Do these images look identical to you?
They are placed on a black and a white background, and if we compare them, we can rapidly notice a couple of things:
Image A looks brighter than image B.
Image A looks a bit washed out, it seems to have less contrast than image B.
Image A looks bigger than image B, by roughly 1/5.
In reality, they actually are the same size and even have the same black and white points. They’ve been treated identically, and they are ultimately the same image. Even after this revelation, the two images continue to appear to be different. This is even more pronounced if you try to compare them while staring at the small dot between them. Weird huh?
Here is another example. We see a seascape background rotated by 90 degrees with a black to white gradation. A horizontal band with a second gradation going the opposite direction has been laid on top. However, that second gradient only exists in our perception, as the whole band consist of an even 50% gray that has become altered by an optical illusion.
Can we really trust our eyes then? Where do these illusions come from?
This phenomenon called simultaneous contrast. We can quantify simultaneous contrast and "measure" its impact only through comparison, but we passively suffer its effects every day, whether we are aware of it or not. It comes from our unconscious processing. We don't make actual decisions, our minds always override our perceptions of contrast and color.
“We should consider color and contrast as merely events.” - Marco Olivotto
They are not things that we can objectively define but they always exist in relation to something else.
Three factors determine the perception of contrast and color:
The observer
The properties intrinsic to what is being observed
The light source illuminating the scene
Contrast and color are just “sensations” created by our minds due to light stimuli, just like sound and music are sensations caused by air vibration. Simultaneous contrast is not only relative to sight but also to all the other senses. Try dipping a hand in a bucket of cold water, after that, dipping it in a bucket with room temperature water will greatly accentuate its warmth.
Now that we are aware of simultaneous contrast, is there a way to control or counter these effects?
Softwares like Capture One and Photoshop use dark grey for their workspace areas by default, which is probably the best choice because it is less fatiguing for the eyes. However, these dark grey work spaces puts us in danger of beign tricked by simultaneous contrast illusions, especially if our images are going to be displayed in a completely different environment. Fortunately, both programs allow the user to change the workspace area color to a custom one by right clicking. If you know that your image will be displayed on a website with a white background, switch the area to white. The same goes for black or any other color.
Whenever possible, try to replicate the same viewing condition that the final observer will encounter. This will allow you to be aware of how your image will be perceived so you can compensate for any unwanted perceptual shifts.
I will likely dive deeper into explaining the different aspects of simultaneous contrast. In fact, as soon as color enters the equation we’ll see illusions that are even more interesting.
Sources:
The Theory of Colours, Johann W. V. Goethe
Interaction of Color, Josef Albers
Marco Olivotto, http:/marcoolivotto.com