Flares - Guillermo Algora - Visual Effects Compositor

Guillermo Algora
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Glowing artefacts caused by light reflecting off (or improperly refracting through) the lens elements. Lens flares can vary widely depending on the lens design and the position of light sources relative to the lens.

They are often intentionally incorporated in post-production for artistic effect, adding a a sense of realism, visual interest, mood and atmosphere to the image. Their strategic placement can enhance the composition and storytelling of the image. However, care must be taken as, in some cases, it can detract from the overall quality of an image, especially when it obscures important details or incorporates distracting elements.
Table of Contents:
1. Veiling Glare.
2. Ghosting Flare:
2.1 Position.
2.2. Shape.
2.3 Color.
3. Filter Flare.
4. Digital Sensor Flare.
1. Veiling Glare:

Glare is a perceptual experience of reduced visibility caused by excessive brightness or reflection of light from a large area, which can even enter from out of frame. It presents itself as a overall haze or fog over a considerable portion of the image, which lowers the contrast and reduces detail. It is caused by bright light sources that scatter and bounce throughout the lenses assembly without forming an image, and behaves in such a way that there is no clear cut-off between it and the rest of the image.
2. Ghosting Flare:

Unlike veiling flare that makes images appear hazy with very little contrast, ghosting flares (commonly simplified to lens flares) represent artefacts from the lens assembly that are more clearly distinctive in the image. Different lenses may produce distinct flare patterns and colors, ranging from subtle streaks to pronounced blobs or circles.

2.1 Position:

The position and orientation of light sources relative to the lens influence its appearance and intensity. Lens flares are more likely to occur when bright light sources are positioned within or near the frame, and from sources at obtuse angles (an angle that measures greater than 90° but less than 180°) relative to the optical axis.

The patterns typically spread widely across the scene and in a direct line from the light source. Since these reflections occur within the lens, they tend to appear along the path that the light takes: a line that connects the point light source and (via) the center of the screen, where the image is formed. The number and position of the artefacts depend on the construction of the lens, as well as its deflection from the source, i.e. the angle between the optical axis of the lens and the line from the lens to the flare source. As the angle decreases and the optical axis comes to point into the source, the circular ghosts are brought closer together. (but in images goes to the bottom corner? not center)

Flare artefacts change location with the camera's movement relative to the light source, tracking with the light position and fading as the camera points away from the bright light until it causes no flare at all.

2.2 Shape:

Lens flare artefacts can manifest in various shapes and patterns, including circles, lines, blobs, rings or polygons depending on the specific configuration of the lens elements and the angle of incident light.

The lens aperture can influence the shape and distribution of flare artefacts. Wider apertures tend to produce softer flares with less definition, while narrower apertures tend to define more the flare's pattern. At the same time, when the diaphragm is wide open, most of the internal reflections happen within the curved glass elements, leading to generally round flares. As the lens is stopped down (narrower aperture), internal reflections caused by the aperture blades are greatly amplified, introducing a higher chance of an effect called aperture ghosting, which can result in artefacts that closer resemble the aperture's shape, for example, non-circular apertures, such as those with six or eight sides, may produce polygonal or geometric flare patterns.

The number of ghost artefacts may vary by the number of elements within the lens. Typically, the more elements are in the lens, the more ghosts will appear in the image, as these often occur through interreflection between optical elements

2.3 Color:

Lens flare can exhibit a range of colors, from subtle hues to vibrant tones. The colors of flare artefacts may vary depending on factors such as the lens coatings, light source temperature and the presence of chromatic aberration (separation of light colors within the lens).

Ghosts from uncoated lenses are usually similar to the color of the source, while in coated lenses often determined by the anti-reflective coatings of the lens surfaces causing it. Coatings work by producing destructive interference: light is partially reflected from the coating surface and from the glass surface, with the coating thickness equal to some fraction of the wavelength to be cancelled. This causes reflected light to cancel itself out, and multi-coating approaches are able to cancel reflections across a range of wavelengths. The side-effect of these anti reflective coatings is the color they impart on the reflections that remain, usually seen in a variety of pinks, greens, and blues.

In general, fixed focal length (i.e. prime) lenses are less susceptible to lens flare than zoom lenses, because the later generally have more elements, which means that there is more potential for light to bounce around and create flare. Wide angle lenses are often designed to be more flare-resistant to bright light sources, mainly because the manufacturer knows that these will likely have the sun within or near the angle of view.
3. Filter Flare:

Filter flares are unwanted artefacts that occur when light bounces off the surface of a filter (e.g. UV, ND, protective, hot mirror, etc.): between the filter and the front of the lens element or in between multiple filters. They can reduce the contrast and clarity of the image, create ghost images or halos, and introduce unwanted colors or tones.

Although most manufactures use anti-reflective coatings, and decent filters are unlikely to show much or any flaring, filter flares are unfortunately more likely to happen when using strong  filters or multiple ones, when there is a bright light source in or near the field of view or a filter is dirty or scratched, often being the result of a combination of at least two out of the three.

Filter flare commonly occurs directly opposite the optical axis (i.e. center of the frame) from an excessively bright light source, and often portraits an inverted (i.e. rotated by 180°) ghost image of the light source, as it is a reflection under central inversion, with the diffuse properties of it varying in degree. Central inversion (also known as point reflection or central reflection) is a geometric transformation that flips an object across a central point called the center of inversion. This transformation is akin to folding the object over itself so that every point on the object is equidistant from the center of inversion, but on the opposite side. In the context of filter flares, the center of inversion is typically the optical axis of the lens system (an imaginary line passing through the center of the lens and perpendicular to its surface).

Filter flares are most of the time very subtle, as if it happens to fall over an area where there are highlights or more confused tones it may probably be imperceptible, but visible if the light reflects with enough intensity to be picked up brighter than a section of the scene. Although most likely minimal, using a narrower aperture can help reduce filter flares to some extent, since by stopping down the aperture (higher f-number), there is a reduction of the amount of light entering the lens at oblique angles, which can help mitigate filter flares. Might not be true.
4. Digital Sensor Flare:

This form of flare is specific to digital cameras, and in essence caused by the reflective nature of the digital sensor. Due to this, and unlike other lens flares, it is not visible through the viewfinder of the camera.

This phenomenon manifest as a faint grid of "rainbow" artifacts, although most often a series of red, green and blue dots that may blend with each other, projecting under a flare form from the light source. The pattern arises from the physical characteristics of the imaging sensor: an arrangement of discrete photosites, arranged in rows and columns, separating the light into its component colors (red, green, blue).

It only happens when the light source is very intense and the lens aperture small (typically f/11 or above). With a narrow aperture, the light rays take a more indirect path through the lens compared to a wider aperture, increasing the chance of light bouncing around internally and reflecting off the sensor. As the light rays reach the sensor, pixels (maybe a better name, micro lenses?) on the sensor that get hit with the bright source of light, especially at odd angles, reflect some of the light back to the rear element, creating the characteristic grid pattern. The magnification of the pattern from the sensor is due to the flange distance (distance from the rear lens element to sensor). The small pixels from the sensor enlarge as they bounce back to the rear lens element and once again as they reflect back from it to the sensor.

Technically, this happens for every ray of light entering the camera, but only perceptible when the light source is bright enough to reflect with intensity, as there is a degree of attenuation for each round trip of reflection, which can be observed by comparing how darker the artefacts are in relation to the light source that causes the issue.

While the effect can be seen on pretty much any digital camera, it is highly amplified on mirrorless cameras with short flange distance, as the intensity of reflections is higher at such short distances. If the flange distance is doubled like on DSLRs, the intensity of the sensor - rear element reflection is much more diminished. However, this does not mean that DSLRs are immune to this particular issue, particularly under narrower apertures.

Modern lenses designed for digital cameras incorporate specialised coatings to solve this problem to a certain extend, and therefore the issue is often magnified when using film lenses.
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