Stare at each image of the Mona Lisa for about 5-20 seconds. You will notice that she starts to change and move around. Pay special attention to her expression and hands.
The same picture seen at different distances (sizes) will be perceived differently because more or less of the image is being apprehended by our vision. You are able to take more in and see a smaller image as a "whole" rather than seeing a larger image that your eyes focus more on specific areas or details.
If you stare long enough it's possible that the entire image will fade to where you don't see it anymore - it just looks white. This phenomena has to do with our peripheral vision and how our eyes operate. Some things that we see out of the "corner of our eye" are seen differently when looked at directly. Stars are an example of this.
When your eyes look around the Mona Lisa - depending on the size/ distance you are viewing from (your perspective) will enable this peripheral phenomena. Meaning that if you are looking at her face or her eyes - her hands will appear to move - and visa versa. As your main area of focus moves, it causes the rest of the painting to seem to 'animate, or change.
This is something Leonardo knew about and implemented in his art, especially within the Mona Lisa. Combined w his sfumato technique (painting with imperceptible outlines -a gradient) the painting tricks your eyes as if it were actually an optical illusion. (it truly is) This is why it's difficult to agree on her facial expression or that her eyes seem to follow you. This was intentionally added by Leo as a 'special effect."
The background in the painting also adds to the effects. This will be explained further in my book, which should be available soon! In the mean time check out more on My site
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