81 Series Filters

[J. Anthony Gonzales]

This might sound like a lame question, but I am not certain about the effect of 81 series filters. Do they simply reduce ANY color temperature, or do they only reduce fixed temps down to 3200?

 

For instance, if I wanted to warm up a scene a bit more from 3200, will an 81A bring me down to 3000?

 

Thanks,

 

John G.

[Chris Keth]

They will reduce any color temperature by the mired value for that filter. Doesn't matter what color temp you start with.

[Timo Haapasaari]

Filter--Exp.increase----Conversion----mired shift

 

81---------1/3--------3300K to 3200K-------+9

81A-------1/3--------3400K to 3200K-------+18

81B-------1/3--------3500K to 3200K-------+27

81C-------1/3--------3600K to 3200K-------+35

81D-------1/3--------3700K to 3200K-------+42

81EF------1/3--------3850K to 3200K-------+53

 

As you can see, these filters are designed with type B tungsten balanced films in mind. However, they are not limited to this.

Edited February 1, 2005 by Timo Haapasaari

[Timo Haapasaari]

A very useful concept is the mired shift. Mathematically, this is defined as

 

1000 * (1000/T2 - 1000/T1)

 

where T1 is the color temperature you have and T2 is the color temperature you desire (for example the color temperature of your film). The mired shift is sometimes called the light balance (LB) index of the filter, and it is listed in the table above.

 

There are two great things about the LB index for a filter:

 

1. It doesn't depend on the color temperature, and

2. It is additive.

 

The first point means that an 80A (LB -131) filter will not only change light with a photographic color temperature of 3200K to 5500K, but also for example 2256K to 3200K - also a shift with a mired of -131.

 

The second point means that you can obtain a LB index that is not in the table by combining filters (i.e. placing more than one filter before the lens). Suppose as an example that you need to convert 11,000K (a clear bright day) to 5500K. Using the formula above you know you need a LB index of 91 mired, so you might use a 85C with a 81 filter (+81+9 = +90 mired).

Edited February 1, 2005 by Timo Haapasaari

[J. Anthony Gonzales]

Thanks Timo! Excellent info. I appreciate your help and time in writing this up.

 

John G.