This article introduces camera profiling and potential sources of error. Successful profiling requires good photographic technique, a reliable target and a model that fits the device behaviour. I use Argyll CMS for camera profiling.
Digital camera sensors do not “see” colour in the same way as human vision. Raw photo processing applications need input device profiles to interpret camera RGB.
There are five or six elements in profiling a digital camera:
- Camera and lens.
- Camera profiling target.
- Photographic technique.
- Profile model.
Each of these elements has some impact on the accuracy of the resulting profile. Camera profiling targets were evaluated in another post. I don’t have a spectrophotometer. I use manufacturer’s reference data and assume that manufacturing is precise.
Some people are good with computers but only mediocre photographers. Getting a good photo of the target is the most difficult step in camera profiling. Garbage in = garbage out.
Reflected light (e.g. trees, buildings, vehicles and, indoors, furniture) will contaminate the colour of the light source. Set-up the target away from any light coloured objects and don’t wear brightly coloured clothing.
I set the target on a large plywood sheet (roughly 1 m x 1 m) which is spray-painted matte black. I also use “shutters” to block light from the sides and reflections from the ground or floor.
Glare is reflection of the light source and imparts a lightness to the image. Viewed at at 100%, glare can be seen as light-speckling in darker parts of the image. Glare should not be mistaken for dust or noise.
A 0/45 degree geometry is used for spectrophotometer measurements to exclude glare. Likewise, the camera profiling target should be photographed from a different angle to the direction of the light source. In direct sunlight, I face the target towards the sun and shoot from below. I sometimes lay on the ground to get a satisfactory angle.
With artificial lighting, it’s usually not possible to place the light source high above the target and photograph from a low angle. I place the light directly in front of the target and then photograph from the side.
Use a lens hood to avoid flare. Stand back to avoid casting a shadow and lens distortion at wide focal lengths. Fill only the centre 1/2 to 2/3 of the frame to avoid any vignetting and resolution loss towards the edges of the frame. Shoot with low ISO to minimise noise.
The reference illuminant for the ICC profile connection space is D50. Most camera profiling targets are supplied with D50 reference data only.
Direct sunlight is a convenient approximation to illuminant D50 except that sunlight has more ultraviolet, especially around noon and in summer. I’ve found that applying manual white balance makes a very good D50 approximation from direct sunlight at any time between about mid-morning and mid-afternoon.
I prefer to shoot with direct sunlight at about one or two hours after sunrise:
- When the sky is clear and sunlight more directional, with less diffuse daylight. It’s easier to control glare and maximise contrast in the photo.
- When sunlight is nearest to D50 and UV is lower than at noon.
Strictly speaking, colour changes with illuminant and a camera profile is accurate only for a specific illuminant. In practise, camera profiles created for sunlight can deliver pleasing results for most other daylight situations and some other continuous spectrum light sources.
The sunny 16 exposure rule predicts a shutter speed of 1/(4 × ISO) = 1/400 at f/8 and ISO 100. A faster shutter speed suggests glare or very bright sunlight.
I actually set my base exposure by metering off a 18% reflectance grey card. I then shoot a series of photos over the base exposure (e.g. 1/400s, 1/320s, 1/250 s, 1/200s). The raw photo RGB channels should be bright, but not clipped. The response is very linear and final exposure adjustments will be made during profiling.
Choosing a profile model depends on the behaviour of the camera and how much data is available for characterisation. Argyll CMS v 1.6.0 has seven profiling models to choose from (in order of complexity):
- Single gamma + matrix.
- RGB gamma + matrix.
- Single shaper + matrix.
- RGB shaper + matrix.
- XYZ colour look-up table.
- L*a*b* colour look-up tabl.
A matrix profile is simply a 3×3 matrix that transforms white-balanced linear RGB to D50 XYZ. I recommend simple matrix profiles:
- The raw photo RGB response for digital cameras are very linear.
- White-point scaling can be applied to retain highlight details.
- Matrix profiles are not limited by the colour gamut and dynamic range of the test chart. Matrix profiles are useful for real world photography. Colour look-up tables can be more accurate for studio work with standardised lighting.
- Most commercial profiling targets have not enough patches for colour look-up table profiles. At least 200 to 600 patches are recommended.