Using Flash Gels

I suppose the first question to answer is, “What are flash gels?”.

A gel is basically a coloured sheet of heat resistant acetate that is used to change the colour of a light source. They work by absorbing the frequencies of some colours of light and letting others pass through. Fortunately, because the light from strobes and portable flashes is a similar colour to daylight, you will get the colour change you expect - such as a blue gel making the light blue. Because the gels absorb light, you will lose some of the intensity of the light. The number of stops of light that a particular gel absorbs is available from the vendor or the manufacturer of the gel.

Gels normally come in square shaped sheets, for use with studio strobes or theatre lights, but you can cut these down to size for use with portable flashes. Some companies sell pre-cut gels that are the right size for the front of a speedlite. Gel manufacturers, like Roscoe or Lee, do sample books of the different colours of gel that they make. These contain strips of each gel that are about the right size for the front of your flash. The sample books can be difficult to get hold of though, due to the obvious popularity of ‘free’ flash gels. I usually buy the pre-cut gels from www.flashgels.co.uk.

Gels come in a multitude of different colours but, from a photography point of view, they can be classed into two types. Colour correcting gels will change the colour of the light your flash to match the colour of another source of light, such as incandescent bulbs or fluorescent strip lights. Theatrical gels change the colour of your light to give you a particular creative effect, such as using red gels to make something appear hotter or blue gels to make something appear more sterile or technical. You can also use the colour correcting gels to get certain effects but I will go into that in more detail later.

Colour correcting gels come in three main types, CTO (Colour Temperature Orange), CTB (Colour Temperature Blue) and Window Green. They also come in different strengths, often called cuts, so you can control how much colour correction is applied. The strengths are usually described as a full gel, a 1/2 cut and a 1/4 cut. Using two quarter cut gels together will produce the same effect as using one half cut gel.

CTO gels are used to make the light from your flash approximate the colour of light emitted by incandescent light bulbs, which is a warm amber colour. A lot of homes are now switching to the energy saving bulbs, but most of these also produce a warm light. CTO gels can also be used to approximate the light from a low sun or you can use a lower strength gel to produce a flattering, warming effect on skin tones when shooting a portrait.

CTB gels are used to change the colour of an incandescent light to approximate daylight. Since speedlite flashes are balanced to daylight already, these are more useful when used for effect, in the same way as the theatrical gels.

Window Green gels are used to approximate the colour of light that is emitted by fluorescent tube lighting, which is pale green colour. This is the sort of lighting often found in offices or hospital buildings.

As well as those three you can also get ND (Neutral Density) gels. The ND gels are a little different from the others because they only change the intensity of the light and not the colour. They are a neutral grey in colour and are rated in stop values, indicating how many stops of light they will absorb from your flash. They are used on flashes that have a limited (or none existent) control over the power output.

The theatrical gels come in many different shades and colours, each one having a name and an associated number for that colour. These are used for effect and their use is only limited by your imagination. Experimentation is the name of the game, to see what effects you like or don’t like when using them. Their use tends to work best when it is fairly subtle instead of blasting everything with bold colours. The strength or saturation of colour that you get from these gels depends on the power of your flash. A flash set to a lower power will produce a more saturated colour than one set to a high power. For example, a red gel can be used produce a light ranging from almost pink to a rich regal red depending on the flash power.

Once you have some gels, you have to find a way to attach it to your flash. The most common way is to attach some velcro to the ends of the gel and fasten it to some velcro tabs (or a velcro strap) at the top of the speedlight. You can also attach them with an elastic band, as in the picture above. I often use a velcro strap around my flash for attaching various light modifiers and I will put this around the ends of the gel to keep it in place. The important thing is to make sure that the whole light emitting surface of the flash is completely covered by the gel, you don’t want any light leaking from the above or below the gel that would hit your subject.

Ok, so assuming we have some gels and a method to attach it to our flash, the next question is, “When do we need to use them?”.

As mentioned previously, the use of theatrical gels is a creative decision - you can use them when you like. Color correcting gels do, however, have a particular purpose. Imagine the scene, you are shooting a portrait of someone indoors, in the evening. The curtains are drawn and the room is lit by incandescent bulbs. You want to add some light to your subject so you break out the flash. For your shot, you want to capture the room that the subject is in, so you set a shutter speed that will allow you to record the available ambient light. As it stands, your room is lit with incandescent lights and will be a warm colour, but your subject would be lit with a daylight balanced flash, which will look cold in comparison to the room. Mixed light sources are difficult to fix in software afterwards, so it is better to get it right in-camera. If you took a CTO gel and attached it to the flash, the colour of light on your subject would be very similar to the light in the room. The result would be more consistent and any changes to white balance that were applied would affect the whole image equally.

To sum up that scenario: the colour correcting gels are used to balance the colour of the light from flash with the colour of the ambient light. This is assuming that you have a long enough shutter speed to record some of the ambient light.

The photograph above is another example of how a CTO gel can be used. When shooting this image of the ball ornament on top of our gate post, the low sun was casting a warm light onto the park in the background of the shot, but the ball was in shade. If I had shot it without any flash, I would have ended up with either the background being blown out (exposing for the light on the ball) or the ball being dark (exposing for the light in the background). I wanted my background to be exposed correctly in the shot so I used some fill flash to bring the exposure of the ball to a similar level. Since the light in the park was a warm tone, due to the low sun, I used a half cut CTO gel on the flash to make the light from it a similar colour. To complete the effect, I had to fire the flash off camera so that the direction of the flash mimicked the direction of the sunlight, keeping the shadows in the right place.

All well and good, but what if you have a situation with mixed colours of light sources? The office I used to work in had fluorescent strip lights in the ceiling but large windows that let in a lot of daylight. There are a couple of ways to deal with a scenario like this, the first and most ideal is to remove light sources. In the office I could have closed the blinds over the windows, leaving me with just florescent light. Using a Window Green gel would balance my flash to this ambient light. Alternatively, I could have turned the ceiling lights off and just used the daylight from the windows as my ambient light, shooting using an un-gelled flash. If you can’t remove any of the light sources, shoot an image, using just the ambient light, with your camera set to a daylight white balance. You can then check on the screen to gauge the overall colour of the ambient light and pick a ‘best-fit’ colour of gel for the flash. Sometimes it is worth doing this anyway because you can’t guarantee that all incandescent bulbs or fluorescent tubes are the same colour as each other. Fluorescent tubes especially can vary significantly in colour, based upon the make and age of the light.

The image opposite is an example of using a theatrical flash gel when lighting a shot. The photograph was taken inside of a washing machine drum and lit with a blue gelled flash.

I chose to use blue because of the metallic nature of the subject. A cool tone, such as this, can make metal seem more technical or more futuristic - plus I like the colour.

I have been asked if I could have just shot this with an un-gelled flash and then then changed the colour in editing software afterwards. In this example you could probably do it that way, but I prefer to get as much of my image as I can in-camera, to save on time when processing the images. If I was using more than one colour of light source, this would be more difficult to replicate in software.

One thing to bare in mind when using gels, especially in this way, is the white balance setting on your camera. If  the camera is set to use an auto white balance, it will try to compensate for the colour(s) of the flash(es) with unpredictable results. If you set the camera to the daylight white balance then you will get the correct colour of the gel in the final image.

If you are using a colour correcting gel, the choice of white balance depends on the effect you want. Choosing the appropriate white balance for the shooting conditions, such as the florescent setting in an office (or using the auto setting), will remove the colour cast from the light and give you a daylight balanced image. If you were shooting an image in low sunlight with a CTO gelled flash, you might want to retain the warm colour of the light and would use a the daylight white balance setting in this instance.

Hopefully this post will have given you some background info on gels and their uses. As with most things in photography, the best thing to do is take some pictures and experiment. Have fun