We've all done it, bought a fresh pack of gels, opened them up to see these vibrant and saturated colours that match the brightly coloured swatches on the back of the packet only to find that once we use them the actual colours appear nothing like we first thought. There is a couple of reasons for this like micro-managing white balance settings for example but the main reason our coloured gels appear different to what we expect is due to exposure.
Correctly exposing coloured gels is arbitrary at best and the sooner you come to terms with the fact that there actually is no right way to correctly expose them the happier you'll be with the results.
Lets just assume you wanted to colour a background, a background that ordinarily is a white wall, nothing complicated. As a rule I usually tend to underexpose all my gels, this results in a far more saturated colour but let me define what I mean by 'under-expose'. If we are lighting that blank white wall normally we 'may' want to over-expose it by a stop to make it 'white' compared to our subject, so if our subject was metered at f11 then the meter reading for the light that is lighting the white wall behind may well read f16. With this lighting the models face will be correctly exposed with the camera set to f11 and the wall behind will appear nice and bright as a white wall.
To colour that white wall using gels we have to let go of the notion of exposing it correctly and literally go with what looks good.
The diagram above illustrates this point by showing that the colour and tone of the gels changes drastically with exposure and although this might seem obvious enough it's easy to fall into the trap of trying to 'correctly' expose something. All 13 of the gels used here were all shot with the gel relatively close to the wall as I wanted to include the hotspot and its falloff of light within the image, as a result you will probably witness many stops of exposure difference within any single frame but it helps to illustrate just how many different tones can be achieved with exposure alone. If you would like to even out the colour and maintain a more uniform tone throughout the frame (no hot-spot or vignetting) then you will either need to move your gelled light further away from the wall or make the gelled light source larger.
As I mentioned earlier I tend to always under-expose my gels and as you can see from the gel diagram above the colours of the gels exposed at f11 or f16 are a lot more saturated and deeper in colour (remember these are just numbers, try to think of that as two or three stops darker than your models key light or camera setting). With the gels appearing darker like this you tend to have a more consistent colour throughout your frame as well and they appear to have a more even exposure too.
Another major factor to bear in mind is how other lights in the shot will affect the exposure and the colour of your gels.
Trying to minimise the spill of light from other light sources onto your gelled colours will certainly help to maintain and hold their saturation, if for example your models key light is spilling onto your gelled background no matter how much you under-expose your gels they will always look washed out. There are a few ways to combat this, you could flag all the lights that might be spilling onto your background or increase the inverse square law theory (essentially move your key light closer to the model thereby increasing the falloff rate of light to the background) this is a last resort as this will of course change the properties of light i.e hardness of light falling onto the model. Alternatively the easiest and usually the most overlooked solution is to move your model and all other lighting away from the gelled background. Try to treat it like you are lighting two completely separate sets, your model and your background, no light from either set should affect one another. When your background was a white wall it obviously matters very little if your models key light is spilling onto the background, with gels however this additional light falling onto the colour will 'contaminate' it and drastically change its appearance.
One other key point when exposing for gels is to remember that not all gels were created equal. As you can see from the gel diagram above, even though they were all shot at the same time with the same settings, some gels are blown out and some are very dark. This is because some highly saturated gels like reds and blues need to be physically quite dense or thick, this in turn reduces the amount of light that passes through them which gives them the impression of being darker and more saturated. It may even be a good idea to do a similar test yourself with your own gels and see what colours can be obtained with your gels and get a feel for what they look like over and under exposed (lighter and darker relative to the cameras exposure).
Key points to take away:
- There is no correct exposure for your gels only lighter resulting colours and darker resulting colours relative to your cameras exposure setting.
- Your gels will appear more saturated the darker they are in the image.
- Darker gels appear to give a more even tone throughout the frame.
- Moving your gelled light further away from your background will give you a more even exposure and a more even colour throughout the frame.
- Minimise the contamination (light spill) of other light sources on the set, this is especially noticeable on some of the thinner gels.
- Not all gels were created equal, some of them are very thick and will need a lot more light to create certain effects, these are usually the very saturated colours like red and blue.
Check out my gel packs here: http://jakehicksphotography.com/products/