A recent post – Shapes & Contrast, 2 – displayed this image
and asked which of the three ovals was brightest. The surprising answer was that all three are the same – pure white. The post suggested a way to verify this fact.
Here’s another example of the same visual perception problem. The question is which of the three smaller squares is the darkest?
After the three ovals you’re probably tempted to say – they’re all the same (regardless of what your eyes say). You’d be right. It’s another example of perceived tonal values being affected by the tonal values of the surrounding area. This can be verified in the same way as suggested previously for the ovals.
There’s a second “illusion” present as well for the visually acute. Look at the large square in the center. Doesn’t its left side appear to be lighter that the right side? Same phenomena.
For both color and tone there can be quite a difference between true and apparent values.
Although we haven’t said much about image editing thus far, and won’t until finishing the 4-C’s, these illusions can cause problems for you when editing your images. You might either misjudge your “white point” dealing with images such as the top one, or you might be tempted, incorrectly, to lighten/darken areas when their tonal values are correct and it’s your eyes that are wrong (bottom image). When in doubt use an image editor’s color picker (or equivalent) to measure the respective RGB values before leaping to conclusions.