lens flare
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All but the very simplest cameras have compound lenses composed of several small lens elements. Zoom lenses and high-end professional lenses may have many components.

When light strikes a glass surface, most of the light passes through the glass (refracts) but a little bit of it reflects off of it. In compound lenses, light can bounce between the lens components and create spots of light on the final photograph.

If the light source is behind or to the side, not much light from it falls directly on the lens. This is what you want. You want your light to bounce off your subject and into your lens. But if you point your lens so that the light source shines into it a little, you'll get a lens flare.

But the lenses used on the Apollo missions were the finest available, custom made by Zeiss. They shouldn't have shown any lens flare.

Manufacturers of high-quality lenses (including Zeiss) have developed chemical coatings which reduce the reflectiveness of their lenses. Under most conditions this would have reduced or eliminated lens flare. Even with low-end lenses, it requires a very bright light source and a certain small cone of camera-and-light angles to produce a lens flare.

The primary responsibility for eliminating lens flare lies with the photographer, not the lens maker. The person behind the camera should be aware of lighting angles and adjust the scene accordingly. This is what all professional (and even most amateur) photographers do, and so you rarely see lens flare in other types of photographs. But the astronauts were not concerned with aesthetics. Their overriding goal was to document the mission, and that called for them to take pictures at whatever angle was appropriate. And they didn't have the benefit of a viewfinder either.

Fig. 1 - Cross section of the lens elements in the modern Zeiss Biogon T 4.5/38, a descendent of the lens developed for the Apollo missions. The horizontal line represents the optical axis. (Courtesy Zeiss, used by permission)
Fig. 2 was taken on the lunar surface with a Zeiss Biogon lens, the best available wide-angle lens. Fig. 1 shows a cross-section of a modern Biogon lens for a Hasselblad SWC camera.

The dotted horizontal line represents the optical axis. The curved white shapes are the lens elements. Each is a precisely ground disc of glass with the cross-section shown, coated with transparent anti-reflective coatings. The vertical lines represent the f-stop aperture.

Some of the lens elements are fixed in position in the lens barrel. Others are mounted in a moveable frame attached to the focus ring. They can move backward and forward along the optical axis to change the focus.

Ironically the Biogon lens was chosen because it was the wide-angle lens least susceptible to focal and chromatic distortion. Correcting for that can only be accomplished by having the many lens elements shown in the diagram. The number of elements increases the chance that a lens flare will occur.

Fig. 2 - A typical Hasselblad photograph showing several lens artifacts. (NASA: AS11-40-5873)
Fig. 2 displays several lens artifacts. The two very bright white spots at upper left are images of the sun reflected between the lenses. They are distorted by the curved surface, otherwise they'd appear circular. If you look carefully you can see a diagonal line of weaker reflections that includes the two bright ones.

The large irregularly shaped area of increased brightness is a reflection of the aperture, the mechanical device adjusted by the f-stop control (see Exposure).

The streak at the upper right is probably the reflection of another internal component.

The soft-edged patch of brightness taking up the top part of the sky is the image of light striking some substance on the lens. It could be the anti-reflective coating, or more likely fine lunar dust on the lens. The astronauts had to constantly brush the camera lenses to keep dust from accumulating on them.

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