I would like to know if, when darkening (or blackening) a piece of glass with a candle:

  • the filter thus made is "neutral"?
  • if it catches UVs or IRs?
  • there is anything else I am missing?

I would suggest not; darkening in this case would be by means of tiny particles of soot, which in themselves are opaque. Any darkening would be by means of scattering, and as evidenced by blue skies and red sunsets, particulate scattering is wavelength dependent, with transmitted light taking on a reddish tinge as opposed to the bluish tinge from refracted light. You can also demonstrate this by taking a glass of water, adding a small amount of milk, and shining a torch through it - the orange hue will be clear if you look directly at the torch through the milky water.

If you use a glass filter darkened in this way, I would expect an orange hue in photographs taken with it - which though possibly not as desirable as a true ND filter, may still produce interesting effects, especially if the coating of soot is uneven. Give it a try and see what happens!

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  • Interesting ... I guess you are referring to the Rayleigh diffusion ? I'll definitely give it a try. Wondering if it will allow me to take nice pics of the Sun... – Skippy Fastol Jul 2 '12 at 12:09
  • +1 "interesting effects, especially if the coating of soot is uneven." – Vorac Jul 5 '12 at 8:39

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