the corners, thereby offsetting light fall-off in the file itself. However, extreme corrections can also exacerbate image noise in the corners.
Subject matter also plays a key role with the visibility of vignetting. Scenes with low-key or uneven lighting can often tolerate substantially more vignetting:
Other times the visibility of vignetting is not a concern. Optical and natural vignetting are often beneficial, for example, since these can draw attention toward a central subject or make the edges of a frame appear to terminate less abruptly. For these reasons vignetting is sometimes added intentionally in post-production:
Although some vignetting is unavoidable with any optical system, the optimal amount will depend on image content and available camera equipment. The benefits of a shallow depth of field at wider apertures, a brighter exposure toward a central subject, or an ultra-wide angle perspective are often all worth it in exchange for a little more vignetting. In other situations, such as with high-key or evenly-illuminated scenes, even small amounts of vignetting is unacceptable. In those cases, the use of radial GND filters, either in post-production or as a physical filter on the lens, are a proven technique for removing moderate optical and natural vignetting.