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
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. 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.
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)
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
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 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.
Fig. 2 - A typical Hasselblad photograph showing
several lens artifacts. (NASA: AS11-40-5873)
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.