You’ve probably noticed from looking at photos or movies that no photograph is absolutely, 100% pristine. Each one has a speckly, spotty texture — usually barely perceptible if the photographer’s done their job right — which is formed as a technical artifact of the film or image sensor.
For pictures or movies taken on film, it’s called film grain, and it’s determined by the physical structure of the photographic film. On a digital photo, it’s image noise, which is an often random pattern created by the circuitry of the camera’s sensor.
Grain usually has to be very, very extreme for our brains to immediately perceive it; at normal levels, we often don’t even notice it unless we’re looking closely. But our brains are generally quite skilled at perceiving small visual patterns — the pages of a closed book, the bumps of paint on a wall, etc. — so does the average case of grain or noise fail to register? Perhaps it’s because we’ve learned to ignore the noisy, grainy pattern that we’re constantly seeing all the time.
Yes, our eyes have a film grain of their own.
So is this grain caused by a physical texture in our eyes, like film grain, or by something in our circuitry, like image noise? A little of both, in fact.
Like a camera, the human eye has a lens in the front, which collects light and sends it into the photographic medium behind it. In a camera, it’s film or a digital image sensor; in an eye, it’s the retina. However, in between the lens and the retina is squishy gel called “vitreous humour“, and anything inside the vitreous humour gets in the way of the light passing through the eye. The blood vessels in the humour never change position, so our retina learns to disregard them soon after we’re born. However, there are always little bits of material that don’t stay still: floaters. These little deposits of protein and debris don’t stay in one place, so our retinas never figure out how to tune them out.
You’ve probably seen floaters in your eyes before; in most people they look like small, faint dots traveling down your field of vision from top to bottom. You can usually get a good look at them by rolling your eyes all over the place for a second and then staring at one spot for a while.
Floaters are only part of the patterns we see every day, though. The brain’s visual cortex, which interprets the information sent to it by the retina, isn’t 100% accurate. In fact, it’s constantly hallucinating.
Close your eyes, and then gently press on them both with your fists. After a few seconds, you should start to see a strange, kaleidoscopic pattern. This is a geometric visual hallucination, or form constant.
This pattern is generated by your visual cortex as a by-product of all the neural activity going on in and around it. It’s amplified by closing or pressing on your eyes, and even more by taking hallucinogenics (not that I’d know that from experience or anything), but it’s actually present all the time. Again, try staring at one spot for a while. It usually helps to stare at something very low saturation; mostly gray, white, or black. You’ll probably start to see a fainter version of that same geometric pattern you got from pressing on your eyes. If you close your eyes after staring for a while, you’ll still see it.
It’s interesting to know how much our vision has in common with the photos and videos we look at every day. Not only do we capture images the same way as cameras do, but the results are similarly imperfect and grainy.
Creative Commons-licensed image from Wikimedia Commons, modified by me.
Do Human Eyes Have “Film Grain”?
You’ve probably noticed from looking at photos or movies that no photograph is absolutely, 100% pristine. Each one has a speckly, spotty texture — usually barely perceptible if the photographer’s done their job right — which is formed as a technical artifact of the film or image sensor.
For pictures or movies taken on film, it’s called film grain, and it’s determined by the physical structure of the photographic film. On a digital photo, it’s image noise, which is an often random pattern created by the circuitry of the camera’s sensor.
Grain usually has to be very, very extreme for our brains to immediately perceive it; at normal levels, we often don’t even notice it unless we’re looking closely. But our brains are generally quite skilled at perceiving small visual patterns — the pages of a closed book, the bumps of paint on a wall, etc. — so does the average case of grain or noise fail to register? Perhaps it’s because we’ve learned to ignore the noisy, grainy pattern that we’re constantly seeing all the time.
Yes, our eyes have a film grain of their own.
So is this grain caused by a physical texture in our eyes, like film grain, or by something in our circuitry, like image noise? A little of both, in fact.
Like a camera, the human eye has a lens in the front, which collects light and sends it into the photographic medium behind it. In a camera, it’s film or a digital image sensor; in an eye, it’s the retina. However, in between the lens and the retina is squishy gel called “vitreous humour“, and anything inside the vitreous humour gets in the way of the light passing through the eye. The blood vessels in the humour never change position, so our retina learns to disregard them soon after we’re born. However, there are always little bits of material that don’t stay still: floaters. These little deposits of protein and debris don’t stay in one place, so our retinas never figure out how to tune them out.
You’ve probably seen floaters in your eyes before; in most people they look like small, faint dots traveling down your field of vision from top to bottom. You can usually get a good look at them by rolling your eyes all over the place for a second and then staring at one spot for a while.
Floaters are only part of the patterns we see every day, though. The brain’s visual cortex, which interprets the information sent to it by the retina, isn’t 100% accurate. In fact, it’s constantly hallucinating.
Close your eyes, and then gently press on them both with your fists. After a few seconds, you should start to see a strange, kaleidoscopic pattern. This is a geometric visual hallucination, or form constant.
This pattern is generated by your visual cortex as a by-product of all the neural activity going on in and around it. It’s amplified by closing or pressing on your eyes, and even more by taking hallucinogenics (not that I’d know that from experience or anything), but it’s actually present all the time. Again, try staring at one spot for a while. It usually helps to stare at something very low saturation; mostly gray, white, or black. You’ll probably start to see a fainter version of that same geometric pattern you got from pressing on your eyes. If you close your eyes after staring for a while, you’ll still see it.
It’s interesting to know how much our vision has in common with the photos and videos we look at every day. Not only do we capture images the same way as cameras do, but the results are similarly imperfect and grainy.
Creative Commons-licensed image from Wikimedia Commons, modified by me.