Using Gels With Your Flash/Strobe – Part 4 of 4

Up to now we’ve talked about how to think about color temperature of a light source and how gels change their color temperature.  And we’ve done some staged tests to make sure our theory on “Red, White, and Blue” works like it should – which it has.  But what about doing on some real photos outside, can we use this to change how the photo looks to make some interesting effects?  We’ll, that’s what we are going to do now!

My girlfriend volunteered to be the model for these pictures.  We went out to a local park overlooking some water.  The time was about 1-2PM in the afternoon, clouds in the sky with some blue patches.  We started off with the camera set to shade white balance and no flash just to see what the ambient light would give us:


No Flash - WB Shade

Here we have the result.  No flash, just ambient light, white balance set to shade.  The sky you can still make out the clouds but she’s a little dark.  I had put her in the shade for this test.

Next we add some flash to her, but no gel on the flash.  Since she’s already a little dark compared to the background adding the flash to her works well because it’s just increasing her brightness, it’s not affecting the background at all.


Flash - No Gels - WB Shade

Here’s what we get when we add the flash.  She brightens up, background stays the same.  This isn’t too bad, but let’s stop and think about what we’ve got here.  There are two light sources in this image.  We have the sun which is coming through partly cloudy skies.  The sun’s color temperature is going to be somewhere between sunny and shade then, we know sunny is about 5200K and shade is about 7000K.  Let’s say the color temperature of our sun then is half-way between, 6100K.  Our second light source is a bare flash, roughly 6000K.  And finally, the camera white balance is set to shade, 7000K. That means the color temperatures in the image are:

Color Temperature
Sun 6100K
Flash 6000K
Camera WB 7000K

Now think about “Red, White, and Blue”.  Our camera is 7000K and both the lights sources are less than 7000K, so they are on the left side of the camera:

Color Temp Chart: Partly Cloudy - Bare Flash - WB Shade

That means both those light sources are, relative to the camera, a warm light source.  They are on the red side of the chart compared to the location of the camera.  Notice in the image there’s a slight warm tone to it.

What if we want to put a little more interest and color in the sky.  We know the partly cloudy sky is a fixed color temperature.  All we have to do is think about the Red, White, and Blue chart.  If we slide the camera to the left on the chart then we can make the sky more blue.  Let’s try setting the camera WB to tungsten which will slide the camera to the left side of the chart:

No Flash - WB Tungsten

Here we have turned the flash off and set the camera WB to tungsten.  First thing we notice is everything has turned blue.  We already knew this would happen from the Red, White, and Blue chart:

Color Temp Chart: Partly Cloudy - WB Tungsten

From the camera’s perspective the light source is now to its right on the blue side, so we expect and do get the image to have a blue/cool tone to it.

Next we’ll add the flash back in so our model isn’t so dark:

Flash - No Gel - WB Tungsten

Brightens up her face, but no real difference in the color temperature of the light. That makes sense because we know that the flash is about the same color temperature as the ambient light (partly cloudy).

Let’s not have her look like a smurf or character from Avatar by gelling the flash to bring the color temperature of her face back to something more normal.  Think about our chart again, the camera is on the left side since it’s set to tungsten, we need to get the flash over to that side too.  Let’s put a 1.5 cut CTO gel on the flash:

Flash - 1.5 CTO Gel - WB Tungsten

Here’s our result.  Let’s take a look at the Red, White, and Blue chart to see what we did:

Color Temp Chart: Partly Cloudy - 1.5 CTO Flash - WB Tungsten

The sun and the camera stayed in the same position, we didn’t change them.  But covered the flash with a gel and slide it to the left on the chart.  If we had of put a single cut of CTO over the flash then it would be at the same position as the camera, but instead we put on a 1.5 cut of CTO.  This causes the flash to be slide even further to the left.  We did this to counter act the ambient light as a cool light source.  Since the flash and the cloudy sky are different color temperatures we have to take into account that when they combine the final color temperature is somewhere between the two light sources.  So for our model the color temperature of the light on her face won’t just be that of the flash and it won’t just be that of the cloudy sky.  It’ll actually be a mixture of the two.  To counter act this fact we slide the flash more to the left.  By sliding the flash to the left of the camera when we combine the two light sources together we’ll get something that’s close to the camera’s white balance position.

Next we’ll drop the ambient light some to darken the clouds:

Flash - 1.5 CTO Gel - WB Tungsten - Drop Ambient

What we did was close down the f-stop of the camera, this let less light in, so everything got a little darker.   We could make the models face a little brighter if we wanted by increasing the flash output.  The flash will only affect how bright she is, it won’t change the brightness of the sky or background that’s way way in the distance.  Our result here looks pretty good.  We’ve taken a picture in the middle of the day and changed it by using gels on the flash to make it seem like it’s much later in the day.

How about doing the opposite?  Let’s make the sky and background more warm looking.  Let’s think about what we want then from the Red, White, and Blue chart.  In order for the background/sky which is coming from the sun we need to place the camera to the left of the partly cloudy sky color temperature.  Because, relative to the camera if it’s to the left then it will be warm (red side).  We also want the model to be fairly neutral color temperature, maybe just a tad warm.  That means the flash color temperature needs to be where the camera is set or actually a little to the right of the camera.  Again, we’ll be mixing the two light sources and we want the mixture to be close to the camera.  On the chart this would look like this:

Color Temp Chart: Partly Cloudy - CTB Flash - WB Shade

Here we see that the partly cloudy sky is locked down, we can’t change its color temperature.  By adjusting the camera white balance we’ve placed it to the right of the sky.  This means from the camera’s perspective the sky will be warm.  We’ve also put a CTB gel over the flash to slide the flash to the right side of the camera.  This will make the light from the flash appear cool to the camera, but that light will be mixed with the sky light source and the color temperature will even out.

Let’s start by keeping everything we had before the same and just switch from a CTO gel to a CTB gel and see what happens.  Thinking about the chart we know that will slide the flash to the right, past the camera, and into the cool color temperature:

Flash - CTB Gel - WB Tungsten

And it did just that, our white balance is tungsten (3200K), so we’re on the left side of the chart.  Both the sky (partly cloud, ~6100K) and the CTB gelled flash (~7800K) are a high number, so they are on the right side of the chart.  Results is all the light sources in the image will appear cool from the camera’s perspective.

Not quite what we wanted, we need to move the camera to be on the right side of the chart, but we need to move it far enough that the sky will be to the camera’s left.  Partly cloudy we figured is about 6100K, let’s try setting the camera white balance to shade (7000K).  When we do that we’ll have the sky on the left of the camera and the CTB gelled flash on the right:

Sky Camera CTB Flash
6100K 7000K 7800K

We already know what the result of this will be from the Red, White, and Blue chart.  The sky/background will appear warm and the model will appear a little cool since she’s mostly lit by the flash:

Flash - CTB Gel - WB Shade

Here we can see that background does have a warm appear to it and the model looks a little cool.   We can fix this by going to a custom white balance where we set the kelvin.  The max I can set the white balance of the camera to is 10,000K.  Let’s go ahead and do that, this will place the camera as far right on the chart as possible so the background will be as warm as possible.  This will also mean the flash color temperature will be to the left of the camera so it will have a slight warm color temperature also.

Flash - CTB Gel - WB 10,000K

And there it is.  Once you start thinking and picturing in your head the Red, White, and Blue chart you’ll easily be able to change the color temperature of the various light sources.  This way you can envision what you want the end result to be in your head and then know what to do to achieve that result.


Checkout the previous posts in this series:

Gelling your flash, as easy as Red, White, and Blue

White Balance and Color Temperature

Gel and Slide

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4 thoughts on “Using Gels With Your Flash/Strobe – Part 4 of 4

  1. This is a great article, which made it very easy to understand such a complicated concept. Awesome work. Couldn’t find anything on the net that explains how white balance really works and how we have full control of it by understanding the simple concepts you’ve explained so well.

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