Product Review: The Human Eye (Four Stars)

The human eye is the most widely owned imaging device in the world, with over sixteen billion units in circulation, almost always sold in pairs. It comes pre-installed, cannot be returned, and has no manual. We spent a week with ours to see if it lives up to the hype.

The unit is roughly 24 millimetres across, about the size of a large grape, and is filled with a clear jelly that you cannot top up. The front lens, the cornea, does most of the actual focusing, which surprised us, since there is a separate part called “the lens” sitting just behind it. The lens mostly handles fine adjustments. It’s like buying a car and discovering the steering wheel is decorative and the real steering is done by the bonnet.

Inside, the retina carries around 120 million rod cells for low light and about 6 million cone cells for colour. On paper, that’s impressive. In practice, the cones are packed most tightly into a tiny patch at the centre called the fovea, and the really sharp part of your vision covers only about the area of your thumbnail held at arm’s length. Cones are spread across the rest of the retina too, just far more thinly, so the edges of your vision still see colour, only in much less detail. The periphery is built for spotting movement and seeing in dim light, not for reading. The device hides this by constantly flicking itself from point to point, several times a second, and stitching the results together. You are doing this right now. You didn’t notice. That’s the point.

Then there is the wiring. In a design choice we can only describe as bold, the light-sensing cells are at the back of the retina, facing away from the light, with all the nerves and blood vessels running across the front. Light has to pass through the cabling to reach the sensor. The cables then need to leave the eye somewhere, so they all gather and exit through a single hole, where there are no light-sensing cells at all. Every unit ships with a permanent blind spot in each eye. The manufacturer’s solution is to have the brain fill it in with a guess. It is a good guess. It is still a guess.

The picture arrives on the retina upside down and back to front, exactly like it does in a pinhole camera. The brain corrects for this in software, and it does so so smoothly that no owner has ever been bothered by it. We tried to find something to complain about here and failed.

Low-light performance is excellent, once it gets going. Switching from bright to dark conditions takes around twenty to thirty minutes to reach full sensitivity, as the rods slowly rebuild their supply of a light-sensitive pigment. This is why you walk into a cinema and see nothing, and why you shouldn’t check your phone while stargazing. One glance at the screen undoes much of it.

We were genuinely impressed by how the sensor works. Each rod is packed with a pigment called rhodopsin, and at its heart is a small, bent molecule called retinal, made from vitamin A. When a single particle of light hits it, the molecule snaps from its bent shape into a straight one, and that tiny change of shape sets off a chain reaction inside the cell, amplified many thousands of times, until a nerve signal goes out. A fully dark-adapted rod can respond to a single photon, the smallest amount of light there is. No camera at this price point comes close. Admittedly, the price point is zero.

Colour performance is strong. There are three kinds of cone, each most sensitive to a different range of wavelengths: short, medium and long, often loosely called blue, green and red. Every colour you have ever seen is your brain comparing how strongly those three types respond. That’s how three kinds of sensor produce around a million distinguishable shades, and it’s also why a phone screen can fake almost any colour using nothing but tiny red, green and blue dots.

Ageing is where the fine print in the warranty kicks in. The lens is soft and flexible in childhood, which lets the eye refocus from the horizon to the end of your nose in a fraction of a second. It stiffens slowly over the decades, and by their mid-forties most owners find themselves holding menus at arm’s length. The condition is called presbyopia. The manufacturer’s recommended fix is a pair of reading glasses, sold separately.

Our verdict: it has a blind spot, wiring on the wrong side, a sharp area the size of a thumbnail and a twenty-minute boot time for night vision. It is also self-cleaning, self-repairing in minor cases, focuses automatically from your nose to the horizon, and tells apart around a million shades of colour while running on a few drops of blood. It is the only camera that has ever been used to review itself. Four stars. We would have given five, but we couldn’t find where the fifth one went. It must have been in the blind spot.