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DV101: Light Reading: Learning to Create and Control Lens Flare

2/20/2013 10:59 AM Eastern

Back when I was a wee lad and an electrician working in films, a cinematographer once looked at me and said, “Do you know the difference between an accidental flare and an intentional one?” I thought for a moment and admitted I did not. He said, “Your day rate.”

A vibrant example of ghosting flare. The additional orbs of light in the image are direct reflections of the bright sources reflected off of other elements in the lens.

I had to laugh. What is an aberration and a fault of the optics can also be a quite beautiful and organic addition to the frame and mood of a scene.
A lens flare results from non-imaging light entering the lens elements, reflecting off the various surfaces within the lens and being re-photographed. The flare is not part of the scene being photographed; rather, it is created by the lens itself as an artifact of stray light.

To Shade or Not to Shade
Avoiding flares requires shielding the lens from stray light. The first line of defense against flares is a lens shade: a piece of plastic (or metal) that attaches to the front of the camera lens to extend the barrel slightly and cut off stray light.

A mattebox does this as well, and to a more efficient degree. Mattebox covers, French flags and side flags go further to eliminate stray light. If you’re looking to eliminate flares in your shot, any of these items will help for all kinds of flares except those created by a light source that is directly in the shot.

An example of spot flare from the sun over the shoulder of model Becka Adams. The long line of orbs is a set of reflections from various elements within the lens. Spot flares usually take on the shape of the lens’ aperture.

A Little Top Coat
Most modern lenses are manufactured to minimize flaring. A flare reduces the performance of a lens by reducing imaging contrast, therefore reducing resolution and sharpness. Lens manufacturers reduce flaring by adding a thin coating on the lens element that cancels out reflections. The lens coating works based on the laws of physics governing the cancellation of light waves that are at opposite frequencies.

Some light that strikes the surface of the lens is reflected back, which reduces the amount of light transmitted through the lens. This effect is mitigated with the application of a thin coating on the lens surface—usually silicon monoxide (SiO) or magnesium fluoride (MgF2). Most of the light will pass through the coating and lens surface with minimal refraction.

Some light will pass through the coating and be reflected off the surface of the lens, however, and some light will be reflected off the surface of the coating. The thickness of the coating refracts the reflecting light slightly so that the surface reflection and the coating reflection actually end up in opposite phases to each other and cancel each other out, thereby eliminating the reflection.

Some lenses have only one coated surface, some have multi-surface coatings. The more coatings, the more transmission of light and less flaring.

Seeing the Light
There are three primary categories of lens flare: ghost flare, spot flare and veiling flare.

Ghost flare happens when a bright source within the frame reflects off the rear of a lens element onto the front surface of another element at a different position, creating a dimmer mirror reflection of the light source in a different part of the frame.

Veiling flare. Indirect light on the lens does not create a distinct shape to the flare but rather serves to reduce overall image contrast—filling in the shadows with light—in this photo of model Carina Trinidad.

Ghosting flare is very hard to fight as it’s caused by a light source or a significant highlight within the frame that is reflecting off one or more elements within the lens and being re-photographed. If the sun or a bright light source is in frame, you’re likely going to have a ghosting problem. This often happens at night with car headlamps—and there’s not a lot you can do about it. The only immediate solutions are to eliminate the source of the flare or change lenses.

Spot flare occurs when a strong light hits the lens surface from off-axis and creates two or more reflections within the lens elements. These reflections generally take on the shape of the lens’ iris. Spot flare can be avoided by incorporating a lens shade or mattebox, flags, etc.—anything that will cut the offending light off the lens.

Veiling flare (sometimes called veiling glare) occurs when indirect light is reflected off one or more surfaces within the lens, reducing image contrast and sharpness. It is generally shapeless. Just like spot flares, veiling flare can be controlled by preventing errant light from hitting the lens.

Although lens coatings were first developed in the 1930s, the first major refinements to the process came in the 1960s and coatings have been improving steadily since the 1990s. This means that older lenses tend to flare more and have less light efficiency than newer lenses. There are occasions when flaring is desirable to the overall style of the image, however, and some very modern lenses can be very hard to flare, which leads the savvy cinematographer to reach for vintage glass.

Three images from an intensive lens test I conducted with Christopher Probst. Model Anne-Michael Smith. Testing services courtesy of CamTec Motion Picture Cameras. Assistance from Alan Hill

In this lens test we can note the difference between lenses of two different eras. The newer Zeiss Master Prime holds its contrast much better even under harsh flaring conditions. The spot flare, especially, of the Kowa is more pronounced than the Master Prime, and the Kowa’s veiling flare affects more of the overall frame than the Master Prime.

Even though a flare is an optical aberration, it can be a wonderful addition to any shot or scene. A flare can add a magical quality and is generally accepted by the audience as an organic part of the frame.