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In watching videos and reading advice for photographing a solar eclipse, most recommend to use a Solar Filter. Searching online, it is unclear to me if there is a difference between a Solar Filter and an ND-Filter?

Many of them in stores are described as 16-stop, 18-stop, 92,000X or 100,000X ND Filters but apart from the strength, is there a difference in optical properties? If not, would a stack of 2 or 3 ND-Filters produce similar results to a Solar Filter?

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Main Differences

The main differences between most neutral density(ND) filters and a solar filter(white light) come down to the filtering strength and the filtering properties. The strength of the more common ND filters range from 1-10 stops, where as for safe solar eclipse viewing you want to use 13 or more stops for imaging and 16 or more stops for direct viewing. More information of that topic can be found here (Can I photograph a solar eclipse using a 10-stop Big Stopper (+ extra ND?))

As for the filtering property differences, standard ND filters typically only cover the visible spectrum where as solar filters will also cover Infra-red(IR) and Ultraviolet(UV) radiation. I'm not aware of a regular ND filter that includes IR/UV attenuating and is 13+ stops, so I would be wary of both your camera sensor and eyes using any ND filter setup.

Less Critical Differences

Some solar filters actually render the sun in different colors such as yellow, orange, blue, and white - something you won't see in a standard ND filter.

Other Thoughts

You can read anecdotes all over the internet about people who do capture eclipses without proper solar filters, but ultimately it is not recommended if you want to protect yourself and your equipment.

There are many ways to measure the transmissiveness of filters. Be very careful comparing ND scales to optical density (OD) scales for solar filters (i.e. ND 8192 = OD 3.9 and ND3 is nowhere near OD3!), and when in question, don't ever bet your eyes on something you aren't sure of!

Sources 1,2,3,4

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    Infrared from the sun can damage your eyes without you even knowing it. The retinas have no pain receptors. Viewing the sun through a camera not properly filtered can literally cook the surface of the retina. Most of the effects on vision don't begin to show up for hours afterwards as scar tissue begins to form on the surface of your retinas. – Michael C Jul 1 '17 at 4:23
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    Good stuff all around. Just to add to the caution, when buying a solar filter for your camera, make sure it says it's "safe for viewing". There are several filters that are OD 3.8 (about 12 1/2 stops) that are intended for digital imaging. They are not considered safe for direct visual viewing (including through an optical viewfinder). For those types of filters, the safe-for-visual is usually around OD 5.0 (about 16 2/3 stops). – scottbb Jul 1 '17 at 13:43

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