Idaho Airships, Inc.

Suppose you are selecting a relatively homogeneous area of an image that is bounded by high contrast edges. You set your Magic Wand Tool to Sensitivity 3 and click near the perimeter of your intended Selection, but the result is that the Selection not only crosses the boundary but extends wildly into other portions of the image. You lower your Sensitivity to 1 and try again, with little difference in the result. What is the problem? Most likely, the problem is that your sample size is exceeding the area you expect it to, and influencing the Selection to be superfluous. If you click on your Eyedropper Tool, you are presented with a Drop Down Menu in your Options Bar (below your Menu Bar, upper left corner of the application interface). That Drop-Down Menu will contain the following (CS3) options:

• Point Sample
• 3 By 3 Average
• 5 By 5 Average
• 11 By 11 Average
• 31 By 31 Average
• 51 By51 Average
• 101 By 101 Average

These settings apply to your Magic Wand Tool, too. If you have a 31 By 31 Average sampling size selected and click only 15 pixels from the perimeter of your intended Selection area, you are going to include characteristics from the edge or perimeter feature in your sampling calculation. Please keep in mind that the Pixels you are calculating are Image Pixels (actual pixels in your Document), not Monitor Pixels, unless the two happen to coincide-for example, when you’re viewing a Document at 100% or Actual Pixels (View>Actual Pixels, or Ctl-+/Ctl– to Zoom at Photoshop’s preset Increments).

If you are using the Black Point, Gray Point, or White Point Eyedropper in Curves or Levels to correct for global bias, remember that they too are subject to the Magic Wand Tool setting. Incidentally, Double-Click on any of those Eyedropper icons in Curves or Levels. Were you aware that you could customize their target color?

The default setting for the Magic Wand Tool is to Sample only from the Active Layer, but “Use All Layers” may be selected in the Options Bar. If setting an appropriate sample doesn’t resolve all bizarre behavior by the Tool, check to ensure that “Use All Layers” is deselected.

Our example image is of a concrete pour at a new Boise State University parking structure on University Dr. at Lincoln, 6/16/07:

Sampling the red box (51×51 Pixels) yields a sample color of R103 G99 B94. Sampling the green box (5×5) yields a sample color R97 G98 B98, indicating the influence on the Sample by the blue concrete boom and green safety vests et. al. within the larger sample dimension, even peripherally. In this case, the result would be to constrict a Selection made with the sample since the relatively uniform concrete will not respond as well to R103 G99 B94 as it will to the R97 G98 B98 Selection criteria unless the Tolerance is coarsened.

Other important points to consider about the Photoshop Eyedropper Tool:

• The Eyedropper Tool samples by calculating the average value of each Channel’s luminosity and does not feather or reduce sensitivity toward the periphery.
• The Eyedropper Tool is available by pressing Ctl (PC) when the Brush or Pencil Tools are active.
• Navigating Edit>Preferences>Cursors and Selecting “Precise” for Other Cursors will provide a “Bullseye” cursor for sampling.
• Setting a judicious Tolerance in the Magic Want Tool Options Bar will do much to control Selections, but they are still primarily dependent upon the area you sample.
• Select>Grow (contiguous Pixels) and Select>Simliar (entire image) use the same mechanism as the Eyedropper Tool, and therefore the Magic Wand Tool.

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