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Digital Camera Colour Management with Adobe Camera Raw
By David Harradine

The first rule of colour management is that no two devices are the same, no two monitors, printers, scanners or digital cameras will produce the same colour from the same signal out of the box, they all have their own unique finger print.

In order to get a response consistent with other devices this fingerprint must be measured so it can be compensated for. This measurement process is known as profiling.

Most devices come with canned profiles, which are averaged measurements of that particular models fingerprint or colour response. This reduces the variables to that model but not to the exact unit that you purchased. Canned profiles are not particularly useful for monitors as monitors will drift over time and need to be profiled on a regular basis. However, more stable devices like scanners, cameras and printers can produce reasonable results from canned profiles.

 CC in PS.gif  CC in ACR 1.gif

Above: Colour Checker in Photoshop set up to run the calibrator script.

 Above: Colour Checker in ACR with recommended custom settings visible in the calibrate tab.

Why to do it.

Custom profiles, however, are always going to be more accurate and produce the optimal results from your devices, and Adobe Camera Raw has a unique method for allowing you to custom profile your camera without needing any additional software.

ACR ships with canned profiles installed for each of the cameras it supports. The second tab from the right in ACR CS3 and on the far right in ACR CS2 is the calibrate tab, which allows you to optimise these canned profiles for your specific camera and save the settings as your defaults. The benefits of this process may be subtle or significant depending on how far your camera’s fingerprint varies from the one Adobe used to build the canned profile.

How to do it.
 
A popular technique is to shoot a GretagMacbeth 24 patch colour checker and tweak the RGB values for each patch in the calibrate tab to customise camera Raw to your camera’s behaviour. A detailed description, by the late Photoshop colour expert Bruce Fraser, can be found at - http://www.creativepro.com/story/feature/21351.html

However, the process of manually adjusting each patch is a little daunting as once you have one correct and start working on another, the original one changes again, creating something of an insanity inducing tail chase. So luckily there is now a free script available to automate the process, which is very generously created and supplied by the very clever Tom Fors at - http://fors.net/chromoholics.

The link above includes full instructions on installing and using the script, which is very easy to do. Depending on the size of your camera and speed of your computer the script can take anywhere from one to three or four hrs to complete it’s procedures, so it’s the perfect job for automation.

There are many more scripts popping up on line these days, just Google; ACR Calibration script. Most of them, however, are based on Tom’s original.

 ACR-Calibrated 1.gif  ACR-Calibrated 2.gif

 Nikon D200 ACR Defaults

 Nikon D200 ACR calibrated


Once you have run the script you will be left with a Photoshop text document listing all the necessary adjustments to get the target to it’s expected appearance. The important numbers are the ones you type in to the calibration tab, which are the last 7.

The basic tab adjustments are not relevant, they are just for that particular image and will not relate to others, whereas the calibrate adjustments relate directly to the colour rendering of your sensor.

Once you have entered the custom settings in to the calibrate tab you either save them as a preset or apply them as the new camera defaults by selecting save new camera raw defaults from the small fly out menu to the right of the tab selector. Note: this will apply all your current settings as your new defaults, so make sure you only change the calibration settings and not any others for this step. If you make a mistake you can easily choose “reset camera raw defaults” from the same menu.

Once you have saved your new defaults they will be applied to all files from that model camera. However, if you have two of the same model camera and you wish to calibrate each of them you can select “make defaults specific to camera serial number” in the ACR preferences and you will now be able to make a separate profile for each camera.

When to do it.

The idea is to produce the optimal daylight profile for your camera so it’s a good idea to shoot your colour checker in direct midday sunlight some time between 12 and 2 pm. A little bracketing is also useful to ensure you get the best possible exposure.

Colour checker in Photoshop set up to run the calibrator script
Colour checker in ACR with recommended custom settings visible in the calibrate tab

Calibrating different lenses and different times of day is not necessary, as we are measuring the sensor not the lens. And if we were to build a sunset calibration the results would eliminate the golden glow of this time of day.

However, if you regularly shoot under tungsten lighting it is worth building one for these conditions as the sensor can behave quite differently. So just like the days of film where we had daylight and tungsten balanced film stocks we now need daylight and tungsten profiles.

Nikon D200 ACR defaults
Nikon D200 ACR calibrated

The differences above are subtle, but the custom profile does produce a slightly richer colour rendition that seems to benefit skin tones and blue sky’s. Thus making for a file that requires less adjustment to reach it’s optimal appearance.

David Harradine is a photographer, trainer and Photoshop beta tester.
David regularly runs training seminars throughout Australia and New Zealand on Photoshop, Digital Photography and Colour Management.
To get more information on David’s current training events please visit http://www.whack.com.au/

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