Neutral Density Filter Tests

Hitech 10-stop neutral density filter in action
I recently added some neutral density filters to my set of gels.

I picked up a used set of Cokin P graduals, a Hitech 0.6 (ND4), 0.9 (ND8) and a 10-stop Hitech 3.0 (ND1024).  These are all the standard (not pro) series filters from Hitech.

Many complain about color casts being introduced by the standard 3.0 filter.  This inspired me to do a few tests digitally before I commit any film to these new filters.

My experience was quite interesting.  So far I've run into three challenges.
  • Heavy magenta color casts when the 3.0 filter is stacked with either the 0.6 or 0.9 filters.  No stacking the 10-stop.
  • Light flare coming from the back corners of the Cokin P holder
  • Vertical bars of tonal difference when the 3.0 filter with others

My first attempt was disastrous, never mind the boring subject.

Hitech ND1024 and ND4
I then made a hood out of duct tape to shade the Cokin holder and reduce the flare.

The following were all shot at f8 using a Canon 50mm/f1.4 lens on a Sony NEX-5 with an FD adapter.  I used ImageMagick to resize the JPGs produced by the camera.  I have raw files but didn't process them as I am only interested in the relative differences between images.

Hitech ND4, 1/160s
Hitech ND8, 1/100s

Hitech ND8 and ND4 stacked, 1/25s

Hitech ND1024, 1s

The ND4 and ND8 don't have a very strong color cast, even when stacked.  The ND8 seems to increase color saturation slightly.  The 10-stop filter is significantly cooler, with a blue-green cast but that should be easily correctable.  This is not the magenta cast that people complain about.

This rest are quite interesting.  I can't quite explain what I'm seeing yet.

Here you'll see what happens when the 10-stop filter is mixed with other Hitech NDs.

Hitech ND1024 and ND4, 2.5s

Hitech ND1024, ND8 and ND4, 6s
Notice the increasingly severe magenta color cast and the mysterious bar down the left side.  The sun was over my left shoulder at the time.  The color cast alone is enough to keep my from trying to stack these.

Next I tried stacking the NDs with the Cokin linear polarizer.

Cokin polarizer and Hitech ND1024, 4s

Cokin polarizer, Hitech ND1024 and ND8, 10s
With just the polarizer stacked with the Hitech ND1024, the color is correctable but the vertical bar is back.

Stacking the ND1024 with the ND8 gives the now-predictable magenta cast.

The last test shot incorporates a Cokin gradual ND.

Cokin linear polarizer, gradual ND and Hitech 1024, 5s
The blue sky was significantly muddied by the addition of the gradual ND.  Aside from the vertical bar issue, I think I'd want to edit this one from RAW before I make a practice of stacking a gradual with the Hitech 3.0.

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