GreySquareIllusion_graphic

Here’s another comprehensive example — ripped-straight-from-the-workshops — in our on-going, occasional series featuring Photography’s Dirty Little Secrets  …

This ripped-straight-from-the-workshop lesson comes from our Digital Black & White Zone System: Yosemite Spring & Yosemite Fall courses. It’s one of those dirty little secrets that, once we explain and show it, you hear a lot of “Holy Cow! I never realized that!”

Is this a trick? An optical illusion? Square B is obviously lighter than Square A. It couldn’t be more obvious, right?

What if I told you that it it is true? Square B is the SAME as Square A!

Seriously. They are the same. No joke. No trickery. Not a play on words. Both squares are the exact same shade of gray. And not only can I prove it .. I’ll explain HOW, and WHY it’s oh-so-important as photographers that you can learn and identify when this happens!

First off, here’s the proof …

THE PROOF

There are three ways you can prove to yourself that the two squares are the same shade of gray. Trust me, I did it all three ways because I didn’t believe it either! And even though each method proves it scientifically — not necessitating any other tests! — our eyes and mind tell us that it really can’t be so … so we do the next test.

#1: The first way is really the easiest. Just color sample the two squares with either a digicolor meter or using the info tool in Photoshop or Lightroom.

GraySquareIllusion_Proof1

As you can see in the above image, both squares come up with the same RGB value of 120. “But … but … that’s impossible!”

#2: If both squares truly were the same shade of gray, then two gray bars of that value (created from a color spec of R: 120, G: 120, B:120) placed alongside both squares at the same time … would reveal that there is no difference between the shade of gray and the bars.

GraySquareIllusion_Proof2

“OK, so that shows they are same, but it still doesn’t look the same!”

#3: The end-all proof to show that they are the same. Simply take the image into Photoshop/Lightroom/Your-Image-Editor and cut Square B out and overlay it onto Square A. You already know what’s going to happen …

Spoiler Alert! Spoiler Alert! Spoiler Alert!
The squares will be the same. The animated gif below illustrates this.

GraySquareIllusion_Proof3

I don’t think we can argue anymore that the two squares aren’t the same shade of gray. But why do we see it differently? Even right now, when we KNOW that they are the same, we still see them as two separate shades of gray!

THE EXPLANATION

The best person to explain this is the man who created this illusion in the first place, Edward Adelson.

The visual system needs to determine the color of objects in the world. In this case the problem is to determine the gray shade of the checks on the floor. Just measuring the light coming from a surface (the luminance) is not enough: a cast shadow will dim a surface, so that a white surface in shadow may be reflecting less light than a black surface in full light. The visual system uses several tricks to determine where the shadows are and how to compensate for them, in order to determine the shade of gray “paint” that belongs to the surface.

The first trick is based on local contrast. In shadow or not, a check that is lighter than its neighboring checks is probably lighter than average, and vice versa. In the figure, the light check in shadow is surrounded by darker checks. Thus, even though the check is physically dark, it is light when compared to its neighbors. The dark checks outside the shadow, conversely, are surrounded by lighter checks, so they look dark by comparison.

A second trick is based on the fact that shadows often have soft edges, while paint boundaries (like the checks) often have sharp edges. The visual system tends to ignore gradual changes in light level, so that it can determine the color of the surfaces without being misled by shadows. In this figure, the shadow looks like a shadow, both because it is fuzzy and because the shadow casting object is visible.

The “paintness” of the checks is aided by the form of the “X-junctions” formed by 4 abutting checks. This type of junction is usually a signal that all the edges should be interpreted as changes in surface color rather than in terms of shadows or lighting.

As with many so-called illusions, this effect really demonstrates the success rather than the failure of the visual system. The visual system is not very good at being a physical light meter, but that is not its purpose. The important task is to break the image information down into meaningful components, and thereby perceive the nature of the objects in view.

THE REASON THIS MATTERS

If you’ve been skimming over this article, stop here and start reading!!!

In a nutshell, it’s obvious we can’t trust our eyes. But as Adelson pointed out above, that’s not what are eyes are supposed to do. They aren’t a physical light meter.

Which is exactly what this is all about. Why your light meter is so important.

But here’s the kicker. Your matrix meter or whatever other fancy meter setting your camera manufacture calls the process will NOT see this difference either. It’s just averaging. It’s giving the best guesstimate — and sometimes it’s right — for how the scene should render.

This next part is straight from our Digital B&W Zone System: Yosemite Spring and Yosemite Fall courses …

When our eyes try to see what a value of gray might be for the final photograph, we’re just guessing. And the matrix meter is doing the same. But the spot meter doesn’t. It says it like it is! It reads the value of the tone based on what is 18% gray. (To understand how a handheld light meter or the in-camera spot meter works, read our “Is a Handheld Lightmeter Better than the In-Camera Meter” post from Nov. 2012.)

When you are shooting for a end-product that is B&W, you NEED to know where your tones are going to fall on the gray scale. You need to be able to read the tones your eyes see and determine exactly how they are going to render in your final B&W image.

In the workshop, we go over the extensive process of learning to read not only the B&W scene but also in reading and evaluating tones. Key to this is knowing that the surrounding values (the surrounding light squares and dark shadow, in the above example) can “contaminate” and trick your mind in how you read a tone. To properly execute and maximize the Zone System, you need to be able to identify these tricky scenes.

And to pour salt into this wound of visual weakness, since our eyes can’t see in B&W, we have to “guess” what those color tones in the scene will do when viewed and valued in B&W. So not only can this optical illusion cause us to stumble in a grayscale scene, it is exponentially made more difficult when reading and evaluating a color seen for the B&W digital zone system!

But as our Zone System course alumni can attest to, it is not all guesswork. The B&W Digital Zone system is a very methodical process in evaluation through previsualization … that works! Just ask Edward Smith, from last year’s Digital B&W Zone System: Yosemite Spring course. His photo below of the Merced River shows perfect execution of tonal rendition in B&W, with the subtle nuances each color value was evaluated and process for a stunning and visually dynamic full tonal range image.

EdwardSmith_YosemiteB&WCourse

To see more of Edward’s photos from the workshop, you can view his gallery here on SlickPic.

To learn more about our zone system courses, click each course option here: Digital B&W Zone System: Yosemite Spring and Digital B&W Zone System: Yosemite Fall