Thursday, December 13, 2007

Flash Gels for Correcting Color Temperature: Using the Gels (Part 2)

On part 2 of this article I will be showing you some practical examples of the most common used gels CTO (Color Temperature Orange, converts Daylight to Tungsten), CTB (Color Temperature Blue, coverts Tungsten to Daylight) and for Fluorescent I will use the Plus Green (Provides a green cast when used on Daylight and Tungsten for balancing Fluorescent).

But first things first let's take a look of what Color Correction and Color Temperature means so you can get an idea how you can explore and use it.

Color correction by using color gels, is a process used in stage lighting, photography, television and cinematography, the intention of which is to alter the overall quality of the light measured on a scale known as color temperature.

Without color correction gels, a scene may have a mix of various colors. Applying color correction gels in front of light sources can alter the color of the various light sources to match. Mixed lighting can produce an undesirable aesthetic when displayed on a television or in a theater.

Conversely, gels may also be used to make a scene appear more natural by simulating the mix of color temperatures that occur naturally. This application is useful especially where motivated lighting is the goal. Color gels may also be used to tint lights for artistic effect.

The main color correction gels are CTB (color temperature blue) and CTO (color temperature orange). A CTB gel converts tungsten light of 3200K to 'daylight' color. A CTO gel performs the reverse. Note that different manufacturers' gels yield slightly different colors. As well, there is no precise definition of the color of daylight since it varies depending on the location (latitude, dust, pollution) and the time of day.

Gels that remove the green cast of fluorescent lights are called minus green. Gels that add a green cast are called plus green. Fractions such as 3/4, 1/2, 1/4, and 1/8 indicate the strength of a gel. A 1/2 CTO gel is half the strength of a (full) CTO gel.

The color temperature of light is measured on the Kelvin scale, which we may have used in our high school science classes, but have long forgotten. The scale is an extension of the Celsius scale. It’s based on the color of the light as it is emitted from a hypothetical black body.

The Kelvin scale (abbreviated as “K”) is confusing, because it runs counter to the way in which we use some photographic terms. For example, when photographers talk about adding “warmth” to a scene, they usually mean to add some reddish tones. Conversely, when photographers talk about “cool” tones, they’re referring to the bluish side of the spectrum. This logic is just the opposite of the Kelvin scale of color temperature.

At this point, you may be scratching your head, wondering how light can be described by its color temperature, well consider a log fire. When the fire/flame is at a low temperature, the color of the flame is red. At higher temperature, the flame changes to bluish tones.

Got it ?

So let’s look at a Kelvin temperatures chart for various lighting conditions. Please note that the values are approximations because many factors affect color temperature. In outdoor conditions, the angle of the sun and condition of the sky (clouds, haze, dust particles, etc.) can raise or lower the color temperature. Indoor conditions such as lamp age, voltage, type of reflectors, etc. affect the color temperature of light.

Here are some common light sources:

Note: Fluorescent light does not operate in the same manner as the black body model of Kelvin temperatures. Additionally, there are six different types of fluorescent lamps. White fluorescent lamps, for example, have a color temperature of 5200 K. That’s why it’s difficult to photograph under fluorescent lighting, even using a fluorescent filter for your film camera.

Since this is not intended to be a class about WB or Color Correction let me show you some examples I got using the gels and ambiance light.

In the following pictures you can see the several lights as well as on the background you can get a glimpse of what it would look like using the gels to correct ambiance and you can even use that to create other colors at your taste.

The weather is not so nice here in Luxembourg but it looks like when I started to get some pictures for this tutorial the sun came up ;)


Here is the setup picture and one of my cats that join me on this setup.

On camera left you have the SB-26 with gels, the Tungsten and a Fluorescent lights (I put the black banner behind the lights so you can see them changing with different settings) and on the right you have a white piece of paper so you get the feel of what the light gel is affecting and you can also see the changes it produces to the background when I change each gel.

I will be holding 6 pieces of paper that correspond to each camera White Balance I used for this testing shots this way will be better for you to understand what's going on. So what we will have is WB, Sunlight, Shade, Cloudy, Tungsten and Fluorescent.



First sequence is with CTO (Color Temperature Orange) on the flash.

Is never too much to mention that this one converts Daylight (5600K) to Tungsten (3200K), you can also find the some complements as 1/2 CTO (5600K-3800K) and 1/4 CTO (5600K-4600K).



Then to the normal CTO (Color Temperature Orange) I added a CTO 1/2 on top of the other. I made this on purpose so you can see the difference between the normal CTO and a little bit more warmer tone. You can also try it with 1/4 CTO for a not so pronounced orange tone.



Then we have the CTB (Color Temperature Blue).

This one we will use to convert Tungsten (3200K) to Daylight (5600K), as you are seeing here it's the opposite of the CTO and you can also find it in 1/2 CTB (3200-4300K) and 1/4 CTB (3200K-3600K).



Made the same thing with the CTB (Color Temperature Blue) and added another CTB 1/2 so you can see the heavy blue tone and what it happen to the lights and background.



And the last one is the Plus Green (Equivalent to Color Correct 30 Green)

This one provides a green cast when used on Daylight (5600K) and Tungsten (3200K) for balancing Fluorescent (3600K). For providing partial green cast when used on Daylight (5600K) and Tungsten (3200K) for balancing Fluorescent (3600K) you can adjust it with 1/2 Plus Green and 1/4 Plus Green.

Hope you have enjoyed these practical examples now, if you already have the Gel Pack, go and experiment yourself on different light situations either inside, outside or both.

Part 1 - Flash Gels for Correcting Color Temperature: The Gels (Part 1)

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