Gel and Slide – Part 3 of 4

From the last article we had the problem of two light sources in a scene that had different color temperatures:

We saw before that if we changed the camera white balance to either match the light bulb or the flash we’d have a color cast in the image.  This is because the two light sources are at different positions on the chart, they aren’t producing the same color temperature light.

How can we fix this issue so the image will look correct? Let’s start with the control table from last time:

ItemAmount of Control
SunNone
Light BulbsNone
Camera FlashGel Filters
Camera White BalanceTotal Control

We know the light bulb is locked down, we can’t control it at all. The camera we have total control over, and we’ve already tried controlling it but it doesn’t fix the issue. The only thing we can do is gel filter the flash so it will slide to a different position on the chart. If we put a gel filter over the flash to change its color temperature to match that of the light bulb we’d get:

Here we see that the light bulb hasn’t changed position, it’s locked. The flash we put a gel filter over it causing it’s position to slide to the left on the chart. The camera we can control where it’s at on the chart, so we change its white balance to match that of the light bulb. End result is everything is lined up! Now it doesn’t matter whether something is illuminated by the light bulb or the flash, it’s color temperature will be neutral, we won’t have a color cast to the image.

This all sounds good in theory, but does it work in real life? It’s easy enough to test. What I did was setup a white background with a black partition. On one side I put a 60W incandescent bulb and on the other side a flash. The partition down the middle keeps any light from the bulb getting onto the flash side, and any light from the flash from getting onto the bulb side:

As you can see pretty simple setup, but it does what we need it to do. Now to show that no light from windows or rooms change the color temperature of the white board, here’s a picture of the white board with both the flash and bulb off, as you can see it’s all black meaning no room lights were changing the results:

Now we need to make sure each light stays on its own side. Here’s a picture of the bulb turned on, the flash turned off, and the camera white balance set to tungsten:

You can see that the color is just a little warm. This is pretty normal. I’ve set the camera to tungsten, but are all tungsten light bulbs the exact same color temperature? No, so there will almost always be a slight warm or cool look to it in a test like this. A custom white balance would remove this completely, but for our tests it’s fine.

Next, let’s try with the flash on, the bulb off, and the camera white balance set to flash:

Looks good, no light from the flash is getting onto the bulb side. Here we can see that the camera flash white balance is closer to the color temperature of the flash.

Now, let’s start doing some tests to see if Red, White, and Blue really work like we’ve learnt so far. Here’s a picture of both the flash and the bulb turned on. Camera white balance set to tungsten:

The white background on the flash side, which is to the right on the chart (the Blue side) is cooler than the bulb side. Pretty much what we expected.

Let’s leave everything the same and just change the white balance on the camera to flash:

Again, looks like what we expect and what the Red, White, and Blue chart told us it’d look like. Since the camera white balance is lined up with the flash that side is white. The bulb now is to the left of the camera on the Red Side, so it’s going to be warmer.

Let’s try setting the camera white balance to be between the bulb and flash color temperature and see what happens. Here the camera white balance is set to 4600K:

Yet again exactly what the Red, White, and Blue chart told us it’d be! From the camera’s perspective the bulb is on its Red side of the chart, on the warm side. We would expect that side to be a warm color temperature then and it is. The flash, from the camera’s perspective, is on the Blue side of the chart, the cool side. Therefore that side should be a cool color temperature and it is.

Now according to the Red, White, and Blue chart, if we place the camera all the way to the left, then the bulb AND the flash will be on the blue side from the camera’s perspective, so we’d expect both sides to be a cool color temperature. Let’s try that out, let’s set the camera white balance to 2500K:

It worked!  We were able to take a light source that normally is thought of as being warmer and made it cool.  All we had to do was follow what the Red, White, and Blue chart told us.  We changed the white balance of the camera so that from the camera’s perspective both the bulb and the flash were on the Blue side and therefore their color temperature would be cool.  Notice something else, the flash is further away from the camera then the bulb is and it’s side is more cool/blue then the bulb side. That’s because the degree to which a light source is warm or cool is dependent upon how far away that light source is from the camera’s white balance. The further away it is, the more warm/cool it will appear. The closer we place the light source on the Red, White, and Blue chart, the closer it will be to neutral white.

Let’s do the opposite, let’s place both the bulb and the flash on the Red side from the camera’s perspective. Here we changed the camera white balance to 10,000K:

As we thought, if we place the bulb and the flash on the Red side from the camera’s perspective they will both have a warm color temperature. The bulb being the further light source from the camera has a more warm color temperature to it then the flash side. So far our theory has worked pretty well.

Let’s try sliding the flash over to where the bulb is at by gelling it. Here we set the camera white balance to tungsten and we’ve put a gel over the flash:

Worked just like we thought. The color temperature of both are almost identical.

We’ve shown that the color temperature of a light source is really about the perspective of that light source relative to the camera.  By adjusting the camera white balance we can make a light source appear to be warm or cool.  We’ve also shown that certain light sources (flashes) we can gel and slide them to different areas on the chart.

Next time we’ll show how to use this in real life pictures to get some pretty cool effects when it comes to the sky and clouds.

Checkout the first and second post in this series, Gelling your flash, as easy as Red, White, and Blue  and  White Balance and Color Temperature.

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