How not to watch a solar eclipse
Be careful about how you watch a solar eclipse. It is not recommended to view it in the following ways:
Smartphone: Watching a solar eclipse on your smartphone camera can put you at risk of accidentally looking at the sun when trying to line up your camera. It could possibly also damage your smartphone camera. Don’t take the risk.
Camera viewfinder: Never look at a solar eclipse through the optical viewfinder of a camera. It can damage your eyes in the same way as looking directly at it.
Unsafe filters: Unless specifically designed for viewing a solar eclipse, no filter is safe to use with any optical device (telescopes, binoculars, etc). All color film, black-and-white film that contains no silver, photographic negatives with images on them (x-rays and snapshots), smoked glass, sunglasses (single or multiple pairs), photographic neutral density filters and polarizing filters are unsafe filters to watch a solar eclipse. Also, solar filters designed for eyepieces that come with inexpensive telescopes are also unsafe. All of these items can increase your risk of damaging your eyes.
(See original article: https://www.preventblindness.org/solar-eclipse-and-your-eyes)