Is Photographing a Solar Eclipse Feasible with ND Filters?
Photographing a solar eclipse is a thrilling experience, but it requires special precautions to ensure both the accuracy of the image and the safety of the photographer. Specifically, using Night and Day (ND) filters for solar photography can be a viable approach, but it comes with its own set of challenges and considerations.
The Role of ND Filters
Specialized ND filters exist specifically for solar photography. These filters provide an attenuation of up to 1 million times, which translates to about 20 stops or an optical density of 6.0. Most ordinary ND filters offer
Attenuation of Near-Infrared Light
A significant concern with ND filters is their ability to attenuate near-infrared (near-IR) light. Near-IR light, which falls in the wavelength range from approximately 700 nm to 1500 nm, is invisible to the human retina but can still pass through glass and the optical components of the eye. If an ND filter attenuates visible light by 16 stops, it might mistakenly seem safe to look through, as the sun appears not to be very bright. However, if the filter is not effective against near-IR light, the remaining IR light could still be harmful and lead to permanent retinal damage.
Therefore, using a photographic filter for visual observation is risky, as these filters do not make claims about IR attenuation. Instead, mirrorless cameras and DSLR cameras in Live View mode are recommended, as the camera's IR filter can reject a modest amount of IR light, saving the camera sensor and shutter while exposing the user to potential hazards. Direct visual observation through a DSLR optical viewfinder or a telescope with a photographic ND filter is especially dangerous, where permanent eye damage might occur.
Pitfalls and Findings
During the search for solar filters before the April 8 eclipse, I encountered two types: filters intended for visual viewing and photographic ND filters. The visual filters, while explicitly stating that they attenuate IR and UV for safe viewing, often resulted in a bright haze around the sun during photography, an issue that hampers image quality. In contrast, photographic ND filters provided a clear image, but they did not claim to filter IR effectively.
To ensure both safety and image quality, I ended up using two different solar filters. One was safe for visual observing but not ideal for photography due to light scattering, while the other was excellent for photography but potentially hazardous for visual observation. Ideally, a filter would attenuate both visible and IR sufficiently for both purposes, but no such filter was tested. It's worth noting that some high-end metal-coated film filters, such as the Baader Planetarium filters, might perform better in both aspects, but more research is needed.
Conclusion
Photographing a solar eclipse with ND filters can be feasible, but it requires careful selection and understanding of the properties of the filters. While these filters help achieve the necessary exposure for clear images, they must also protect the photographer from harmful near-IR light. For the best results, it is crucial to use the right tools and techniques, ensuring both the image and the photographer's eyes are protected.