Vignette - Guillermo Algora - Visual Effects Compositor

Guillermo Algora
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VIGNETTING
Vignetting is an imaging phenomenon that happens with virtually every optical system and appears as a radial darkening of the image toward the periphery of the frame.
Table of Contents:
1. Natural Vignetting.
2. Optical Vignetting.
3. Mechanical Vignetting.
4. Pixel Vignetting.
1. Natural Vignetting:

It is a result of the geometry of the optical system and the light falloff that naturally occurs as the angle of incidence increases towards the edges of the lens. This variation in angles causes radial dimming, because light incident at the edges of a film/sensor has to travel a longer distance than light incident in the center, thus becoming progressively dimmer. At the same time, light hitting the image center at normal incidence is stronger than striking the image corner at the angle (this can be compared to a late afternoon sun, which warms the earth less than the midday sun because the same beam of sunlight is spread over a larger area). Finally, the lens aperture (pupil), as seen by the light rays, is more elliptical and therefore narrower at an angle, compared to a wider circle straight on.

It typically manifests as a gradual darkening towards the periphery of the image (which is usually smooth and blends in naturally) that results in a uniform vignette due to the cosine forth law: light falloff is proportional to the fourth power of the cosine of the angle between the peripheral light ray and the optical axis (the angle at which the light impinges upon the film/sensor, measured in image space at the rear end of the lens).

It is inherent to "any" lens design and most significant with wide-angle lenses. It cannot be stopped by reducing the aperture (higher f-stop) and is less pronounced on smaller camera sensors (due to the implicit reduction in the lens focal length).
Light at oblique angles distance
and dispersion
Aperture seen at an angle
Natural Vignetting
2. Optical Vignetting:

This type of vignetting occurs due to limitations in the optical design of the lens and shading from the lens barrel itself. Outer portions of the lens are less effective at transmitting light than the center, due to rear elements being shaded by elements in front of them, which reduces the effective lens opening for off-axis incident light.

It may appear as a more abrupt darkening towards the edges of the image, exhibiting more distinct and pronounced darkening patterns, often circular, but could take on a slight polygonal shape (similar to the shape of the diaphragm) according to the lens design, although this type of pattern may most be virtually imperceptible to the human eye.

Optical vignetting tends to be stronger in wide-angle lenses, due to the larger covered field of view, although the effect can be noticed with most photographic lenses (e.g. zoom lenses tend to have fair amount of optical vignetting), and at large apertures. At the widest apertures, due to the length of the lens barrel, peripheral light rays travelling at extreme angles are partially blocked by the lens barrel itself. Consequently, the light that reaches the image plane at such angles naturally falls off (decreases in brightness) towards the extreme corners of the frame.
The remedy is to reduce the lens diaphragm (higher f-stop): the darkening observed at full aperture already improves greatly when the lens opening is decreased by one stop, and a complete cure often requires two or three stops depending on the design. The darkening that remains at small apertures is due to natural vignetting.
Comparison of wide vs narrow aperture, front vs sideways
Simulation of optical vignetting at f/1.4 vs f/5.6
Optical vignetting
3. Mechanical Vignetting:

Mechanical vignetting occurs when physical obstructions "external" to the lens system reduce the amount of light reaching the film/sensor. This can be due to lens hoods, filters or adaptors that intrude into the light path, as well as certain lens designs that have physical barriers near the edges.

This type of vignetting typically happens abruptly only at the very corners of the frame due to the physical obstruction, and may present more defined edges, irregular shapes and patterns depending on the nature of the obstruction. It visually appears less abrupt at wider apertures due to the increased amount of light and with longer focal lengths as the angle of view narrows. However, under such context, there might be an increment in the presence of natural and optical vignette.
Mechanical obstruction
Mechanical vignetting
4. Pixel Vignetting:

Specific to digital cameras, pixel vignetting is caused by angle dependence in the digital sensors. Light incident on the sensor at a right angle produces a stronger signal than light incident at an oblique angle. It may appear as uneven darkening or lightening of pixels at the corners and edges of the image sensor, where light falloff from the lens combines with inherent sensitivity differences between pixels, which may result in color shifts or artefacts in the affected areas.
Micro-lens array and angle of incidence
Pixel vignetting
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