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Sunday, May 15, 2011

••◊ Neutral Density and the Fader-ND, part II

When I was writing my last blog post a question came to mind; "How do I quickly adjust for the Fader-ND's green tint if I'm already white balanced?"  A quick experiment was devised in my living room the next afternoon.  First we'll start with a simple setup that includes the camera in live view and a white balance card.  I did a custom white balance to the card, which is illuminated by simple afternoon daylight through the window.  I figured this was a reasonable test configuration since I typically use the Fader-ND outside in bright daylight conditions.  If you click on the photos you'll see that the histogram shows equivalent RGB values, indicating a good white balance. 

Next I put on the Fader-ND at the "Min" setting, which as we know from the last post means about -1.6 stops of "neutral" density.  Therefore I had to open the aperture to compensate.  I ideally I wanted to keep the exposure at middle gray since this is steepest part of the tone curve and the most likely place to show a color shift.  The first picture below shows the Fader-ND installed on the end of my 24-70 lens.

The surprise here is that we also see red shifting less than blue.  Although there is a slight attenuated shift in red, it appears that the greatest shift is with the attenuation of blue.  My goal here was to develop a quick rule of thumb for working on set, so the easiest solution when I'm not necessarily white balancing to a white balance card is to use the white balance shift/bracket function (WB SHIFT/BKT) built into the camera.  I started off with the white balance shift set to neutral (0,0).  When you enter this menu item it gives you a grid which represents white balance shifts in the typical green, blue, magenta, and red (don't know why they label it "A").

Through trial and error I found that shifting the white balance one step toward blue (i.e. one step away from red) and five steps toward magenta (i.e. 5 steps away from green) evened out the histogram.  Ah...back to normal.

Another aspect of this experiment involved seeing if the colors shift around when more neutral filtering is applied, but the color rendition seemed reasonably independent of the filter setting.  So now when I'm on set and need to make a quick change I know to shift the white balance one additional step toward blue and five steps toward magenta when I put on the Fader-ND filter.  That's a much cheaper fix than buying the Singh-Ray or the Tiffen filters.

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