Name: But What IS A Lens Flare?
Uploaded: 2025-06-19T17:54:24.000Z
Duration: 5 min 18 s
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I saw some pictures of the recent solar eclipse in North America, 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 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 the 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.

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.

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 the 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 to notice, and lens makers typically work very hard to engineer lenses to minimize the reflections and absorptions that lead to flares, though defects are impossible to eliminate entirely.

You only start noticing flaring when the light causing the flare 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 even really 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 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.

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.

In a very real 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.

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 pass in front of the sun enough to darken it for the camera to take a photo directly without glows or flares to obscure the image.

And if you're actually out in the real world photographing bright objects, if something's a flare, you can put your hand behind it because the flare is happening inside the lens, while if the bright thing really is the object itself, 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.

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