I recently received an e-mail from a new filmmaker who was shooting an action sequence for a low-budget independent feature with 15 DSLR cameras. It was a daylight exterior sequence — he knew he was going to be fighting the changing sun all day long and he was trying to find an expedient way to keep the cameras white balanced throughout the day to help reduce the color timing requirements later on.

As I explained to him, there are several options.

A) You can white balance once in the morning and once after lunch and fix it later.

B) You can continue to white balance all day, but this practice can introduce bias as well if the cameras aren’t all balanced in the same place and under the same light. If clouds arrive between white balancing camera 1 and camera 15, you’re going to have different balances.

C) You can use color-correction filters and quickly apply them throughout the day. He liked option C but didn’t know anything about color-correction filters.

I realized through our conversations that most digital shooters have probably never considered correction filters. With digital cameras able to white balance in a wide variety of situations, why would you ever consider using the old-school filters? This shoot was a perfect example of how classic concepts still apply.

Color correction comes in two primary “flavors”: orange and blue. In lighting gels, these would be CTO and CTB; in camera filters, they’re identified by Wratten filter numbers, named after Fredrick Wratten, who invented the filtration system that was eventually purchased by Eastman Kodak.

Filters are manufactured and sold by Tiffen, Schneider, Formatt, Cokin and many others. The two primary Wratten numbers are 80 (blue) and 85 (orange). Like CTO and CTB, color-correction filters are also available in varying strengths: 80A, 80B, 80C, 80D, 85, 85B, 85C. Knowing when to use which requires a bit of math.

Color correction is not a linear process. An 85C will convert 5600° Kelvin to 3850°K, a change of 1750°K. That same filter, however, will only correct 5000° Kelvin to 3560°K, a change of 1440°K.
Knowing how a filter will convert any given color temperature requires the use of mireds, or microreciprocal degrees. This requires converting Kelvin temperatures to mireds by dividing 1,000,000 by the Kelvin temperature:


You take the target mired and subtract the mired to be converted, as seen in this chart:

The use of correction filters requires knowing what temperature you’re under and what temperature you want to convert to. The best way to know your color temperature is to use a tri-color color temp meter. But you should keep in mind the temp of some typical lighting elements:

So 5600°K has a mired value of 179 and 3200°K has a mired of 313. We want to correct the 5600° to 3200°, so we take our target mired, 313, and subtract the mired to be converted, 179. We find a mired shift of 134.

Looking at the filter list above, an 85B will correct +131, which gets us nearly exactly where we want to be.

Sometimes, in order to achieve a conversion, you need to stack multiple filters. Filters are available in a variety of shapes and sizes, such as circular screw-ons that attach directly to the threads on your lens and square/rectangular filters for mattebox use.

Tiffen also offers decamired filters, which are named for their mired shift values divided by ten (deca) — so a B1.5 is a blue filter with a mired shift value of -15. A R6 is a red decamired filter with a shift of +60.  

Although digital cameras have wonderful flexibility in their white balance function, there are situations where it is good to understand the old-school methods to help make your workflow more efficient.
This shoot was a perfect example where classic concepts still apply.