Your eye is not a camera

Although sometimes credited to the Renaissance artists and engineers, the camera obscura, or pinhole camera was already used by the Chinese in the 4th century BC and the Arabs in the 10th century AD. If you have never seen one in action, you are missing out. The images have a vibrant dreamlike quality, especially when objects in the scene are moving.

Digital photo- and video-cameras are such a commodity these days that you have to force yourself to realize the magic. But whatever your assessment, I want to point out an unfortunate side effect of the immense success of this technology. It is now broadly taken as an explanatory metaphor for human vision.

Of course, in a technical sense the eye is a camera with a lens focusing the image on a photo sensitive surface (thank you, Johannes Kepler!). But the differences between eyes and cameras are more striking and, in my opinion, so important that the camera metaphor for vision is misleading. Let me give you three examples.

In order to take a proper photograph the camera needs to be held “still”. In contrast, eyes are almost continuously moving; the body is moving in the world, the head is moving relative to the body, and the eyes are darting around in the head. Even when you focus directly on one direction, the eye is making micro-saccades, tiny eye movements. “Still vision” dos not exist.

What about the field of view of a camera? Typical numbers are 40-50 degrees visual angle both in the horizontal and vertical orientation. For the human visual field the vertical visual angle is about 100 degrees, and horizontally about 180 degrees!

A proper photograph is usually “sharp” or at high resolution all over the image. The resolution of the human eye is highest in the centre and decreases about 1% per degree from the optical axis. We nevertheless perceive the world as entirely “sharp” even while we know we can not see every detail. You can not read the last sentence below while you are reading this sentence. Though if you attend to it without letting your gaze leave this sentence, it does not seem blurred or unsharp.

The way out of this conundrum is to consider the goals of vision versus photography. Images are produced to be looked at by human observers. Human vision itself is producing visual percepts in the mind that have entirely different characteristics. Photos and videos mimic the physical world. Your eyes are a part of your brain that turns light into conscious experiences in your mind. A camera has no mind. Yet.

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