Friday, 3 August 2012

Gels and Filters

In photographic terms, gels also called coloured filters, are tough, heat-resistant sheets of plastic or polyester film that are attached to the front of the lights to change their colour qualities. They are made in a rainbow of colours with varying degrees of intensity. Their purpose is to modify the colour temperature of the flash or strobe to match that of ambient light sources present in the scene. As a result we get to use an even light balance and that makes our post processing all the more easier.

Gels used for Photography
Gels used for Photography

Tips on Using Gels

Although gels are made of heat resistant materials, proper care should be taken to ensure that they do not overheat so as to melt or catch fire. Remember the modelling lamp on a strobe if left on could become very hot; similarly a flash could also generate a lot of heat when used at full power. When attaching gels to strobes; use gaffers tape instead of other adhesive tapes. They do not leave any residue and are also heat resistant.

Types of Gels

Gels are available in all shades of colours in the spectrum. There is also Neutral Density (ND) gels which are grey in color and do not add any colour cast to light passing through them. Their sole purpose is to reduce the amount of light from a light source. It is very useful when you have to get the power output of a particular light way down than it is normally possible and you are not in a position to achieve this by moving the light source away from the subject.

The most common colours of gels used are CTO, CTG and CTB. CTO (color temperature orange) is a warming gel. It is used to match the flash unit to tungsten light sources. And CTB (colour temperature blue) used to convert tungsten light sources to match daylight. Then there is CTG (colour temperature green) which is used to match the flash unit to fluorescent light sources.

Gels used for Photography
Gels used for Photography

Gels are available in varying degrees of intensity for e.g. a CTO gel is available in ¼ CTO (less orange - lighter), ½ CTO (more orange - darker) etc. you can also stack gels on top of one another to increase their intensity. Rosco makes starter kits with an assortment of colours at a reasonable price. Theatrical supply stores are also great places to find coloured gels. .
Effect of Gels on Exposure

By controlling the exposure values we could make a single colour gel generate many different shades of that colour (from light to dark). For example if we are shooting a white wall and we have applied red gel to our flash, when exposed properly the wall will look the same colour as the gel, when over exposed it will be lighter shades of red and when underexposed it will be darker shades of red. Over exposing to the limit will create white light with no hint of red and under exposing to the limit will create black. Thus you could create all the mid values of red from white to black simply by varying the power output of the strobe and keeping your exposure value constant.

Light Contamination Issue When using Gels

Light contamination occurs when either coloured light or white light accidentally spills on to other elements in the scene and alters its lighting properties. This can also be caused by two or more lights of different colour crossing paths. When this happens if the colours crossing paths are complementary, they could produce a nice effect and when the colours are not complementary the whole scene will be a mess and a post production artist’s worst nightmare.

Gels used for Photography
Gels used for Photography

As long as daylight, flash, or tungsten is reasonably close to “normal,” we may not even notice the colour. But once we start thinking about it and start to play with it, we would realize the potential it holds for creative photography. And instead of ignoring light and its colours and taking whatever colour we get; we will begin to generate our own colours and get what we want.

In the next article we will discuss about Light Modifiers - Bare bulb

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