Whats the effect of doubling up gels?

[John M]

Does doubling up gels just add stops? Or will it also alter color temperature? I.e I have a Lee 442 Straw filter that changes 6500K to 4300K. If I doubled that up would it bring the temperature even lower? Or am I just losing more light?

 

Thanks

[Jax McLennan]

Yes it would - two 1/2 CTBs combined create a full CTB, for example.

[Sean Elder]

Yes doubling a filter will decrease the amount of light that passes through the gel also it will change the value of the light as well. I'm sure there is someone on this forum that can give a a far more technical answer than what I have given.

[Kyle Reid]

It's completely additive, if you double a gel you will see double the effect of that gel (color and exposure).

Although it would be good to note that two halves do not make a full when we are dealing with color temp gels. For example 1/2 CTB creates a MIRED shift of -79, however, full CTB creates a MIRED shift of -137. So if you use two sheets of 1/2 CTB over a light source you will create a MIRED shift of -158, but full CTB only creates a MIRED shift of -137. Gels work in a linear fashion, whereas color temperature does not.

Edited June 23, 2012 by Kyle Reid

[Conor Stalvey]

It's completely additive, if you double a gel you will see double the effect of that gel (color and exposure).

Although it would be good to note that two halves do not make a full when we are dealing with color temp gels. For example 1/2 CTB creates a MIRED shift of -79, however, full CTB creates a MIRED shift of -137. So if you use two sheets of 1/2 CTB over a light source you will create a MIRED shift of -158, but full CTB only creates a MIRED shift of -137. Gels work in a linear fashion, whereas color temperature does not.

 

Thanks Kyle, I was not aware of the numerical values. Learn something new everyday.

[Mei Lewis]

It's completely additive, if you double a gel you will see double the effect of that gel (color and exposure).

 

 

What does 'double' mean there? I guess it doubles the light loss, but in terms of color what number gets multiplied by two?

[Chris Millar]

It is not additive in the way some of you might interpret that - in some headspaces it's more multiplicative.

 

Think about it this way - imagine you had an ND gel that took away %50 of light and your original light source had a value of '1' - you'd end up with 0.5 units of light out of it, if it were additive you'd end up with 0 light with the second gel - and you know that aint correct

 

The math would be as follows:

 

1 x 0.5 = 0.5 (first gel)

 

0.5 x 0.5 = 0.25 (second gel)

 

maybe use the word 'sequential' instead

 

To set up a function with respect to number of gels as you're working in discrete units I think you'd need something like a recurrence relation or otherwise - not enough coffee to do that at the moment

[Chris Millar]

ok, had my coffee -

 

oh gawd, its not as creepy as recurrence relations at all

 

 

x = number of gels

 

% light output = 100(2^-x)

[Chris Millar]

Should say that that function is only valid for each frequency band (which in an ND should be all visible light) - when looking at something with a colour correction on it you'd have to apply that function to every point along the response curve. Easy if you have a tabulated version in excel or some similar spreadsheet...

[Stuart Brereton]

What does 'double' mean there? I guess it doubles the light loss, but in terms of color what number gets multiplied by two?

 

If you using Color Correction gels then it's the MIRED shift that doubles. If it's other colors, then you would just say that effect doubles in strength (although I am sure there a mathematical way of describing it).

[Justin Dombrowski]

Another fun thing to try with stacking gels..

If your ever in a situation where you have a full CTB and no halves or quarters. You can apply the Full CTB & counter act the color temp with a 1/4 or 1/2 CTO.

 

Had this on a shoot where we needed to correct 2 tungsten 650w and we had limited gels. So we put a 1/2 CTB on one and a Full CTB paired with a 1/2 CTO on the other.

 

 

Now I know the light output and color temp wasn't perfectly matched but, it was close enough and served its purpose well.

 

 

 

Are there any other gel combinations that get interesting effects?

[Sean Elder]

Here's a question! (Yes, I know that sentence was a statement.) How would you determine the falloff or mired shift for diffusion? Is there a formula for that or is that something that the diffusion manufactures set? Also is this based on one particular light source(i.e.: a scientifically preset and calibrated light fixture) or a general light source(i.e.: a open-faced quartz light, HMI, Par, Fresnel, Etc.)?

[Stuart Brereton]

How would you determine the falloff or mired shift for diffusion? Is there a formula for that or is that something that the diffusion manufactures set?

 

Diffusion generally doesn't have a Mired shift. It's designed to soften light, not to change its color. Exceptions to this would be gels like LEE's Cosmetic series which combine a light diffusion with subtle color tints.

[rsellars]

Diffusion generally doesn't have a Mired shift. It's designed to soften light, not to change its color. Exceptions to this would be gels like LEE's Cosmetic series which combine a light diffusion with subtle color tints.

 

Stuart & Sean, while Lee & Rosco might not publish mired shift numbers for diffusion material, nearly all diffusion material does make light sources warmer. You can see it with your eyes and you can measure it with a color temperature meter. It varies slightly with each type of material, but I have found that a true 3200 tungsten source aimed through diffusion material averages around 2900 K. I'm not sure what the mired shift is. This is one of the reasons why Kinoflo offers 2900K tubes for their units. This "warmer" tungsten matches better with tungsten studio lights that are softened with diffusion material or bounced off white boards (which also warm light).