The human eye focuses by flexing the lens, changing its focal length. When we switch from looking at a near object to a far object, our lens flexes, moving the focal length such that the near object is out of focus and the far object is in focus. Why, when presented with a blurry photo or video, can we not do the same thing?
[Physics] Why can’t the human eye focus to make blurry photos/video clear
biologyopticsperceptionvision
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You are looking at the outer surface of the eye through the glasses. The eye forms its image on the retina.
If you took a picture of your mother wearing glasses that showed her retina, you could adjust the camera to see the retina in focus. Light from a point on the sensor of your camera would go through both the camera lens, the glasses, and her eye's lens to form a point on her retina. That means light from a point from her retina would follow the same rays backwards and form a point on your sensor.
On the other hand, when you wear her glasses, they cause rays from the camera sensor to come to a focus in front of or behind your retina. Light from that point could follow the rays backward to a point on the sensor, but a point on the retina would be out of focus.
The glasses themselves are usually weak lenses. Usually they have a focal length of a meter or so. Compare that to eye lenses which have (or should have) a focal length of the eye's diameter. Likewise, camera lenses have short focal lengths.
When you focus the camera on a face, light from a point on the face comes to a point on the camera sensor. Light from slightly farther or closer are almost points. The camera uses a small aperture, which blocks rays except those that travel pretty close to a straight line. These rays come to an almost point. This gives the camera some depth of field. A ranges of distances near the true focal plane are close enough to in focus that they look clear.
You can see the effect of an aperture by taking off your glasses and seeing the blurry world. Look through a pinhole and the focus improves. When you get an eye exam, this is why the opthalmologist puts drops in your eye to dilate it. He maximizes your eye's aperture so he can learn about all your eyes defects, not just the central portion.
The weak glass lenses change the angles of rays slightly, as if that portion of your face was a little closer or farther away. But not so much that it is noticeably out of focus, given the camera's depth of field. The eye's lenses is much stronger. The retina would be well out of focus.
Brief summary: the difference between a normal eye and a myopic eye is in how they bend light coming from a point source that is very far away ("at infinity") when the muscles that change the shape of the lens are in their completely relaxed state. A myopic eye bends light from a point source that is very far away "too much" so that the rays meet in front of the retina rather than exactly at the retina.
Part 1: What it means for an eye to be focused on an object
Consider the following picture:
The eye is focusing on the green object, because the lens is flexed in just the right way that all rays emanating from the bottom of the object converge to a single point on the retina, and all rays from the top of the object converge to another single point on the retina. This must be true for all points of the object, that all rays from that point are bent by the lens to reach a unique single point on the retina. In this way, each point on the retina corresponds to a unique point in the (two-dimensional) space in front of the lens so that the brain can interpret "these molecules at this point on the retina fire" as "there is a point source of this color at this point in space".
Part 2: What it means for an eye to be unfocused
Consider the following picture, which corresponds to a too-relaxed eye:
The rays emanating from a point source hit the lens are bent in such a way that they meet behind the retina. According to the diagram, we can see that multiple points on the retina are hit by the different rays, and hence the brain is forced to interpret this object as being smeared out in space, because each point the retina is supposed to correspond to a different point the visual field in front of the lens. Furthermore, according to this diagram,
we can see that there can be multiple point sources out there in the field of vision whose light rays converge on the same point on the retina, and effectively the brain must smear these two points into one.
If instead the muscles in the lens are too flexed, leading to the rays being bent too much, the following occurs:
We can see that light rays from the object that are farther away are bent too much, so that they meet at a point in front of the retina, leading to many points on the retina being activated by these multiple rays and hence the image of the point source begin smeared out on the retina, meaning that the brain must again interpret this light as coming from different points in the visual field. In contrast, an object that is nearer can be in focus, as shown in the diagram: because these rays are coming in at a wider angle, the lens needs to bend them more in order for them to meet correctly at the retina.
Part 3: Focusing on an object that is "far away"
Consider the following diagram.
Light rays coming in from a point source "very far" away come in essentially parallel to each other, hit the lens and are bent so that they meet at the retina. This is what happens in a "normal" eye in the completely relaxed state; that is, the muscles that change the shape of the lens are completely relaxed. Any flexing of these muscles causes the lens to bulge out, and hence the light hitting the lens is bent more when the muscles are not completely relaxed. A point of terminology: the farthest point away on which an eye can focus is called the far point of the eye. For "normal" eyes, this point is essentially infinitely far away (at least to the extent that we can treat the light rays coming from this point as coming in parallel to each other).
Part 4: Myopia
A myopic eye is one that has a far point which is closer than infinity. That is, there some distance $d_{\textrm{far}}$ from the lens beyond which the lens can't create an image in the retina. Thinking mechanically, for an eye that is myopic, the rays that come from far away are bent too much by the lens in the completely relaxed state. There is point a maximal distance away for which the light rays emanating from this point are bent by the lens (in it's completely relaxed state) to meet at the retina. This point is called the far point. The issue is then illustrated by one of the previous pictures, included again here:
Suppose the point labeled "In Focus" is the far point of the lens. Then any light rays coming in "shallower" than those coming from the far point will bend in such a way that they meet in front of the retina; for instance, the light rays coming from the point labeled "Out of Focus" will meet in front of the retina as shown (and we've already explained why this corresponds to the point source being "out of focus").
Part 5: Correcting myopia
I think it helps the understanding of nearsightedness to explain how we use corrective lenses (spectacles or contact lenses) to correct it. Consider the following (busy) diagram:
The purpose of a corrective lens (labeled "lens axis") is to make it so that the eye in its completely relaxed state can focus the rays coming from a point source far away on a single point on the retina. To do this, the lens is designed in such a way that it takes rays coming in parallel and bends them so that they *appear as if they're coming from the far point". Let's explain this using the diagram.
The point labeled FP is the far point of the eye.
The blue dashed line is what occurs without the corrective lens in place: In the completely relaxed state, the lens of the eye will bend light rays coming from this point in such a way that they meet at the retina (Remember, this is the definition of the far point.)
The red dashed line is what occurs without the corrective lens in place: In the completely relaxed state, light rays coming from far away (solid black) are bent at the lens of the eye so that they meet in front of the retina. (Remember, this is what it means to by myopic and is related to the definition of the far point.)
The solid black line from left all the way to the retina is what occurs when the corrective lens is in place: In front of the eye, we place a corrective lens that is designed to take light rays that are coming in parallel and bend them so that they lie along the lines coming directly from the far point. These rays are naturally bent by the eye lens (in its relaxed state) in such a way that they meet at the retina. Thus, the light rays from a point source at "infinity" follow the solid black line in the diagram, meeting at a single point on the retina, and hence the brain is able to interpret that as light coming from a single point in its visual field!
Best Answer
The human eye focusing is resolving all the possible detail it can from a scene that is sharp and not distorted. The details of exactly how your brain forms an image from what your eye does is extremely complex, but the basics are : sharp initial image can be focused on to produce sharp image.
The blurry photo cannot be sharply resolved in that way because (surprise) the data is simply not there to make it focused. In effect the "sharpest image" of a blurry scene is a blurry image. It's a faithful rendering of the scene.
The process for (trying) to deblur a blurry image is called convolution. This is not the same or the reverse of focusing on a sharp scene to produce a sharp image. They work differently. A lens like the eye cannot do that complex operation. Even that deblurring process (which you can see done in many photos nowadays by software) is not 100% and it effectively makes "educated guesses" about what the scene originally looked like – guesses guided by good math and physics but still guesses (I suppose "estimate" would be a fairer word).