Does staring at the sun cause significant, permanent harm?

Question

Of course, I think I can safely assume if an eye is left pointing directly towards sun for a long period of time it does get major damage. However, staring at the sun also hurts very quickly. That pain seems like a very natural^* defense mechanism against eye damage.

Also, I guess technically even a very quick glance at the sun does cause temporary eye damage, as there is a visible distortion spot in sight for a short while afterwards.

So the question is: Is possible for someone to naturally stare at the sun for an amount of time required for significant permanent eye damage, or whether the pain will become too much significantly sooner than any such damage would happen?

Then: If not, is it possible with simple willpower, without special training or medication, to force oneself to ignore the pain for a long enough time to cause such damage?

Also: Is there a difference in this between adults, children and babies?

And as a bonus, are there any special environmental situations where this does not apply? For example: sunset, sunrise, solar eclipse, major fog, underwater, or light filtered through something else.

^{* - Natural is here not in the sense of "everything natural is good", but in the sense of "it is reasonable to assume there would be an evolutive advantage from it".}

Answer 1

An ordinary person can easily stare at the sun or a solar eclipse long enough to cause significant, possibly permanent loss of vision.

Looking directly at the sun for even brief periods of time may cause blindness or severe damage to the eye. Solar retinopathy, damage to the eye’s retina due to solar radiation, and blindness to varying degrees and persistence frequently result from sungazing during a solar eclipse. Although vision loss due to this damage is generally reversible, permanent damage and loss of vision have been reported. Most eye care professionals advise patients to avoid looking directly at the sun. Exposure to ultraviolet radiation, produced by the sun, is associated with damage to the eye, including pterygium and cataracts.

(Sungazing)

Phil Plait recently cited an ‘‘unprecedented rise in the number of cases of solar retinopathy’’ directly linked to a popular Irish shrine where people stared at the sun in order to experience ‘‘visions of the Virgin Mary’’.

(Plait)

Answer 2

The amount of light emitted by the solar corona is far too little to do damage. So during a total eclipse, but only during totality, it is completely safe to look at the corona with naked eyes. NASA is a good source of information about eclipses, including safe viewing and photography techniques. According to the NASA Eye Safety page:

The Sun can be viewed safely with the naked eye only during the few brief seconds or minutes of a total solar eclipse. Partial eclipses, annular eclipses, and the partial phases of total eclipses are never safe to watch without taking special precautions. Even when 99% of the Sun's surface is obscured during the partial phases of a total eclipse, the remaining photospheric crescent is intensely bright and cannot be viewed safely without eye protection [Chou, 1981; Marsh, 1982]. Do not attempt to observe the partial or annular phases of any eclipse with the naked eye. Failure to use appropriate filtration may result in permanent eye damage or blindness!

So it would be a good idea, especially if the eclipse is not near local sunrise or sunset, to be aware of how long totality is expected to last, and be ready to look away as soon as you glimpse the "diamond ring" effect where the disk of the sun first becomes visible through a notch on the moon's limb.

Photography of the eclipse with a DSLR requires that you look through the camera. If the mirror were locked up, then the camera would be projecting a nearly in focus image of the sun onto the shutter curtain, and eventually onto the film or sensor. Clearly, significant damage to your eye and/or the camera is possible. To avoid damage, a solar filter should be used in front of the lens. NASA also has a good resource page related to eclipse photography.

During the 2006 eclipse I was very close to the center of the path of totality on a yacht in the Mediterranean, attempting photography with a hand-held DSLR with a 500mm lens. I had a Thousand Oaks Optical black polymer solar filter on the front of the lens, and it was fun to see the disk of the sun with decent magnification. However with that filter in place, the whole frame was dead black when pointed anywhere in the sky that didn't include the sun. Aiming was a bear. While practicing, I made the mistake of opening my other eye to check the larger view which was not a pleasant experience.

For use aboard ship, I punched one eye out of a cardboard pair of eclipse watching glasses that happened to be made with the same filter material I had on the lens. With the camera on the unfiltered eye, and the other eye filtered I was able to successfully aim and photograph the partial phase without further pain or injury.

During totality, I removed the filter and shot the corona with the same lens.

Answer 3

"Permanent damage to the retina has been shown to occur in ~100 seconds"

Source: http://scienceline.ucsb.edu/getkey.php?key=3269

A similar statement: "sungazing at bright midnoon for 100 s can produce a threshold lesion."

Source: W. T. Ham, Jr., H. A Mueller, and D. H. Sliney Retinal sensitivity to damage from short wavelength light Nature 260, 153-155 (1976) - cited here: http://mintaka.sdsu.edu/GF/vision/Galileo.html