Why can't we see in low light if staring long enough?

Question

For me it seems reasonable that if I kept my gaze on a fixed point in a room with low light, a progressively brighter and better picture would appear before my eyes, just like a camera can see in the dark if the shutter speed is really slow, e.g. 4 seconds exposure. Why can't our brain do this trick as well (accumulate visual information over time)? Or is it a limitation of the eyes?

edit:

To further clarify what I'm after; I will show a concrete example from the world of photography (images taken from this website).

Here is an example where we have a series of underexposed images - this would be what the brain receives:

Now, combining all of them with a simple add-operation reveals one image that has normal exposure.

This seems like a simple trick for our powerful brain - surely it can add incoming signals?

Answer 1

For simplicity's sake, let's really reduce this to something like photography.

A camera's aperture can stay open indefinitely, allowing the plate (or whatever is receiving and recording light) to "collect and save the effect of photons" over time, if you want to phrase it that way. That allows a camera to make images that our eyes never can, for example, of "star trails".

The retina isn't like a photographic plate or a digital sensor's photosites (or pixels). It can't "collect and save" like a camera can. There is a "refresh rate", if you will, that disallows a collection and saving of light that doesn't apply to cameras, because cameras don't care if something in their vicinity is sneaking up on them and presenting a danger to their lives. Not being able to detect change rapidly is something that would be most inconvenient to survival.

It is the time sampling with long exposures that really makes the magic of digital astrophotography possible. A digital sensor's true power comes from its ability to integrate, or collect, photons over much longer time periods than the eye. This is why we can record details in long exposures that are invisible to the eye, even through a large telescope.

_{How Digital Cameras Work}

Answer 2

The simple answer is, that eye is not constructed such way.

The eye have much more "pixels" than "links" to the brain and sends in "preprocessed" image. Moreover the the eye is constantly moving and scanning the "area of vision" and the body and head are supposedly also moving (willingly or not - nobody can freeze totally) so longer accumulation of data would lead to big blur.

And the main purpose of eye is to spot danger - something changing, or moving victim - as we human are not nocturnal animals, we are constructed/optimized to work in mode active on light, passive and sleeping in dark. As there is real need for sleep anyway, there is not good reason to develope secondary system for night vision - meaning duplicate the main vision system completely with totally different mode of work (long time collecting data) which would be used only in very little split of time - when predator find us in night sleeping and we survive the first attack.

So only the main system was slightly modified with other kind of pixels more sensitive to light, but less to color, which allows us work to relatively last night and from really early morning when only split of light is accessible. At the price of color and details. But it is much cheaper, then mainly unused secondary system. And covers more time, than we usually use to move in.

Answer 3

The differences at the photoreceptor level have been addressed by others. The mechanical restrictions of the visual system were shortly hinted at by @gilhad et al., but deserve more attention in my opinion.

First off, in darkness we cannot focus on an object and our eyes will move. And even when we focus on a specific point there is always movement of the eyes due to due to tremor, drift and microsaccades. Microsaccades are involuntary small movements of the eye (Fig. 1) that have received quite some attention lately. It is estimated they occur 1 - 2 times per second and they can reach amplitudes of up to 1 degrees of field of view (Martinez-Conde et al., 2013) and last for about 15 ms (Cui et al., 2009). It is thought that these movements prevent adaptation at the retinal level, and prevent image fading. Hence, images on the retina are constantly mechanically refreshed. The brain in turn stabilizes the image by correcting the image at the perceptual level through oculomotor feedback (Martinez-Conde et al., 2013).

^{Fig. 1. Microsaccades recorded by an eye tracker. Source: Martinez-Conde et al. (2013)}

While a camera must be fixated on a tripod stand to allow for overexposure, our eyes cannot be fixated to the same extent, even when we try. Hence, combining exposures as indicated in the question is impossible and results in image blur. Instead, retinal images are constantly refreshed and when lighting conditions are too dim we cannot integrate photon input in the temporal domain.

Note, however, that photoreceptors do integrate photon input to some extent, given that higher luminance results in brighter perceptions. However, this operates only in the order of milliseconds and doesn't allow for long-term exposures as necessary to obtain images like the one shown in the great answer from @anongoodnurse.

<sub>References
- Cui et al., Vis Res (2009); 49(2): 228–36
- Martinez-Conde et al., Nature Reviews Neurosci (2013); 14: 83-96</sub>

Answer 4

There's probably a theoretical capacity to do so. The brain is amazingly good at signal processing, and could probably pull off such a summation. However, there is a limit. You have to hold very very still for it to work.

Go take one of the time lapse pictures, like anongoodnurse's answer posted. The shutter is open for quite some time (her picture looks like a 30 minute or 1 hour exposure to me). During that exposure, the camera holds perfectly still. All motion you see is motion due to the objects in the scene moving (or, if you prefer the technicality, the stars are holding still, and the camera is rotating... really really really smoothly).

The body does not have such an ability to lock itself down. Try taking one of those pictures while holding the camera in your hands, and you'll see its particularly difficult. Now consider that your eyes are even more twitchy than the rest of your body, capable of darting this way and that. We have good control over our eyes, but nothing close to what you need to create an effect similar to that of a tripod.

Thus, if you were to try to use your eyes in this way, almost all of what you would see is your own motion. Presumably a very well controlled individual might be able to sense that movement and account for it, but there's little reason for the brain to have that ability in "hardware."

Of course we can lock our eyes on to see with incredible accuracy, right? We can read words on an eye chart at 20 paces. Those activities are being done in a scene which permits visual feedback. If its too dark, we don't get enough visual feedback to see where our eyes are pointing and compensate.

Answer 5

What I believe you are referring to, is the phenomenon by which the camera adjusts light exposure by adjusting aperture. We can also do this, but it happens very fast. Go from a dark room to a brighter room and you will be blinded, but that effect soon subsides, and vice-versa.

The pupil opens up in a dark room and production of visual purple or Rhodopsin takes place in the Retina, a pigment responsible for visibility in low light. When you enter a bright area, the pupil contracts and Rhodopsin is photobleached, with production of Iodopsin taking place.

https://en.wikipedia.org/wiki/Adaptation_(eye)

^ Check out the Dark adaptaion and Light adaptation sections

(Sorry I don't have more sources, I had done this from my highschool bio textbook, and I can't find it)

Answer 6

Pretty much all answers which focus on movement of the eye causing blur (something a digital camera does not have to deal with) are wrong. The brain has absolutely no problem processing images in low-light at speed.

The answer is all to do with the fact that the eye is not a camera. Much of the old-school theories which where based on the fact that the eye works like a camera, such as persistence of vision, etc, have been proven demonstrably wrong. The eye does not have a shutter speed - information is constantly being sent back to the brain with no interval delay. (https://en.wikipedia.org/wiki/Persistence_of_vision)

This means that resolving blur/etc is done in the brain, not by the eye. Think of digital image stabilization that actually works, and works in real-time. However, the brain does appear to work on chunks of eye-input, at roughly 16-24 chunks a second. Why this speed? Well, liking the brain to a computer, it probably has something to do with the amount of memory the brain can store for unprocessed eye-data. Long-exposure photos require a lot of RAM to store the raw data, then a lot of time to compile it into a single image. The brain could do no doubt do the compilation from raw data to image, but it very likely cannot store more than 1/24th of a second's worth of data in "memory" before it has to compile.

More importantly, doing so would reduce our reaction time significantly. This is important because you do not want an organism that can see a branch clearly at night, but when they try to grab it, miss by 5-10 seconds.

Answer 7

I wish my computer as capable of sending an illustration of the arrangement of the human eye, versus the hypothetical "camera lens" idea, as organic eyes and camera optics are in NO WAY SIMILAR! Most of you have been in gross error by doing such, in your discussions.

The eye uses a combination of organic optical cells called "rods," and "cones," in order to manifest an image. In addition, there is a "dead spot" on the image perceived by an eye, owing to the insertion-point of the optical nerve. Any discussion of organic vision has to take these facts into account.

Organic eyes MUST go through a variable period of "dark-adaptation" in order to be able to perceive an image,as well. The minimum period is between 50-120 minutes; and even then, even an instant's exposure to "brighter" light will erase all of this adaptation, necessitating "restarting the clock" to dark-adapt the eye, again.

There's an anecdotal tale purporting that pirates had worn eye patches so that they achieved and kept dark-adaptation in one eye. There are many advantages to keeping one eye dark-adapted--- one is going from a brightly-lit deck, down into the very dim below-decks areas of a victim's ship. This is a case where being able to pull off the patch and being able to immediately see the enemy crewman coming in with a cutlass would be very valuable!

Another factor is that the distribution of rods and cones is not uniform, across the eye. Cones deal with colour-perception, and are concentrated at the centre of of the visual field. The concentration of cones rapidly declines, going outward.

Densities of rods in the same eye rapidly increases, going out from about five degrees from dead centre to a maximum of approximately 25 degrees from dead centre. Rods are responsible for our peripheral vision, our "sensitivity" to even seemingly-microscopic motion, AND OUR NIGHT VISION.

Because of the lack of rod-cells at the centre of our eyes' visual field, we are unable to see anything dead-ahead of us, under low-light conditions!

In order to be able to bring the maximum number of rod-cells to bear on an "item of interest" in our visual field, we have to use our peripheral vision and "cheat" off to one side of our visual field by about 25 degrees. This is like watching the front door in the centre of a building by looking "straight" at the middle of the left or right facade.

One would also be able to detect motion far easier than exact shape by looking in such a way. By continually "looking to one side," and by altering our location so as to change the background, it is entirely possible for an astute woodsman (a Native American or a Hillbilly, for example!) to be able to not only spot a raccoon in the top of an oak tree, but also to make out the form of the opossum looking up at him, from a lower limb!

Many animals far better able to operate at night have eyes that are not only better-equipped with rod-cells, but are in fact, much larger than ours! We would see as well as any owl, had only we been born with eyes the diameters of "jumbo" grapefruit!

Plus, watching and closely emulating the owl, with the head-bobbing, the weaving from side to side, and the peering-ahead, we would naturally improve the peripheral-sensitivity and the "looking to one side" conjuring of sharper images of those objects of interest to us!

I am sorry to be critical, as many comments showed great understanding of non-organic optics, as well as great imagination, but it is simply not possible to be able to blythly interchange the principles of non-organic optics and organic optics.