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Filed under: Cameras •Production •Tips •Training •

Optically White Balance the RED with Schneider CTB Filters

Once again, here’s the histogram and gray card under tungsten light and without any filtration:

Red is way ahead of the other channels, meaning that it will clip sooner under tungsten light than in daylight if the camera shoots either a bright neutral tone or a saturated red object.

Let’s take a look at the first, and lightest, CTB filter:

SCHNEIDER 1/8 CTB FILTER

MY METER SAYS: OPEN UP 1/3 STOP

The 1/8 CTB filter is a very light blue, but we can already see the effect it is having on the camera’s white balance: the red peak has moved considerably to the left, showing less exposure in that channel, while green moved a little bit and blue barely moved at all. In theory a blue filter shouldn’t affect the blue channel much because it is passing blue and absorbing the other colors.

By tightening up this grouping we’ve given ourselves a little bit more overexposure latitude, as the red channel will be slightly less likely to clip when shooting bright white or red objects.

The next darker filter is:

SCHNEIDER 1/4 CTB FILTER

MY METER SAYS: OPEN UP 1/2 STOP

The 1/4 CTB filter is absorbing a lot more red. Blue and green are almost in alignment, and the overall grouping is a lot tighter. This filter will allow for even greater overexposure latitude, keeping red from clipping a lot sooner than the other colors and reducing noise in the blue channel due to better blue exposure when the T-stop is opened to compensate for light loss.

Once again, notice that blue has barely moved at all. The changes are most significant in the red and green channels.

The heaviest filter is:

SCHNEIDER 1/2 CTB FILTER

MY METER SAYS: OPEN UP 2/3 STOP

This is an excellent grouping. Blue and green are essentially in balance, and red is only about a half stop ahead of the other channels. While red will still clip a half stop sooner than blue or green, that’s considerably better than clipping one-and-a-half stops sooner—plus the blue channel is getting as much exposure as the green channel, which means it will be a lot less noisy when the exposure is increased to compensate for the reduction in red and green light.

A Wratten 80A filter would bring the peaks into complete alignment but at the cost of another stop of light. I almost never have that much leeway when shooting under tungsten light, and my past experiments with the 80D filter (which falls between the 1/4 and 1/2 CTB filters in strength) show that half correction to daylight is plenty of compensation under most circumstances.

Schneider CTB filters are available from, among other places, Filmtools.

Art Adams is a DP who leads a colorful life. His website is at www.artadams.net.

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