Why does my "ND4" filter transmit about 40% light instead of 25%?

Question

I tested a Fujimi 58mm filter labeled "ND4" and measured about 40–45% transmission across the visible spectrum. I also checked it with a lux meter, my Canon EOS 1100D meter, and RAW histogram comparisons, all showing roughly 38–41% transmission.

I expected an ND4 filter to pass 25% light (about 2 stops). Does this mean the filter is mislabeled, or are there different ND naming conventions where this result is normal?

user58107 · Accepted Answer

The filter is almost certainly an ND0.4 (or ND.4) filter.

Filters specified as ND .4 have a transmissivity of 10^(1/0.4) which is .398. Looking at the 4 measurements the OP made, the reflectance at 520nm (approx. peak of visual response) is .40. The other 3, lux, exposure steps of 1/3 f-stop, and raw histogram range from .38 to .41. This corresponds very closely to an ND.4 filter.

There are several ways neutral filters are designated. The most common is in multiples of half stops. Thus an ND2 filter is 1 f-stop, ND4 is 2 f-stops and so on. This yields transmissivity of .50, and .25 which are not even close. Another convention not in play here is to label the filter with a number followed by "x" which shows the transmissivity decrease factor. Thus one stop of attenuation would be labeling the filter 2x, 2 stops, 4x, 3 stops 8x, etc.

Given the very close results of the 4 techniques used to measure transmissivity, one must conclude the filter is best described as a ND0.4 or ND.4 filter. Perhaps the filter was made by Fujimi to target the Tiffen market which uses this designation. Or some error in their translation or even manufacturing.

Whatever the case, there is little doubt it is a ND .4 filter and not an ND4 or ND2 as they are usually labeled.

Why does my "ND4" filter transmit about 40% light instead of 25%?

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