Journal Number 96
August 2005

ORIGINAL PAPERS

White Balance
By Max Gibbs

White balance is something camera companies have been working hard to accommodate
automatically. What they don't tell you is why. There are two points that digital cameras see
the same: white and black.

All other colours are computed from the three colours red/ green/blue by algorithms which add or subtract the intensity of these three colours digitally. The grey scale accommodates all shades of grey between white and black and the two scales - colour and grey - coincide at the white (all colours, maximum intensity) and black (no colour, zero intensity). The colour the camera produces theoretically faithfully reproduces the colour spectrum under standard conditions. We trust the manufacturer!

Then we put on a UV or sky light or polarising filter to protect the lens and take the camera outside and put this dirty great variable filter between our subject and the camera i.e., air with dust and smoke, and shadows, and rain, water vapour, etc.

Then nature plays its part and changes the thickness of the air layer through which the sunlight passes so that at midday (standard colours correct) there is minimum adsorption but high scattering of blue light (that is why the sky is blue) while at sunrise and sunset, there is high adsorption of blue and scattering of red light (giving red sunrises and sunsets). Water vapour in the air removes red wavelengths (that is why on wet days everything looks cooler and greeny-blue). You will have seen the effects of smoke turning the sky to red as blue light is removed, and tree leaves act as green transmission filters reducing red light to give everything a green caste.

And we all know about artificial light - tungsten, sodium, mercury vapour, fluorescent - most of which will have a factory setting on the camera. Try comparing a manually set white balance with the factory setting sometime under artificial light. You might be surprised.

Given the above, unless you always take your photos at noon in full sun the auto white balance will be slightly wrong: more so on some days than others.

The manual white balance setting allows the photographer to set the white end of the scale on site for the conditions on-the-day. It must be set every time for different conditions and different days. This will then produce a colour which can be related back to the standard colour algorithm set by the manufacturer in the camera. The colour will then be "correct" and can be compared exactly with a photo taken by another camera, using the same manual white balance technique, at another time.

A word of caution: make sure your white reference card is white! A4 printer paper is not white!

The human eye is attached to the most complicated computer we know of (our brain) which has this remarkable ability to synthesise colours that it remembers. We can discriminate between subtle shades of colour much more precisely that the camera. But not quantitatively.

When you view the digital photo on the computer screen, the settings of the computer screen will also affect the colour you see. Consequently, with a known white balance set and the screen set to the correct standard colours (no pigment burnout) we might just get a true colour on screen.

Now we come to printing. The paper we print on has a colour caste and the inks used to print the photo will have a different response on each paper. The print set-up mode in the computer can be taken off auto to use the manufacturer's spectral response curve for each type of paper to correct for the paper type and the inks used by the printer to give true colours.

This coupled with the manually set white balance will result in colours which should be indistinguishable from the original viewed side by side in any light. This aspect is usually left to the professionals. But if you get prints from the photo shop, you have a better chance of getting true colours from your manually set white balance digital images.

Now I know the above sounds like a lot of fiddling, and of course your eye sees the photos you have taken with similar settings to be pretty much the same colour as the colour chart. So why go to the trouble of doing a manual white balance?

If you don't, the error produced by the auto white balance can never be compensated for and future users of your photo can never be sure of the true colour. Setting the white balance manually means that the optical filter conditions at the time have been removed and the original digital image is as true as you can get to the subject. Thus all future users can be certain that the starting point is a standard colour spectrum and work accordingly.

The differences might be small, but that is the reason the manufacturers put in the manual white balance mode. So use it.

You might like to read the comments by "leading" photographers in digital camera reviews on the accuracy of the colours produced by digital cameras.

Mark Moorhouse, photographer of the green Gastrodia "long column" on the cover of Journal 95, commented, "To demonstrate the importance of `white balance' take note of the cover photograph taken digitally with only an automated white balance adjustment then compare it to another photograph of the same plant taken on slide film (see Eric's already published shot a few journals back of Gastrodia 'long column St Arnaud ).

Neither of these photographs actually reproduce the greenish olive colour seen by the naked eye on site! But for further reference in the future, a `white balanced shot' would offer a correct colour reference point by which other thus treated shots could be compared."