An eclipse is one of the few things worth rearranging a day for. It is also the one sky event that regularly sends people to eye doctors afterward with vision they will not fully get back.
The reason is simple and worth stating bluntly. Looking at the sun without the right filter burns the retina, and the burn does not hurt while it is happening. There is no signal telling you to stop.
Why the injury is painless
The retina lines the back of the eye and turns light into signals. It has no pain receptors, so nothing can tell you its light-sensing cells are being cooked.
Your lens focuses sunlight onto a spot on the retina much as a magnifying glass focuses it onto paper. Heat and a chemical reaction in the light-sensing cells then damage the tissue, both within seconds.
The injury is called solar retinopathy, and it lands on the central part of the retina, the part you read and recognize faces with. Symptoms typically appear hours later: a blurred or gray patch in the middle of your vision, straight lines that look bent, washed out color, or a lingering afterimage that never fades.
Some people recover partially over months. Some do not. There is no treatment that reverses it. An eclipse is dangerous precisely because the dimmed sky removes the discomfort that normally makes you look away, while the exposed sliver of sun stays as intense as ever.
What certified eclipse glasses are
Safe solar filters are not dark sunglasses. They are made to a specific international standard, ISO 12312-2, and they block roughly all ultraviolet and infrared light along with all but a tiny fraction of visible light.
Through a proper filter you should see nothing at all except the sun itself. If you can make out room lights, a lamp, or the outline of furniture, the filter is not safe, no matter what is printed on it.
Get them from a reputable source. Counterfeits are a real problem and printed certification text is easy to fake. Astronomy organizations publish lists of vetted suppliers, and libraries, science museums and planetariums often hand out verified ones.
What is not a solar filter
- Sunglasses of any darkness, including polarized ones, and any number of pairs stacked together
- Smoked glass, exposed photographic film, or X-ray film
- Space blankets, foil, mylar balloon material or CDs
- Welding glass below shade 14
- Looking through water, or at a reflection in a window or puddle
- A phone screen showing the sun through its own camera
Using them correctly
- Inspect them first. Any scratch, pinhole, tear or separation from the frame means throw them away.
- Put them on while looking away from the sun, then turn your head up. Turn away before removing them.
- Children need an adult in charge of the glasses, watching that they stay on and stay put.
The only exception, and it is narrow
During a total eclipse, in the narrow band where the moon fully covers the sun, it is safe to look with bare eyes for the brief period of totality only. The instant any part of the bright disk reappears, filters go back on.
That exception does not apply to a partial eclipse, and it does not apply to an annular eclipse, where a ring of sun stays visible the whole time. If you are outside the path of totality, there is no moment during the entire event when it is safe to look without a filter. If you are not certain which kind you are watching or where you are standing, keep the filter on.
Indirect viewing is safer and works well
Projection shows you the eclipse without anyone looking up at all, which makes it ideal with children.
- Pinhole projection. Punch a small hole in card, stand with your back to the sun, and let the light fall through the hole onto a second card or the pavement. You will see a crescent image of the sun.
- Kitchen equipment. A colander or a slotted spoon projects dozens of crescents at once.
- Tree shade. Gaps between leaves act as natural pinholes. The ground under a leafy tree fills with crescents during a partial eclipse.
In every case you are looking at the shadow or the projected image, never back through the hole at the sun.
Cameras, binoculars and telescopes
This is where serious injuries happen. Binoculars, a telescope or a camera lens gather far more sunlight than your pupil does and concentrate it. Looking at the sun through any of them without a proper front filter can destroy central vision instantly.
Two rules cover it. The filter must fit over the front of the instrument, so sunlight is cut before it enters. And eclipse glasses held at the eyepiece are useless, because concentrated light will melt straight through them.
Phone cameras will not damage your eye, but they can damage the sensor, and lining up a shot tempts people to glance up. Solar filter film cut for a phone lens solves both problems.
If you think you looked
Do not wait to see whether it settles. Symptoms are commonly delayed by several hours, so feeling fine right afterward proves nothing.
Book an eye exam promptly, and go the same day if you have a blur or dark patch in your central vision, distorted lines, or a persistent afterimage. An eye doctor can examine the retina, document any damage and follow it over time. Even without a cure, knowing what happened matters for how your vision is monitored from here.
Common questions
How long can I safely glance at the sun?
There is no safe glance. Injury has been documented from brief unprotected viewing, and there is no threshold to aim at. Filter on, or look at a projection.
Are last eclipse's glasses still good?
If they meet the current standard, are undamaged and are not the flimsy kind that has been rattling around a drawer, they can be reused. Inspect them against a bright lamp first. When in doubt, replace them.
Do I need filters for my regular glasses or contacts?
Eclipse glasses go on over your regular glasses. Contact lenses offer no protection at all and are irrelevant to the question.
Watching an eclipse safely comes down to one decision made in advance: a certified filter, or a projection, and nothing else. If you did look, or your vision has seemed off since, it is worth being checked, and you can find an eye doctor nearby who works with your vision plan without much trouble.