As I understand, ND4 filter should transmit about 25% of the incident light. I have measured transmittance spectrum of my Fujimi ND4 58mm filter, and got the following result:
So, transmittance is about 40%-45% in the visible range of wavelengths. That’s quite far from the expected 25%. Is my filter not really ND4 (although I bought it as such, and it has the marking on its rim)? Or is my expectation wrong, and this filter is normal?