White Balance for Digital

White balance for digital capture is actually similar to days of film in some regards. Especially relevant when dealing with tungsten lighting when shooting stills or video. Why?

Because tungsten lighting is a dynamic-range killer due to it being deficient in the blue channel.  Using custom white balance in-camera doesn’t solve the lack of blue light.  The amplification in the blue channel raises the noise floor.  Its bad enough that the blue channel is the weakest on many sensors, but ramping up that channel often worsens the noise even at relatively low ISO levels and goes downhill quickly as you raise ISO (which amplifies all channels).  Solution? 80 series filters.
The 80 series filters were of course for color conversion when shooting daylight color films in tungsten lighting.  The 80 series vary, but I”ve found the three most relevant to be the 80-A, 80-B and 80-C, each varying slightly (chart provided later in the post).  The result of using an 80A for example is that you suppress everything but blue light.  This is what you get when you use a digital camera set to daylight white balance with an 80-A filter in front of it ==>
The following is happening:

—  Red, green and blue levels in the light source are approximately equal, and tungsten light looks white.   Subjects in that light look normal.
—  Blue channel noise is reduced even at high ISO settings because the blue channel is being “fed” per se.   Dark shadow noise is reduced tremendously.
—  Red-channel overexposure halos (also known as “blooming”) that occurs around edges of light sources, is reduced.
—  (THE DOWNSIDE) Exposure time is increased by 2 stops for an 80-A (so 1/250 sec ends up being 1/60 sec, or 400 ISO becomes 100 ISO).  It is best to use color-blind centerweighted metering.

80 Series Filters Color conversion (blue filters)
3200 K to 5500 K 80A Loss of stops = 2
3400 K to 5500 K 80B Loss of stops = 1 2/3
3800 K to 5500 K 80C Loss of stops = 1

Again, tungsten light is a dynamic-range killer, and using  your camera’s custom white balance abilities can’t fight it alone. If you value using every ounce of the dynamic range of your sensor, it is important to make adjustments to light temperatures BEFORE the light hits the sensor.  Dynamic range is measured from the noise floor, which is affected by blue-channel noise. Using a blue filter in tungsten lighting will help allow your camera’s sensitivity perform as well as it does in daylight color white balance where channels are more even (RGB).

In real estate photography this becomes extremely common and relevant where the goal is to find ways to INCREASE dynamic range. You certainly don’t want to make your life more difficult by relying solely on a camera’s custom white balance.  And if you are bracketing for HDR and you are concerned about noise???… well this is just as relevant.  Because most shots are taken from a tri-pod, the loss of stops using an 80-A 80-B or 80-C filter are less dramatic then if you hand hold shots.

Posted by Michael James on Feb 19 2010 in Basics and Terminology, Filters, HDR, White Balance Tags: , , , ,