Worse than this... while I was in Casper imaging the eclipse, one of the club members in my group spotted a guy nearby who was looking at the sun without solar glasses.
We had free solar glasses to give away (compliments of Google & NASA - many of us are invovled in outreach and they were keeping us supplied with glasses to give away). We told him to take a pair.
He told the club members (I wasn't nearby) that he was one of Stan Lee's real-life super-humans and that he believed his eyes could not be damaged by looking directly at the sun. He claimed he had been staring at the sun all morning and was not blind (apparently that was his confirmation that he had super-hero eyes.)
I am not a doctor and have no special training on the physiology of the eye. But as I like to do solar observing and do outreach to schools... I thought it would be a good idea to get educated on the sun & eye-safety (more than just "don't stare at the sun" - I wanted to understand the process and consequences from informed sources.)
There are no pain receptors in the back of your eyes. So as you destroy the retina cells, you wont feel it. But this process is a bit like getting a sun-burn. You don't experience the effects straight away... it sets in a few hours later and that's when you realize you over-did it. It can take 24-48 hours before the effects set in and usually the effects wont be apparent for at least 12 hours. You stare at the sun, think you're ok (you felt no pain) and can still see afterwards. Many hours later the damage starts to set in. The damaged area in the back of your eye begins to swell and your cornea can no longer produce a focused image on that area. Your vision might recover (or partially recover) -- or you might have permanent damage.
Anyone who thinks they may have stared at the sun too longer should seek medical attention.
This guy told us he had been looking at the sun for hours. We kept urging him to take the glasses... he refused. Once we explained the process.... (no pain recepters... and symptoms wont show up for hours and often 24-48 hours) he sort of had this "uh oh" look on his face and then left the area.
A person can get another camera. They can't really get another set of eyes. That was stupid and tragic.
Back on the topic of cameras.... one of the biggest factors is the physical size of the "entrance pupil" of the lens or telescope. This is the phsyical diameter of the objective lens. We sometimes refer to telescope's as "light buckets" - because if you think of the photons of light in the way you might think of raindrops, then all the "drops" hitting that physical area are funneled to the back. The larger the area, the more energy you can collect and focus. ... when you're trying to look at really really faint objects in in the universe, the bigger the light bucket... the better. When you're looking at the sun, you want to try to find a way to reject and/or dump the energy without doing damage.
The filter on the front (or the solar glasses for your eyes) is a basic energy-rejection-filter. An ND 5.0 solar filter is blocking 99.999% of the Sun's light. I'm told the "safe" limit is 99.997% (you must block at least that much or more to be safe for extended viewing.)
I have a refractor with a white light "solar wedge" (also called a "Herschel Wedge" because the device was invented by astronomer John Herschel back in 1830. See https://en.wikipedia.org/wiki/Herschel_wedge ). This device is different because all of the Sun's energy passes through the telescope and the energy isn't dumped untiil it reaches the back -- at the eyepiece end.
Anyway, the caveat for my particular instrument is that it can only safely be used in refracting telescopes with an objective lens aperture of 4" or smaller. Anything larger and it will collect more heat than it can safely dump and the reflection may be too bright to be safe.
And of course... this is a device designed to take the heat and dump it. The camera has nothing in it that's desgined to do that.
They demonstrated the damage using a Canon EF 600mm f/4L IS II USM lens. At f/4 that would put the effective aperture at 150mm. That's even too much for something with a herschel wedge at the back dealing with the heat. The camera doesn't have anything like this.
Tim Campbell
5D III, 5D IV, 60Da
