A lot of people took pictures of the recent solar
eclipse in North America and got photos where there’s a ghostly image of the eclipse floating
in the sky nowhere near where the sun is! The short explanation is that the ghostly
eclipse images are lens flares: the thing that happens when you point a camera towards a
particularly bright light source and you get a glow or streaks or disks... or eclipses?
The eclipse lens flares are cool in two really different ways - first, they allow
you to actually take an image of a partial or annular eclipse without fancy photographic
filters to darken the sun! Though my lawyer tells me that you should never look directly at
the sun or point a camera at it for an extended period of time without a proper solar filter. And
second, eclipse lens flares reveal some really interesting facts about lens flares in general!
This is grossly oversimplified, but lens flares are basically, generally speaking, an optical
defect: they’re light that passes through a lens in an unintended way. Most modern camera
“lenses” are actually multiple glass lenses, called lens elements, working together to function
as a single optical device. In a perfect world, all the light falling on a lens would be bent
in exactly the way the designers intended, passing through each lens element and reaching
the image sensor or film to create an image. But physics is annoying: glass likes to reflect and
absorb and scatter light as well as transmit it, and no matter how careful you are with
anti-reflective coatings, no lens is perfect. Every single lens element, including the aperture,
will in principle “incorrectly” reflect or scatter or diffract a small amount of all light that
hits it, and that light then bounces around in the lens before hitting the inside wall
of the lens, or going back out the front, or hitting the image sensor. But even when it hits
the image sensor, you don’t notice most of this incorrectly bouncing light, especially
if the lens makers did their job well. It’s all about relative brightness - if, say, 99%
of light passes correctly through a lens element and only 1% of it bounces around and causes
a lens flare, then that flare is at most 1% as bright as the correct light, and you probably
won’t notice it in the final image. This is how one-way mirror glass works, for example, or why
it’s really easy to see into a house at night, but hard to see out. Relative brightness!
When you do have a bright light, oh, do lens flares come in all varieties! Glow
and rings and rays and starbursts and disks and rainbow arcs and so on - the appearance of
flares depends on a lot of factors: the shape and positioning of lens elements, the coatings on
them, the focus and zoom and aperture settings, how bright the light is, and even the
direction the bright light is coming from. Anything that has a line of sight to the front
of the lens – whether within the field of view or not – anything that could in principle cast
light of any sort onto the lens is technically speaking causing lens flaring of some type
somewhere in the image. Most of the flares are simply too faint/dark to notice. And
lens makers typically work very hard to engineer lenses to minimize the reflections and
absorption that lead to flares, though defects are impossible to eliminate entirely. You only
start noticing flaring when the light causing the flare - whether the light source is visible
in the image or not – is bright enough that, even when it’s darkened to 1% or 0.1%
or 0.01% of its original brightness, that darkened light is still brighter than - or at
least similar in brightness to - the other things in the part of the image where the flare appears.
The sun, of course, is definitely bright enough that even when you darken it by a factor of a
hundred, or a thousand, or ten thousand, it’s still bright. In the case of the eclipse photos,
some of the light from the sun is presumably illuminating or scattering off of a lens element
somewhere within the lens (rather than passing through as intended), and that creates a flare
that looks like the eclipse. This flaring also happens if you take a picture of the sun not
during an eclipse, but in that case it’s really hard to tell whether the flare is a circle because
the sun is a circle, or because the lens aperture is a circle, or because the bright light source
was blurred into a circle, and so on. And we’re so used to seeing lens flares in images that most
of the time we don’t really even notice or think about them. During an eclipse, though, the lens
flare is different - in the right circumstances, the flare will be an image of the eclipsed sun
itself! Though mirrored across the field of view the way some some lenses flip things upside down.
There are two really cool things about this lens flare: first, the fact you’re seeing the eclipse
in the flare tells you that if you took a picture of the sun with that camera on a normal day, the
flare you’d see IS an actual image of the sun, and not just a weird out of focus orb - which
is really cool! Second, regardless of how the flare was created within the lens, its light
had to have been dimmed enough relative to the powerful brightness of the sun that the
flare doesn’t create its own haze and glow and flaring - it’s a correctly exposed photo
of an eclipse, obtained using a lens defect rather than a solar filter. and this is part of
what lets you see it clearly in a photograph! In a sense, when you take a picture that has a
lens flare that’s an image of the eclipse, you are actually, directly, photographing the eclipse
- just not the way the lens makers intended. ps - here’s a photo that puzzled me at
first: the flare isn’t opposite the sun, but instead is directly where the sun should be!
I wondered what was going on until I realized... this isn’t a flare, it’s just a direct image of
the eclipse! The clouds passed in front of the sun enough to darken it enough for the camera
to take a photo directly without glow or flares to obscure the image. It’s all about relative
brightnesses! And if you’re actually out in the real world photographing bright objects,
well, if something’s a flare, you can put your hand “behind” it (because the flare is happening
inside the lens), while if it’s really the object, then the only way to put your hand behind it
is to actually get behind the object (which, with the sun at least, is kind of hard).
While you’re waiting for the next eclipse to get hands-on with the sun, it’s a good time to
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- and thanks to Brilliant for their support.
