by Steve Upton
Profiling Digital Cameras
A hot topic indeed.
In some ways you would think it's like a scanner. A camera is really just a scanner up on end with a lens right? Oh, but lighting is different isn't it? You bet it is, that and other factors can make camera profiling a much different experience than scanner profiling.
On a basic level, profiling digital cameras and scanners works the same way. A target containing numerous color patches is captured into your computer. Save the resulting RGB file and import it into your profiling application. Then load the text file that contains the Lab measurements for each patch on your target. The profiling software calculates an RGB-to-Lab lookup table, your profile. That is pretty much where the similarities end.
Several different targets are available for camera profiling including the venerable IT8.x (Q60), Macbeth ColorChecker, ColorChecker DC as well as the Hutch Color Target (HCT) and some proprietary ones. Like anything in life there are tradeoffs when choosing any of them. Photo paper targets can produce smoother tones but tend to have a smaller gamut and are subject to fading over time. Paint chip targets (both colorcheckers) have greater longevity and durability but in the case of the Macbeth Colorchecker, do not have enough mid tones to effectively sample a camera's color response. The ColorChecker DC, while having many more mid tone colors has been known to behave unpredictably in polarized light and is under the process of being reformulated by GretagMacbeth. We have found that the profiling software / target combination plays a significant role in profiling results.
Making a scene?
Early camera profiling adventurers found that simply placing the profiling target in the scene can create a profile for use in that lighting condition. But any other lighting condition, even as much as moving a light, can cause enough variance that a new profile is required. This "profiling the scene" can be used to correct color in some situations but it is a very brittle technique and falls out of favor after building 10 or 20 profiles.
A properly built camera profile, in combination with correct gray balancing, characterizes the camera system effectively for a wide range of lighting conditions. Occasionally a new profile is required when lighting is drastically different than the profiled lighting or a camera is particularly sensitive to infrared wavelengths. Still, a couple of profiles for your entire shooting experience sure beats a life of constant profiling.
When setting up RIP systems for customers, we typically say that you can never spend too much time on ink levels and linearizing - it will always result in a better print profile. Similarly, when it comes to building quality camera profiles, you can never spend too much time setting up the lighting and target for capture. This is the process that makes or breaks the quality of the profile. Absolutely even lighting from a single light source is required. Those who've spent many an hour shooting copy work will recall that even lighting is quickly achieved with two light sources. That's fine for copy work but murder for camera profiling. Any deviation in color temperature between the two lights and you will have a color bias across the target that will kill your profile. And as digital cameras can pickup as little as 1/10th stop variation, you have to be very careful.
Often called white balance, gray balancing a camera can be done in several ways including automatically, in camera, in the camera software or in Photoshop. No matter how it is achieved, gray balance is the secret to using profiles in a wide range of circumstances. It effectively calibrates your camera for each lighting situation (an oversimplification but OK for this level of understanding). Calibration, as we know, is the key to devices behaving themselves and profiles remaining valid.
Is that it?
Well, basically yes. The correct target and software combination, combined with good technique will build you a good camera profile that is usable in a wide range of circumstances.
Is it that easy?
Perhaps not. We have spent a considerable amount of time working out the techniques of camera profiling and determining which product mixes actually produce the results our customers expect. The results?
Tools: ColorEyes 20/20 from Integrated Color Corp is the profiling tool of choice. It includes a proprietary reflective target that has a wide gamut, enough color patches to sample the camera space effectively and extra patches in important color zones such as flesh tones. ColorEyes 20/20 works as a plug-in within Photoshop and is pretty much a one-button profiling tool - as mentioned above, shooting the target is everything. Oh - in regards to shooting the target, the software actually includes a real manual. When was the last time you saw a printed, bound manual that contained useful, if not essential information? Overall it's a great package and also the only one we have tested so far that builds great camera profiles that work in a wide range of circumstances.
Technique: What can we say? It is everything when profiling these things (have I repeated that enough?). As a result we have worked closely with the software manufacturers and photographers to develop a comprehensive training program and get our field color geeks up to speed. Please contact us for more information about these services.
Adobe Camera Raw
Some of you may be reading this article and wondering when I am going to mention Adobe Camera Raw (ACR). Recently Adobe released a Photoshop plugin that allows the opening of "raw" digital camera files directly into Photoshop. Typically users have been forced to use the camera manufacturer's own camera software which ranges widely from camera to camera (even from the same manufacturer) and rarely works well in either default mode or using custom profiles.
ACR creates a single interface for a wide range of cameras and also supplies some "tweaking" tools for adjusting white balance and other image characteristics as the file is opened. What ACR brings to the table is a great replacement for the bewildering and problematic software typically included with cameras. ACR uses unmodifiable ICC technology internally that acts as generic profiles. Testing has shown us that while ACR is a vast improvement over existing software solutions, it does not give the results that our professional customers demand. Custom profiles exhibit smoother highlights, fuller flesh tones and more accurate colors for product and branding shots. Some of the effects are subtle but make a noticeable difference, and for our pro customers that is the difference they require.
Without getting into a full book on the subject, that is about as far as we can go on the topic of digital camera profiling. I have always referred to camera profiling as the bleeding edge of color management. It seems that finally we can move away from the edge and get back to work with some great results.