Monday, May 25, 2009

Temperature of Light - part 5 (In-Camera White Balance Settings)

My purpose in writing this posting, is to convince you to use a custom white balance. If you are using jpeg files from your camera, it is the only way to ensure you get correct color rendering. Some time ago I gave a white balance card to a friend and taught him how to use it with his digicam. A few months later I heard from his daughter about how much his photos have improved since he started using the “thingy” that I gave him. Why is that and why couldn't the camera just get it right? Read on to find out.

What is a Custom White Balance?
A custom white balance calibrates the camera's color rendering to the actual light being used. This works regardless of the light's temperature and it works reliably. For most cameras, you take a picture of a known white or near white surface and then program the camera to calibrate its color rendering to that white.

Why a Custom White Balance?
The short answer is because digital camera auto white balance is very unreliable. Take a look at the above images. All of these were taken under the same incandescent lighting, but using different white balance settings as indicated. The one on the bottom closely matches the actual magazine.

Why not use Auto White Balance?
Auto white balance may seem a simple function, yet few if any cameras get it right much of the time.

Take a look at the photo above. It was taken with incandescent light and the camera was set for auto white balance. This magazine cover has a good B&W print that should have been sufficient for the camera to get a decent color balance. However, as is the case for many cameras (especially Canon DSLRs), the auto white balance fell far short of the mark.

The lower photo shows the camera's display, including the RGB and luminance histograms. Notice how red dominates. The camera has chosen a color temperature that is not even close to incandescent.

Auto white balance attempts to figure out the color temperature of the light being used by searching out the brightest thing in the image. It assumes that this is a white surface and so does a color balance based on that. It sounds good on paper, but in practical use it leaves much to be desired.

The problem lies with not always having a true white reference. What if there is nothing white or even close to white in the scene? In this case the camera makes a best guess at what might be close to white and calibrates to that. In reality it does not usually get it right and sometimes it doesn't even come close. There is also the effect of color shift. Under incandescent light, something with a blueish hue might appear as white to the camera, in which case it will make a bad assumption about color. In the case above I cannot imagine what went wrong, but it certainly is not right.

Why not use Presets?
Presets for daylight and incandescent usually get you close, but quite often, not often not close enough, and sometimes they are just plain wrong. The image above was taken with the incandescent preset. The improvement in color compared to auto white balance is obvious. However, it still has a reddish hue that is not found in the real life subject. The RGB histograms from the camera show the improvement, but also show the lack of proper white balance. Because the magazine cover is essentially black and white, all three histograms should look the same.

If without a way or time to calibrate and under known lighting, I would use a preset rather than trusting auto. However, what if I'm in a room that is lit with Reveal incandescent bulbs? From part 4 of this series we know that the Reveal incandescent bulb will produce a light that has more blue than a standard incandescent bulb. If my camera is set to incandescent white balance, then I will get a blue cast in my images.

Fluorescent bulbs are even more problematic, as they have much variability from based on the manufacturer and type. They also shift color with use. Some cameras have two or more fluorescent settings, but that cannot possibly cover all of the variations one might encounter in the real world. Even if there were a preset, how would you know which one to use? When possible (as it usually is), it is always better to get a custom white balance. Daylight varies by time of day and atmospheric conditions.

What About Using a Custom White Balance?
Glad you asked because if you are going to let the camera generate jpeg files for you, this is the best way to go.
The photo above was taken after performing a custom white balance. As you can see, the image correctly depicts the black & white photograph on the cover of the magazine. A look at the RGB histograms in the next photo down confirms an excellent white balance. The simple step of performing a custom white balance has produced a near perfect rendering of the subject, enough said.

Coming Next
In the next posting I will demonstrate how to set an in-camera custom white balance using a Canon 450D. This tutorial should make a good primer even for non-Canon users, so until then...


Continue on to Temperature of Light - part 6 (How to Set an In-Camera Custom White Balance)

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