A previous post introduced digital camera white balance. This post presents results of some white balance testing for my Canon digital SLR cameras and RawTherapee version 4.09. It concludes with some advice on how to achieve a satisfactory white balance. The testing methods can be applied to other cameras and other raw photo processing applications.
The colour temperature and tint white balance (WB) parameters in RawTherapee (RT) seemed to be unreliable and I thought some tests would help my raw photo processing.
I photographed the WB target of an X-Rite ColorChecker Passport in various lighting conditions:
- Daylight at sunrise, morning, noon, afternoon, sunset.
- Shade and shade under trees.
- Window light.
- Tungsten incandescent.
- Halogen incandescent.
- Fluorescent lamps warm white, cool white, 5000K and 6500K.
For each scene I took two photos: Auto WB and Custom WB. I subsequently performed Spot WB in RT version 4.09 and Colour Temperature WB in Canon Digital Photo Professional (DPP) version 3.10 for comparison. I tested both a Canon 350D and a Canon 400D. Findings for these two cameras were similar and for clarity of the presentation I will focus on results from the 400D.
White balance results
Colour temperature and tint for Canon Custom WB and RT Spot WB agreed very closely, as shown in the first two graphs below. This means that Canon and RT applied similar RGB multipliers.
Next, colour temperatures reported by RT were lower than expected. The first graph below compares RT Spot WB colour temperature with Canon 400D WB preset values (e.g. Daylight = approx. 5200 K) and colour temperatures for the lamps tested (often marked on the lamp or on the packaging). I do not have a spectrophotometer and did not measure the light. The second graph compares RT with Canon DPP Colour Temperature WB. Systematic errors in are apparent in both comparisons and the difference can be several hundred K at higher colour temperatures.
According to the RT colorimetry document, colour temperature and tint should be reliable for daylight illuminants and above 4000 K. My testing refutes that claim. Some independent data is available in the RawTherapee 4.0.10 User Manual for comparison. These daylight WB colour temperatures and tints are from camera input profiles:
|Some Canon and Nikon daylight white balance colour temperatures and tints from the RawTherapee 4.0.10 User Manual. Most of these data should correspond to direct sunlight, although a few higher values suggest overcast skies. The expected daylight colour temperatures are the same for Canon and Nikon.|
I assume most of the above data correspond to direct sunlight, which is a practical, common illuminant for daylight profiles. I computed averages and compared these to the manufacturer’s values (also from the RT manual). For Canon cameras, RT daylight colour temperatures averaged 203 K lower than expected. For Nikon cameras, RT daylight colour temperatures averaged only 7 K lower than expected. The practically zero bias for Nikons suggests that the model linking WB multipliers and colour temperature and tint in RT is based on measurements from Nikon cameras.
A second result from the above table is the slight positive bias in colour tint results for both Canon and Nikon cameras. My white balance testing showed unexpected variation in RT colour tint, as shown below. Daylight series illuminants are specified by colour temperature alone and tint should equal unity. However, RT colour tint was strongly correlated with colour temperature. Colour tint was around 0.89 to 0.94 at sunrise/sunset for my Canon DSLRs, which is important to keep in mind when processing raw photos. I think those low tint values might even be misleading.
Next, Canon Auto WB was conservative and biased towards daylight. It tends to exaggerate the colour of lighting that is far from daylight (e.g. photos in tungsten lighting look excessively warm). The two graphs below compare Canon Auto WB and Custom WB. I must acknowledge that taking photos of the greycard with limited surrounding scenery may not be the most realistic test of Auto WB.
White balance in practice
The preceding analysis identified two WB problems:
- Auto WB was unreliable.
- The absolute accuracy of RawTherapee 4.09 WB colour temperatures and tints for Canon cameras was poor, although these WB parameters are meaningful in a relative sense.
The photographer has a choice of using Auto WB, presets, Custom WB or performing WB during raw photo processing. I usually shoot with Auto WB and adjust it later in RT.
I consider only one or two of the presets on my Canon 350D and Canon 400D useful:
- Sunlight WB is useful when using a polarizing filter in bright sunlight. Auto WB will tend to counteract the effect of colour filters.
- Tungsten WB can be useful because incandescent lighting has a narrow colour temperature range from 2700 to 3200 K.
I have used two methods for Custom WB: 1) neutral WB targets (e.g. grey card, white card, foam cup, etc.); and, 2) WB lens caps (mine is a white, translucent plastic cap). The former use reflected light. The latter use incident light. Here are some tips for Custom WB:
- Remember to remove any colour filters, else the Custom WB will neutralise the effect of the filter!
- Beware of reflected light from nearby objects. For example, one of my shade WB tests was contaminated by reflected green light from nearby foliage in strong sunlight.
- I have found the WB cap works very well when taking photos under a forest canopy. Reflective WB targets performs poorly in this situation, perhaps because the light is mixed.
Adjusting WB in RT 4.09 can be a tedious, trial and error process. Here are some strategies for performing WB during raw photo processing:
- Find a neutral white object in the scene and try white balancing off that. However, the object actually may not have neutral reflectance or the light falling on the object is not the same as the scene lighting (e.g. white clouds are not very helpful).
- Set WB based on experience (e.g. colour tint should be near unity in daylight). I could refer to my WB testing results for guidance.
- When completely lost, adjust WB until the image ‘looks right’. First, set tint = 1 and adjust colour temperature. Second, adjust tint as required.
- It would help to take some notes about the light during shooting (e.g. warm/cool, greenish, etc.) because the image often looks very different when loaded with a biased Auto WB as the starting point.
It was disappointing to find that WB and colour temperature especially was biased for Canon cameras in RT 4.09 – this can cause confusion when adjusting WB. The basic problem is that native RGB scaling is a convenience and not based on colour science. Conversion between colour temperature and tint and WB RGB multipliers is not straightforward and seems to be different for cameras from different manufacturers.
Finally, there is a wrong WB, which results in ugly colour casts, but there is no ‘right’ WB. A technically correct WB neutralises the colour of the light and kills the mood – the world is not illuminated by D50 light. For example, increasing the colour temperature slightly for early morning or late afternoon landscapes will emphasise the warmth of the sunlight.