When viewing or photographing a solar eclipse, it is important to use a solar filter to protect your eyes and camera equipment. According to the search results, a solar filter must be used on the lens throughout the partial phases for both photography and safe viewing. The ONLY time the filter is not needed is during the height of a total solar eclipse. During totality, all solar filters must be removed to capture the suns corona.
There are different types of solar filters available, including metal on glass, aluminized polyester film, and black polymer. Safe solar filters for cameras and telescopes are available as either "Full-Aperture" and "Off-Axis" filters. It is important to inspect the filter each time, as even a pinhole in it can blind you when looking through optics.
It is not recommended to use variable ND filters for solar eclipse photography. Instead, a single 16-stop+ ND filter is recommended for non-optical viewfinder solar photography.
In summary, a solar filter is necessary for safe viewing and photography of a solar eclipse, except during the height of a total solar eclipse. Different types of solar filters are available, and it is important to inspect the filter each time. A single 16-stop+ ND filter is recommended for non-optical viewfinder solar photography, and variable ND filters are not recommended for solar eclipse photography.
