Can I photograph a solar eclipse using a 10-stop Big Stopper (+ extra ND?)
Will a 10-stop ND filter (Big Stopper) allow me to photograph a solar eclipse?
I read in another post here that solar filters are around 14-stops - presumably I would need around this strength even for an eclipse, as it's related to the peak brightness, not the size of the sun. Stacking a 10-stop and a 4-stop ND should result in a solar filter, even if it has significant vignetting?
If you have a removable lens, put the filters you intend to use in front and then focus sunlight into a sheet of paper. It should be really dim in the daylight. Compare with the image you get (without filters) for an ordinary scene. This is "back to basics" photography but it should allow you to answer the question safely and cheaply (no risk to eye or sensor). Don't drop the lens as you play with it and don't get fingerprints in the surfaces. If you mount it in a cardboard box with tracing paper at the focal plane it would be easier. If the paper catches fire your filter is too weak.
A "big stopper" reduces light by a factor of 1000x, whereas Baader Astro filter film reduces light by a factor of 100,000x. You may get away with using the big stopper if you're using live view on a dSLR but I'd seriously recommend avoiding viewing through the finder. If you fried your sensor that would be bad, but not as bad as burning a retina.
Stacking ND filters will further cut the amount of light coming, careful trial and error will be way forward to finding out. If you need to use the finder first put a piece of paper where your eye would be - this will give you some idea of the intensity of the light being passed by the filter(s). The Baader film is rated as safe for direct viewing and will therefore be a guaranteed safe choice for your sensor. And it's considerably less than £150...