In this video, let's learn how sharp human eyes are and what resolution is enough for cinema cameras and displays. This video is for filmmakers and cographers. If you're from another field, click away. It's going to get ugly like this chart. Why is an important organ like your eye tested on a chart with alphabets on it meant to be read at 20 ft? If you can read the last line, you have 20/20 vision. That's good enough for passing a driving test, but most healthy human eyes do much better than 20/20. 20/12 is quite common, and there are people who can read the Snellen chart at 40 ft. That's 20/8 vision. Guess how good a hawk's eyes are, supposedly 20/2 vision. We stand no chance. Now here's a great question: of all the letters on the Snellen chart, which is the most popular? It's the letter E. Why? Let's start with this cone. The size of the base of the cone depends on two things: the angle of the

cone and how long it is when you read the Snell and chart 20 ft away. The letter e subtends an angle of five Arc minutes. If you assume every stroke in the letter e is of the same size, you have five divisions. If you study basic geometry, you'll have heard of degrees for angles but not arc minutes. When degree is 60 arc minutes, we use ARC minutes for angles that are too small in degrees. The letter e has five lines, three black and two white. If a person with 20/20 Vision subtains an angle of five arc minutes, each line will be equal to one arc minute. So, drum roll, according to your traffic department, 20/20 Vision means your eye has a resolution of one arc minute. The closer you are to something, the smaller it can be. The further back you go, the larger it needs to be. The smallest detail a person can recognize is called resolution, but it depends on how close or far away you are. One black and one white together make a pair. Some people measure it in lines per

millimeter or line pairs per millimeter. You'll find this number on lens resolution discussions. Lens resolution is a lot more complex, so the numbers are not usually published. An Alexa LF has a resolution of 4448 by 396 pixels, with a sensor size of 36.7 x 2.54 mm. The pixels or lines per millimeter is 121. In terms of line pairs, it's 60 line pairs per millim. On the other hand, the Red Raptor X, this division needs lenses to resolve 100 line pairs per mm. You need sharper lenses on the Raptor to get 8K than you need with the Alexa LF's resolution. Sharpness then, no sharpness is a bit tricky to understand. Did you wonder why we use black and white lines for our measurements? Why not shades of gray? We chose the two colors that are the furthest apart from each other. We call this difference contrast. However, what if the object we're trying to figure out is the same color as the background? If they're both equally gray.

we won't be able to tell the difference even if our eyes are sharp. This means the boundaries need contrast too. Without contrast, we won't be able to distinguish one thing from another. By increasing the contrast of the edges, an image can appear sharper and more detailed. The resolution doesn't change, but it will appear to have more detail and hence sharper. The contrast of the edge, or Edge contrast, is called acutance. If the thickness of each line determines resolution, the contrast of the edge between two lines determines acutance. Some people like to call acutance local contrast as opposed to Global contrast. Global will contrast the difference between light and dark in an image. Local contrast, or acutance, or Edge contrast, is the contrast of the boundary between two details. The eye is drawn to high contrast, and if this happens to be in the edges, the mind is fooled into believing the image is sharper than it really is. Contrast, resolution, and acutance can be manipulated by sensor design, Optical low pass filters, raw demoing algorithms, lenses, etc. If you

zoom in close. Even the sharpest edge will become smooth at some point, kind of like zooming into a really sharp knife. If you zoom out far enough, even the smoothest of edges will appear sharp, like sugar candy. The human eye is more complex than any camera and possibly more complex than every camera put together. Light passes through a lens, through layers of gel-like material, and hits a small spot filled with rods and cones. The path the light takes to the brain is very complex and is nowhere as simple as how light strikes a sensor or film in a camera. How can the eye possibly see so well with such a convoluted design? I don't know, but it does. The black hole in the middle is the pupil. Why is it black? Because the eye sucks up all the light so efficiently inside that none reflects out. The colorful part, the one that comes in many shades, is the iris. It is what decides how big or small the pupil should be. When strong light enters the pupil, the iris shuts it down. If you really want to shut out light

completely. You have eyelids. When the available light isn't bright enough, the iris compensates by widening the pupil. The minimum pupil size is about 3 mm to 5 mm, and the maximum is about 9 mm. This varies greatly among humans. The lens is suspended in space somewhat and helps focus the light on the retina. Unlike camera lenses, the lens in the eye can change shape depending on how far the subject is. They call it accommodation. Why does a lens need to accommodate? Because otherwise, the entire eye needs to expand and contract like bellows on a camera to focus. We don't want that. Once light passes through the lens, it has to travel through a gel-like sea called the vitreous body or vitreous humor. 99% of it is water. The other 1% is the subject of a lifetime of study. The gel keeps the eye's shape. If we had air instead of gel, the eye could easily collapse like a ping pong ball. Light, after having passed through the vitreous body, hits the retina, which covers the inner lining of the eye.

retina comes from the word net. It catches light. The retina acts like the sensor in a camera. The retina collects the light and starts the chemical process of converting this into signals that our brain interprets as vision. It has one hole through which the optic fibers leave the eye to the brain. This hole is what causes the blind spot. The area of the spot is approximately 3 square mm. We usually don't have holes in camera sensors, do we? The most interesting aspect of the retina is its collection of photo receptor cells, the neurons that convert light into signals. There are different kinds of photo receptor cells, two of which are important to us: the rods and the cones. These are spread across the retina like cheese and toppings on a pizza. The entire retina contains about 7 million cones and 75 to 150 million rods. The question is, if there are 150 million or so photo receptor cells spread across the retina, then are there 150 million optical fibers going into the brain? No, they're not. There are only about 1 million.

Optical fibers. This means there must be a convergence or mixing of signals. Strange, but our retina compresses the image in an order of 100 to 1. The retina pretty much explains our vision: blurred at the edges and focused at the center. The fovea is the point on the retina that the eye focuses on. The concentration of cones increases as we move to the center. As we move to the periphery of the retina, the number of rods increases and cones decrease. The macula surrounds the fovea and is actually a yellow spot that acts like sunglasses for the fovea, shielding it from bright light. Rods look like rods, cones look like cones. Rods do well under low light and with black and white color. Cones do well under bright light. Cones are where the color in our vision comes from. Under bright light, our vision is the sharpest and the colors are brightest. At night, our vision isn't as sharp and the colors are muted and almost monochromatic. For any activity that needs a sampling of detail, the eye

uses the fobia, for example, reading, driving, etc. The phobia is also critical for color vision and motion detection. The phobia has the greatest resolution within the human eye. It only subtains about 2° of human vision, and even though it is only 1% of the retina, it has access to about 50% of the visual cortex. One funny fact is that you could expect the fobia to be located on the optical axis, in line with the center of the lens, but it's not. It's actually located about 4 to 8° to the side. Rod cells are sensitive to low light levels. A rod cell is sensitive enough to respond to a single photon of light and is about 100 times more sensitive than cones. Rods are indispensable for night vision and are most sensitive to wavelengths of light around 498 nm, which is around green-blue, the color of night for most of us. This causes the pori effect, which is the tendency for the human eye to shift towards the blue end of the color.

spectrum at low illumination levels. This is why we accept blue or green as the light for night automatically, while other colors seem wrong intuitively. For our purposes, we will stick to one of the widest known and simplest models of the eye, the mly reduced schematic 1952. The Emsley model simplifies the eye to a single lens model. It also disregards accommodation, rays from infinity, focus on the phobia, and therefore the focal length of the eye is 22.22% to 22.22%. Therefore, if we consider the diameter of the inner eye to be 22.22% of this area, since we see a little more horizontally than vertically, we can assume the mapped area of the retina to be an ellipse rather than a sphere. The average visual acuity of the human eye is one arc minute. The maximum is about 4 arc minutes. Therefore, we can safely say that the average resolution of a good eye is between 0.4 to 1 arc minute. Before these figures can be translated to pixels or displays, one

needs to realize that the size of the pixel would vary with distance. A very young child can focus at about 2 in, but the average adult can focus no closer than 4 in. So, how many of her eye pixels can fit into an inch at point 4 arc minutes? It's 2,190 PPI or pixels per inch. If a healthy adult brings a display or printed paper or whatever 4 in from his or her face, the maximum resolution he or she can see is 2190 PPI. It doesn't get any better than this for 99.99% of us, except maybe during pre-kindergarten years. But the legally accepted Norm of 2020 Vision only asks for 876 PPI at 4 in. If the average reading distance is 1T, the resolution required at one arc minute is 89 microns or about 300 DPI or PPI. This is why magazines are printed at 300 DPI. It's good enough for most people. Fine art printers aim for 720 PPI, and that's the best it needs to be. Very few

people stick their heads closer than 1T away from a painting or photograph. The average computer display viewing distance is about 2.5 ft. At 4 arc minutes, it's 300 PPI. At 1, it is 115 PPI. Now you can understand why most consumer computer monitors are between 100 and 300 PPI. What about TVs? Assuming the average viewing distance for television is 6 ft, 4 is about 120, and 1 is about 50 PPI. If your television gets smaller in size, then the higher PPI doesn't really help. A 55 in 4K TV gets you about 75 PPI, and an 8K 65 in TV gets you 130 PPI. That's the limit, even for the best eyes. The further you go from your 8K TV, and doesn't take a lot, the more it will look like a 4K TV. Cinema is different. The width of a cinema screen can vary from 30 to 70 ft. The closest viewing distance recommended is about 40 ft, about three

times the height. Those look like awfully small numbers, but that's all we need because the viewing distance is far, just like Billboards. But what if you're sitting in the front row at color grading facilities and QC facilities? The projection rooms are quite small. I was watching my film on these screens and could see the perforations in the screen. It really takes away from the experience. A 30 to 70 ft screen at 8K gives me from 9.75 to 22 2.8 PPI. Strangely, as the screen size gets bigger in cinema, something weird happens. To get the best possible resolution for a 70 ft screen, we'll need a horizontal resolution of 15 or 16k. This is about 128 megapixels. The problem is Cinema screens have been shrinking for decades. I don't see a trend for Cinema to get bigger, but that's probably what we need to revive the cinema business. Make the screens bigger, just like true IMax. IMax film is supposed to resolve between 10K to 12K.

remember the sharpest part is the fovea. The cone density in the fovea is about 350,000. You could translate that into 250 line pairs per millimeter or about 12,700 PPI. That's an incredible resolution at the center. However, we cannot extrapolate this to calculate the megapixel size of the eye because it's just a small part of the retina. We already know the eye has about 150 million rods and cones. That's 150 megapixels right there, but this is spread over an area of more than 1,000 square mm. A 35 mm sensor has an area of 864 mm. If we take this ratio, we're left with about 116 megapixels. If we consider the angle of view, the megapixel count can vary from 75 to 470 megapixels. Anyway, I think from a practical standpoint, 100 megapixels is about good enough, and it translates to roughly about 12K, which is good enough for large cinema screens like true IMAX and IMAX.

Dome theaters. It also is a good resolution to scale down to 8K for 8K TVs, for home systems, and any consumer-oriented display. Depending on our age, our eyes are really sharp or just mead. Most people and cinematographers past the age of 40 can see better than 4K anyway. I believe cameras and lenses should be better, and 12K is a good final resting place for the resolution battle. In the still photography world, we already have 100-megapix backs and lenses that supposedly are good enough to resolve this resolution, so we're almost there at 12K. And that's how many likes I need for this video. Wait, are you still watching? Don't tell me you're still titillating your rods and cones. Think about that while I figure out my next video.

Is the Resolution of the Eye better than 4K or 8K?

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