So would not mind some recommendations for a replacement 3-stop ND filter. It will be awhile before I get one, unless I drop it in the sand one too many times (it being fall now and my subjects being lightly clothed humans, probably won't be at the beach at sunrise until spring at the earliest). At least not in my location.
Hi Tim,
What a coincidence. To further reduce the power of my studio-flash units when used at shorter distances, I just purchased a 3 stop ND filter. Since I wanted to try the Haida filters because they are relatively affordable, I also purchased an ND 0.9 (3-stop, 8x), Pro II coated version.
I've recorded it's spectral transmission characteristics by measuring the spectrum of daylight (6586 k) with and without filter, and plotted the absorption of the filter expressed as Density (-Log10(transmission%) ), because I wanted to know how accurate it would match the intended 0.9 density. Here is the result:
It shows that there is a slightly warm response, a bit more UV and Blue density (less transmission), and a bit less Red density (more transmission), but overall it hits the 0.9 density pretty well across the visible spectrum. The slightly higher IR transmission will be compensated for by the IR-filter that covers the sensor (combined with the Optical Low-pass Filter, if present). The exact color response therefore depends on the particular camera it is used with, but it looks pretty neutral to me.
I also compared the spectral transmission, and density, of both the Lee Big Stopper and the Haida Pro II version of these 10-stop (ND 3.0, 1000x) filters. Here are the results based on the absorption characteristics of Daylight (6568 k), as a Transmission plot and as a relative Density plot:
As can be seen, the Lee filter is slightly smoother across the spectrum, but both have a similar profile which allows to correct the Whitebalance with a simple Kelvin and Tint correction. Some spectrally very pure colors may be hard to get exactly right without targeted post-processing due to the more bumpy absorption spectrum in the yellow-orange region. As always, shooting Raw-file data is preferable, also because these filters do introduce additional light fall-off towards the corners (especially on wide-angle lenses), which can be compensated for by a Flat-field compensation or Lens Cast Calibration which is best done in Linear gamma Raw before demosaicing.
The average density of my Haida Pro II ND 0.9 filter across the visible spectrum is calculated to be 0.95 (3.16 stops, 8.9x) which is pretty accurate for such a density. The Lee BigStopper is 2.51 (8.3 stops, 323x) and the Haida Pro II is 2.64 (8.8 stops, 443x), both fall a bit short of the intended 10 stops in the center of the image, so make sure to check the histogram of your image on location to avoid overexposure. Especially with wide angle lenses the images will be a bit darker in the corners, so one should try to avoid underexposure there.
Cheers,
Bart