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Understanding Lens Vignetting

Vignetting is an imaging phenomenon that happens with virtually every optical system. It can even 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:

Optical & Natural Vignetting
Mechanical Vignetting

  • Natural vignetting appears as a gradual darkening and is primarily caused by light reaching different locations on the camera sensor at different angles (see next section for more). This type of vignetting is most significant with wide angle lenses.

  • Optical vignetting is also gradual, but is primarily caused by intrinsic lens characteristics and shading from the lens barrel itself. It's also what ultimately determines the size of a lens's imaging circle. This type of vignetting is more pronounced at wider apertures, and is strongly influenced by the particular lens design.

  • Mechanical vignetting typically happens abruptly and only in the corners, since this is caused by matte boxes, filter rings or other objects physically blocking light in front of the lens. This type of vignetting appears less abrupt at wider apertures and with zoom lenses, and can often be eliminated by using a longer focal length.

    • 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 farther 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 farther 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 when viewed straight-on).

    CONTROLLING VIGNETTING

    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.

    Higher Resolution
    (Lower Crop Factor)
    Lower Resolution
    (Higher Crop Factor)

    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 35mm lens is unlikely to vignette on a Super 35 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.

    Radial GND Filter

    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.

    Substantial Vignetting
    Removed in Post-Production

    SUBJECT MATTER

    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:

    Imperceptible Vignetting with a Low-Key Exposure

    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:

    Without Vignette
    With Vignette

    ON BALANCE

    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.

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