MAKING SENSE OF "WHITE BALANCE"

Because of the way our eyes respond to light, all the colours that we see are some combination of red, green and blue (RGB for short). For a given combination of red, green and blue, the more light there is, the brighter the colour appears. The less light there is the darker the colour appears. Your computer monitor also uses combinations of red, green and blue to produce all the other colours that it displays. Your camera records information in red, green and blue "channels". Red, green and blue are called "additive primary colours" because they are "added" to produce the other colours.

When we see all three colours in a balanced amount (equal intensities of red, green and blue) we see white light. Low levels of white light appear as grey. We sometimes use the term "neutral grey" to emphasise that it has equal amounts of red, green and blue.

When you look at a white (or grey) object under different lighting conditions (bright sunlight, shade, fluorescent lights,etc) your brain interprets the informaton from your eyes and the objects that you expect to be white (or grey) will probably appear to be white (or grey). However, under different lighting conditions a "white" object may actually be reflecting light that is not exactly white, even though your brain has been tricked into thinking that th eobject looks white.

When you photograph a white object under different lighting conditions your camera may not recognise that the object is supposed to be white- it may record the white object (and everything else in the image) with a colour cast. This is actually an accurate representation of the "real" colours in the image, but it will not look "right" when you print the image.

Most digital cameras have a "white balance" control. This allows you to compensate for expected colour casts (such as the green cast that you often get under fluorescent light) and "balance" the red, green and blue light that is reflected from objects that are supposed to be "white" (or grey). The purpose of this is to allow you to record colours the way your brain sees them, rather than the way they really are.

If you have set your camera on "auto white balance" it will attempt to analyse the colours of the light falling on the sensor and "guess" what objects are supposed to be white. It will then adjust the overall colour of the image to try to remove any colour casts. Most cameras do a fairly good job of automatically correcting the white balance under most daylight shooting conditions. However, this automatic setting may not give you satisfactory results under artificial light or under unusual lighting conditions (such as sunsets).

For these unusual lighting conditions you might be able to set your camera to a special white balance setting (such as "fluorescent" or "tungsten") or set it to a specific "colour temperature". Refer to your camera manual for details of how to do this.

If you change the white balance setting on your camera from "auto" to some preset value DON'T FORGET TO CHECK IT before every shot. The image above shows an image taken in daylight on auto white balance (left) and the same image taken with the camera set to "tungsten" white balance - the strong blue tint is because the camera was expecting the scene to have a strong yellow tint from the tungsten light (so it compensated for the non-existent yellow tint and produced a blue image).

The white balance is "wrong" when objects that should be white (or grey) with equal intensities of red, green and blue light being reflected from them appear to have a colour tint. The resulting image is sometimes said to have a "colour cast".

In the image above, the original (on the left) has a slight yellow cast on the flower petals which are supposed to be white. Correcting the white balance gives the result on the right. Some ways of achieving this are described below.

When you open a RAW image in ACR and select the "basic" tab in the right hand column the white balance informatoin is displayed just below the histogram. The white balance information recorded by your camera is referred to in Adobe Camera Raw as the "As Shot" white balance. The information displayed is the "colour temperature" (5300 in this example) and the "tint" (+39 in this example).

If you have set your camera on "auto white balance" and it has given you a satisfactory result then you will not need to make any white balance adjustments in ACR. However, under unusual lighting conditions (such as fluorescent lights) you may get an unwanted colour cast - in this case you need to adjust the white balance. Normally what you are trying to achieve is the "correct" white balance - that is, neutral greys that are really neutral so that there is no unwanted colour cast on the image.

To do this you have three basic options in Adobe Camera Raw - presets, the eyedropper and the sliders.

If you click on the double-headed arrow to the right of the white balance preset selection window it will reveal a drop-down list from which you can select a number of preset ways of changing the white balance. These will compensate for any unwanted colour casts in the image (which may have resulted from you having the wrong white balance setting on your camera or from some unusual lighting conditions - such as shooting under fluorescent lights).

Note: If you open a JPEG or TIFF image in Camera Raw the only presets available are As Shot, Auto and Custom.

If you select "As Shot" (which is the default) ACR will apply the white balance setting that was recorded by your camera (provided ACR can read that information in the data attached to the image file - if it can't then the "As Shot" option will not be available).

If you select "Auto", ACR will attempt to evaluate the colour information in the image and set an appropriate white balance.

If you select Daylight, Cloudy, etc, ACR will apply preset values of colour temperature and tint to compensate for the colour cast that it expects in the image. Don't be surprised if the presets on this drop-down list do not give you the white balance that you want - they apply fixed temperature/tint values that will not necessarily suit your individual images. You will often get better results with one of the following methods of adjusting the white balance.

The white balance eyedropper is the third tool from the left (at the top left of the ACR screen).

When you click on it the cursor changes to an eyedropper.

If you identify an area of the image that you think should be neutral grey and click on it with the eyedropper Adobe Camera Raw will attempt to correct the colours in the image so that the point you clicked on is made to be neutral grey (the red, green and blue values will be made equal).

If the result is not quite what you wanted you can simply click the eyedropper on a different part of the image.

As you make these changes, you will notice that the white balance present changes to "custom" and the two white balance sliders (for colour temperature and tint) move and their readout values change.

Sometimes the white balance that you get using the eyedropper doesn't quite look right - you may want to vary it slightly. If you want maximum control over the white balance setting, you can manually adjust the "temperature" and "tint" sliders - either starting from the "As shot" white balance or starting from a preset or a value set with the eyedropper.

1. If you move the colour temperature slider to the left (towards the blue end of the scale) it will "cool" the image by introducing a blue tint. If you move this slider towards the right (yellow end of the scale) it will "warm" the image by adding a yellow tint.

2. If you move the "tint" slider to the left it will introduce a green tint. If you move this slider to the right it will introduce a magenta tint.

If you just randomly move these sliders around you may not have much success correcting the white balance. If you want to be accurate you can do it this way:

First, select the "Colour sampler tool" (4th from the left in the group of tools at the top left of the screen).

Next, look for an area in the image that you think should be neutral grey - if possible select an area that is not too light or too dark. In the sample image I am using here, there are parts of the bird's beak that I think should be grey.

Then click once on the area that you think should be neutral grey.

Notice that a small circle with the number 1 has been placed on the image. This is called a "sample point". If it is not exactly where you want it you can click on it and and drag it to a new location.

If you look immediately below the row of tools you will now see that the RGB values for this sample point have been displayed.

In this example, the red and green values are almost the same and the blue value is higher. This is suggesting that there is a blue cast on the image. I could try to correct this by simply selecting the eyedropper and clicking directly on top of the sample point (as I described previously) but here I want to illustrate how to do the adjustments manually.

If I move the colour temperature slider to the right (away from the blue end and towards the yellow end of the scale) at the same time as I watch the RGB values displayed for the sample point I can try to get the RGB values equal (so that the colour at the sample point is a neutral grey).

In this example, as I move the colour temperature slider to the right I get to the point where the red (R) and blue (B) values are equal (the B value has come down from 73 to 64 and the R value has increased from 56 to 64). The green (G) value has changed just a small amount and the image has lost some of its blue tint.

However, the image now has a slight magenta tint because the RGB values at the sample point are still not all equal.
To fix this, I move the "tint" slider slightly to the left (towards the green end).

At the point where the R, G and B values are equal (in this example when they are all equal to 58) I have achieved a neutral grey at the reference point.
[By the way, at this point the colour temperature value was 6450 and the tint value was +4.]

After you have tried this process on several images you will find that you are able to make these adjustments fairly quickly.

It is important to note two things here:
(a) There is some interaction between the two white balance sliders. When you move the "temperature" slider it alters the blue value at the reference point (which I wanted to do in this example) but it also alters the green and red values. This is because yellow (the right hand end of temperature slider) is a mixture of green and red. Likewise, moving the "tint" slider alters the green value at the reference point but it also alters the red and blue values. This is because magenta (right hand end of the tint slider) is a mixture of blue and red.

(b) If the reference point you have selected is not really neutral grey, then forcing it to be neutral grey (using the method I just described) may introduce an unwanted colour cast elsewhere in the image. In the example I have shown above, when the reference point (on the bird's beak) was set to neutral grey the overall image appeared to have a very slight green tint. If this happens, you can create a second reference point in a different part of the image and do some further "fine tuning" until you have a visually acceptable image. This might take a bit of practice. Of course, if your monitor has not been calibrated correctly the image may look good (appear to have correct white balance) but the image might still have a cast when you print it.

If you use any of the methods described above for correcting white balance you have the problem that you have to guess what parts of the image should be neutral grey (or white). This is a particular problem for images such as the one above where there is no obvious point to select as your neutral grey reference point.

One way to solve this problem is to photograph something that you know is neutral grey and use it as a reference for adjusting other images that you take under the same lighting conditions. I will now describe how to do that.

A "reference card" is a card that is printed with "standard" or "reference" colours. Two types are shown above. The simple one contains just three reference areas - white, grey and black. (Actually the "white" is not totally white and the "black" is not totally black.) The other card has thirty colour patches, including some that are neutral grey.

Other cards like these are available. A popular one is the GretagMacbeth colour chart, but it is expensive. You could also use a simple 18% grey card (from the old film days).

If you take a photograph that includes a grey/colour card like this you can use the mid-grey patch as a reference point for setting the white balance on the image that contains the card. Just use the eyedropper and click on the grey reference patch.

You can then copy the adjusted white balance setting to other images that are taken under the same lighting conditions.

After you have corrected the white balance on your reference image in ACR, return to Bridge (by clicking "done" at the bottom right of the ACR screen.

In Bridge, right-click on the thumbnail for the image and the above menu will appear.

Select "Develop Settings" and from the next menu select "Copy Settings".

Next, select the thumbnail for the image (or several images) to which you want to apply the new white balance setting. These must be images taken under the same lighting conditions as the reference image.

Right-click on this thumbnail and select "Develop Settings" and then "Paste Settings".

The following window will appear:

If you click "OK" all the settings that are "checked" will be copied from your reference image to the image(s) you currently have selected.

If you just want to copy the white balance setting, click on the double-headed arrow next to the word "Settings" and from the list that appears select "White balance". You will then see this window:

Just click OK to appply the white balance settings.

If you think it is necessary, you can still make further adjustments to the white balance on the new image(s). Of course, you can also make any other adjustments that you want (contrast, cropping, etc).

Some digital cameras allow you to set a "custom" white balance rather than "auto" or one of the presets. One way of doing this are to use an "Expodisc" which is a filter that you temporary place in front of the camera lens while setting the camera's custom white balnce. An alternative is the "CBL lens" which you hold in front of the camera and photograph in order to produce a custom white balance reading. Both approaches can be quite effective if you are taking several images under the same lighting conditions.

Setting the "correct" white balance is just one part of displaying and printing images that faithfully reproduce the colours in the original scene. There are many other aspects of colour management that you need to consider - most important of those are calibrating your monitor and your printer.

Comments on this tutorial are welcome.

Roy Killen
Updated 17th April 2009
Email: roykillen@mac.com