Profiling a Camera with an IT8 Target

Background

I am often asked about camera profiling in one context or another, and even challenged by other professionals as to whether or not it is even possible to develop ICC profiles for digital cameras. As I often say, the answer can be complex and may depend on many factors, but let's break it down into a few key points that are relatively easy to understand.

What is a camera profile?

I've done numerous articles in the past about color management and color profiling, so if you need a refresher on the subject of color management and ICC profiles, please check out my articles from August 2004 and February 2005.

First, it is important to realize that the only time a custom or "home made" camera profile is needed is when the camera or raw developing software needs a little help to get more accurate color. Due to differences in lighting and viewing conditions, the term color "accuracy" is a subject of some debate since you are unlikely to be able to reproduce the exact lighting (color temperature) and exact colors from the original scene. To those who will be viewing your photos on screen or in print, color "accuracy" can best be defined as the reproduction looking as much like the original scene as possible to eyes of the observer. Sometimes cameras add a little "pop" by increasing contrast and saturation a bit, and that is normally not objectionable unless it is extreme. What most viewers object to are noticeable hue shifts: color shifts that make a blue sweater look purple, a red flower look orange, green grass look yellow, and so forth.

When hue shifts are significant enough, viewers may remark that the subject is the "wrong color". In my 5+ years of developing href="http://www.ddisoftware.com/qimage" rel="nofollow"> Profile Prism, software that can discover the color characteristics of almost any device to produce ICC profiles, I have developed methods that allow for accurate profiling of digital cameras, with some caveats (under some conditions). These profiles can be used to improve color "accuracy" and reduce or eliminate complaints about color problems when your raw conversion software falls a bit short. I mention "raw" because it is nearly impossible to create a usable profile that works under a variety of shooting conditions when shooting in JPEG/TIFF mode with your camera. When the camera stores a JPEG/TIFF, the image has already been "profiled" (in a sense) by the camera and producing a profile that second-guesses the camera is usually of little use due to the fact that results/colors are often inconsistent when shooting in JPEG/TIFF mode under different lighting and exposures.

Profiling a camera: the process

In the early days of digital cameras, it was possible to produce a profile for cameras shooting in JPEG/TIFF mode mainly due to the fact that some cameras produced gross errors that could benefit from correction, even if the result wasn't completely "accurate". Now, most cameras comply reasonably well with the sRGB color space and many more advanced cameras even offer an option of sRGB or Adobe RGB as the color space used by the camera. When we have a relatively recent camera model and/or a color space selection, it is rarely beneficial to try to develop ICC profiles for the camera shooting in JPEG/TIFF mode because it is difficult to impossible to produce corrections that result in any consistent improvement. If we shoot in raw mode, however, most raw conversion tools offer an option to turn off color management so that custom ICC profiles can be created/used. With color management turned off, the raw data offers a much more consistent starting point, and profiling becomes not only possible, but often quite beneficial.

The process, at least conceptually, is very simple. Take a shot of a color target in raw mode, develop the raw image with color management turned off in the developing software, and use a profiling tool to create a profile from the image of the target. The profile can then be activated in the raw developing tool. That said, the actual process itself can get a bit complex if we want to ensure a quality profile. You need to get a good shot of the target under good lighting, and you need to use a profiling tool like href="http://www.ddisoftware.com/qimage" rel="nofollow">Profile Prism that was designed with camera profiling in mind as camera profiling requires specialized options like the ability to normalize tone curves and let the device dictate white balance. There are other high-end (read expensive) tools that allow you to develop camera profiles. These tools offer specialized targets and software, but I find that with some care, it is possible to match or even exceed the performance of these "high dollar" tools with Profile Prism and a standard IT8 target!

The problem, the solution

Before we start with the details, it is appropriate to inject a bit of reality here. Many raw developing tools, while they are designed to produce the best color possible, just weren't built using any real "scientific" means for color accuracy. Some use a simple color matrix to tweak color so that it looks acceptable and many don't employ reasonable tone curves to ensure good shadow detail. In layman's terms, this is the reason that it is often possible to develop ICC profiles for raw images that result in better color reproduction than the raw tools offer out-of-the-box.

If we can develop a profile that improves color over the "default" color reproduction of the raw developing tool, we can say we have a successful/useful profile. Some may question whether or not it is possible to develop a single profile that works under all lighting conditions, or whether it is imperative to develop one profile for each lighting condition: sunlight, fluorescent, incandescent, mercury vapor, etc.. Again, the true scientific answer here can get complex, but I've found that when profiling the true raw data, a "generic" profile can be developed using direct sunlight. As lighting conditions (color temperature) shift from direct sunlight to warmer lighting such as incandescent lighting, the profile will become less accurate but the shift is not normally so extreme as to cause gross errors. This is, in part, because the color filters used on the image sensor aren't changing under different lighting. Their overall response is the same under different lighting and color temperature only affects the proportions of red, green, and blue recorded by the sensor. A good profiling tool can discover the overall color characteristics of the sensor which tend to be valid over a wide range of lighting conditions. Here, the closer you can get to the actual raw data the better, because up-front color corrections only tend to multiply color shifts, so a raw tool that offers the ability to process the raw data without injecting color corrections will work best.

While some may choose to develop different profiles for different lighting, and that's certainly optimal, a generic profile for sunlight should work under a variety of conditions. Shooting the IT8 target in direct sunlight helps to reduce any metamerism of colors on the target and ensures a good match to the data file that tells the profiling software what the color on the target should look like. Shooting in direct sunlight also offers the ability to eliminate glare as the IT8 target is a glossy target that, when not shot under the proper conditions, can certainly produce glare which will make the profile useless. Shooting with the light hitting the target at an angle is imperative to eliminate glare/reflections and due to the fact that our light source (the sun) is so far from the target, we don't have to worry about the light being brighter on the side of the target closest to the sun as we would with angled studio lighting! Here's how to shoot an IT8 with no reflections or glare:

  1. Of course, a lot depends on your location and the time of year, but in general, the best time to shoot the target is either 1-2 hours before mid-day or 1-2 hours after mid-day. Try to shoot on a day with minimal clouds so the sun isn't changing intensity/color as you shoot.

  2. It is helpful to attach your IT8 target to a piece of thick cardboard using small tacks or pins at the corners or even tape at the corners as an IT8 will tend to curl and bend when it heats up in sunlight.

  3. Try to find a room where light is entering a window/door at a sharp angle and hitting a wall adjacent to the window. If you can open the window to reduce lighting variations caused by the glass, all the better! Here in the northern hemisphere, a south facing window often works well in the afternoon. If the sun doesn't hit a wall, a palette, chair, or other object may be used to place your cardboard w/IT8 in the sun.

  4. Make sure the room is as dark as possible and that the only light entering the room is coming from the window. Also try to avoid the direct sunlight hitting bright colored (non-neutral) surfaces such as red walls, blue floor tiles, etc. as these reflections can cause color shifts on the target.

  5. Place your target in the sunlight so that the sun is hitting the target at an angle and you can sit in the shadows while taking the shot. The following is a typical setup for shooting an IT8 target in direct sunlight. Notice how the sun hits the target at a sharp angle so that the camera can sit in the shadows, thereby eliminating glare on the target:

  6. If your camera has a custom white balance feature, using a white/gray card or a white sheet of copy paper (don't use photo paper with brighteners), place the card at about the same location as the IT8 and make sure it is in the sunlight. Use the custom white balance on your camera to white balance on the card.

  7. Take several shots of the target in raw mode. Take one "normal" shot and then increase exposure incrementally, taking several more shots with brighter exposures making sure to stop just before the exposure gets "blown out" in the highlights. Camera settings like aperture usually have little influence, but smaller apertures often produce more even lighting across the frame. Note that camera lens and ISO speed can make a slight difference in profiling, so be sure your ISO speed is set appropriately and you are using your most-often-used lens. If you and/or the camera are sitting in the shadows of the room, you can take the photo straight-on at the target and you should get no glare or reflections. When taking the photos, fill only about 3/4 of the frame with the target. Don't zoom in so far that the target covers the entire frame because light falloff from the edges of the lens can be a factor here.

Once you have the shots of the target, turn off color management in your raw developing tool and develop the photos. Depending on the raw tool you are using, turning off color management may entail selecting a color management tab and selecting "Embed camera profile", or selecting "None" in the "color management" dropdown. Whatever you do, the important thing to remember is that you need to be able to turn off color management to develop the profile. Then, once you are done creating the profile, the profile can be activated in the raw tool by selecting the ICC profile that you created. Of course, this assumes that the raw tool you are using allows selection of custom profiles. Not all tools allow use/application of custom profiles so be sure the tool you are using has this feature. The more popular third party tools like Bibble, Capture One, and (the now discontinued) RawShooter allow the use of custom profiles. When developing the images, develop to TIFF (you can use 8 or 16 bit/channel TIFF format).

In Profile Prism, click "File", "Open" and open one of the developed images of the IT8 target. Next, make the following selections on the Profile Prism main window (description and file name are just an example):

Parameter Set to
Type of device to profile Camera/scanner
Reference target Choose the file for your IT8 target
Profile description Something like "Canon 5D Generic"
Printer target N/A
File name Choose a name like canon-5d.icm
Profile for Highest Accuracy
White balance Device dictates WB
Tone reprod. curves Normalize
All other options "Normal" or zero (0)

The above parameters are appropriate for profiling a camera. Once you have set all the parameters, mark the 4 corners of the target on the image of the IT8 target. The step by step procedures for profiling a camera or scanner in the Profile Prism help will show you how and where to place the crop markers on the IT8 target. Once placed, there should be a white punch-out in each of the color squares on the IT8 including the gray scale at the bottom. If the punch-outs don't align inside each color square on the target, the corner markers have not been placed properly. Finally, click "Create Profile" at the bottom left and Profile Prism will create your camera profile. You can then test the profile by selecting the profile in your raw developing tool using the file name you used in the table above. Once the new custom profile has been set, simply redevelop the photos and evaluate them for color accuracy/appearance.

Since some raw tools like Capture One and RawShooter apply some "pre-curves", it isn't possible to profile based on truly raw data. As such, you may have to create a profile for each of the exposures (the one normal exposure and several brighter ones) and then pick the profile that has the tone curve (shadow and highlight detail) that you prefer. Usually, the best result occurs when the curves displayed in Profile Prism (after clicking "Create Profile") end as close as possible to the upper/right corner of the graph. If the curves end on the top edge or the right edge of the graph, you may need to try a different/better exposure. Note that it is best to pick a different shot with a different exposure as opposed to tweaking the exposure of a single shot in the raw developing tool! With a little practice, the above process can produce excellent profiles for any camera shooting in raw mode. The above are the procedures we used to develop our own camera profiles for numerous raw tools. These profiles have gotten many positive reviews and are often compared to profiles produced with much more expensive equipment/targets from other sources.

Summary

This article can be described as a "secrets revealed" on how to create IT8 based ICC profiles for digital cameras shooting in raw mode using my inexpensive but highly effective href="http://www.ddisoftware.com/qimage" rel="nofollow">Profile Prism software. With the right tools and a little experience, it is possible to develop excellent ICC profiles for digital camera raw photos using a standard IT8 target. It is possible to rival or even beat results of software/targets costing 10 to 20 times as much as Profile Prism. Although there are thousands of satisfied Profile Prism users out there who have created excellent scanner and printer/paper profiles, some users may have been reluctant to try Profile Prism for profiling their cameras in raw capture mode. I hope that this article will be helpful in getting people started who wish to create custom camera profiles for raw developing tools. While this article has been tailored to my own Profile Prism software, the techniques can certainly be used by anyone using any software capable of creating camera profiles. Regardless of the profiling tool you use, the saying "it can't hurt to try" applies here. Just remember that the whole point of creating a profile is to improve color in the developed images, so always evaluate your results against the "default" color produced by the raw developing tool. You want to make sure you aren't going backwards, which is a possibility when developing camera profiles!

Since the initial release of Profile Prism in 2001, it has been shipped with two targets: a glossy IT8 and a matte target. We will be dropping the matte target soon and will be shipping Profile Prism with only the IT8 target as we have found the IT8 to be the most accurate under all conditions and with the tips in this article, the matte target should no longer be necessary for camera profiling. We use some of the most accurate IT8 targets in the industry and I feel that with a little care, the IT8 can and should be used to profile all devices: cameras, scanners, and printers. With the proper setup, a matte surface is not necessary to eliminate glare. While you may hear words of disbelief regarding the ability to profile a camera, especially using a standard IT8, just follow the steps outlined in this article and you may be surprised at the results. Many times, camera profiling difficulties come from using a tool that doesn't offer the features (like "device dictates WB" and normalization of tone curves) needed for camera profiling. We've proven with our own custom raw profiles that with the right tools and the right setup, camera profiling can be beneficial and cost effective.

-- Mike Chaney