A Flower for DM, UniWB, and Pekka Potka

Started Apr 17, 2012 | Discussions thread
gollywop
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Creating a UniWB: step-by-step
In reply to gollywop, Apr 17, 2012

There are several ways for creating a UniWB for your camera. For those with the right software, there are easier ways than that described below, but this method should be rather universally applicable.

First note: the EXIF info for an image's WB is reported differently for different cameras. For the D300, there is a field called "WB_GRBGLevels" that gives 4 numbers, like 256 260 258 256, which are, respectively, the relative values for G, R, B, and G. The G figures are the base and will always be 256. The multipliers for R and B are determined as the ratios of their respective values to G. Thus, in the figures I gave above, the R multiplier would be 260/256 = 1.0156 and the B multiplier would be 258/256 = 1.0078. The info for the OM-D is reported in a field WB_RBGG; it is the same kind of information, used the same way, but given in a different order.

The following is an iterative procedure. It is written with PS in mind, but you can likely adapt it to any decent photo editor:

1. Open a new document in PS (create a new image): RGB color mode, 8-bit, white background and large enough to allow you to more than fill the view finder of your camera while shooting the screen at some appropriate zoom.

2. Double-click the foreground color icon to open the color picker and set the color to a middle gray, say, R = G = B = 128. Now select the entirety of your document (select all) and fill (with paint bucket) with this color. You now have a nice gray document.

3. Set your camera on a tripod aimed at right angles to this image and zoom in to fill the view finder with the central part of the gray image (I toggle the f key until the monitor background is black). Shoot manual mode with manual focus, about 1/2 second exposure (to remove screen flicker), and an F stop to achieve a good exposure (typically somewhere between f5.8 and f11), focus blurred. Set a custom white balance from this gray image using the one-touch method. It is best (necessary) to do all shooting, here and below, in a darkened room and avoiding all sources of glare. Note: this method is independent of monitor calibration.

4. Using this WB, take a shot of this same image. It's best if you don't move the camera or the zoom so that all readings are taken from the same screen area.

5. Remove memory card (try not to move camera) and examine the image in an EXIF reader that reports the WB Levels information (such as WB_RBGG). Let's assume the quartet of values is (r, b, g, g). Then r/g is your red multiplier, and b/g is your blue multiplier (g, of course, will be 256).

6. Using these multipliers, adjust the R and B values for the gray document in PS to match. If your gray was R=G=B=128, then multiply R=128 by r/g (round to nearest integer) and B=128 by b/g. Do not change the G value. You will now have a magenta colored document.

7. Not having moved your camera, and in the darkened room, replace your memory card and set one-touch custom WB from this image, and then shoot an image of it with this WB. (The resulting shot should look pleasantly gray. If you had shot something other than the magenta screen, the image would have a decidedly greenish cast.)

8. Examine the EXIF WB info for this shot. The numbers should have moved closer to the desired goal of 256/256/256/256 (implying multipliers of one -- or UniWB). This magenta image will be a good starting point for further refinement. But further refinements will likely be necessary.

9. For further refinements, you can simply adjust the R and/or B values for your magenta document, several values at a time (G left unchanged). If your new b value is too low(high), decrease(increase) the B value in the gray document and likewise for the r value. With these new numbers (and a new magenta image), repeat steps 7 and 8, and continue iterating till you're done. The number of iterations depends on how smart you are at figuring how much to adjust the R and B values in the magenta document at each iteration. Once you get close to the 256/256/256/256 goal, you're probably safer moving only one color at a time, one or two values. Multipliers within 1% (0.99-1.01) are fine; they don't have to be exactly unity. Thus, figures WB_RBGG figures like 254/257/256/256 are quite acceptable. Such results are sometimes referred to as Near UniWB.

10. Once you get a WB that results in an image with your (near) UniWB, it will be residing in your one-touch custom WB slot. If you have only one such slot, never let it go or you'll have to go back and re-establish it. Be sure to record your final image RGB figures so you can recreate the magenta target in the future if need be.

For OM-D owners, you might start with the magenta image that gave me final figures of 254/256/256/256. Specifically, these were a PS document with R = 197, B = 223, and G = 128.
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gollywop

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