Last week I went to the optics laboratory of my local university (for other reasons) and I Measured this filter from 300 nm (deep ultraviolet) to 900nm (IR). It turned out the performances are very bad for all Wavelengths:
- 800-900nm the absorption is about 10%, very high
- 550-800nm the absorption is variable from 7% to 10%, it Should be constant and lower (5%)
- 400-550nm the absorption Increases linearly: 10% for 550nm and up to 20% for 400nm !! from the hoya catalog It Should Be Still less than 10%
- From 400nm to 380nm (still visible light: violet colors) the absorption is too high but constant, then ... the absorption 380nm-340nm for Decreases, It Means it lets more UV through visible light than the last! (The 380-400nm).
- Below the 340nm absorption Increases, but it's normal for most glasses, nothing new here. At 330 nm the absorption is only 60%, it Should be more.
The linear increase in absorption from 550nm down to 400nm makes the filter yellowish and changes the white balance (most of the time it gets automatically corrected, but still ... 20% light is lost for nothing).
This is not even good as lens protector, Given the amount of light you lose without any advantage. Filtering more violet (NOT ultraviolet) light than the UV light is ridiculous.
This is a chinese copy of Hoya filters. Do not buy: buy a real one (if it's cheap, it's probably a copy) or buy simple transparent glass for protection (cheaper and original).
