question: Why is depth perception messed up in low light?
comment: The sensors in the eye are not spread out uniformly, the eye is not a camera. The ones that feed into depth perception best, the cones, are denser in the center of the field of view. The ones that see in low light best, the rods, are denser around the edge.
passages: BULLET::::- Shadow Stereopsis : Antonio Medina Puerta demonstrated that retinal images with no parallax disparity but with different shadows are fused stereoscopically, imparting depth perception to the imaged scene. He named the phenomenon "shadow stereopsis". Shadows are therefore an important, stereoscopic cue for depth perception.
   BULLET::::- Lighting and shading : The way that light falls on an object and reflects off its surfaces, and the shadows that are cast by objects provide an effective cue for the brain to determine the shape of objects and their position in space.
   BULLET::::- Texture gradient – Suppose you are standing on a gravel road. The gravel near you can be clearly seen in terms of shape, size and colour. As your vision shifts towards the more distant part of the road it becomes progressively less easy to distinguish the texture.
  
  Recent advances in computational machine learning now allow monocular depth for an entire scene to be algorithmically estimated from a single digital image by implicitly using one or more of these cues
  
  Section::::Balance.
   BULLET::::- Defocus blur : Selective image blurring is very commonly used in photographic and video for establishing the impression of depth. This can act as a monocular cue even when all other cues are removed. It may contribute to the depth perception in natural retinal images, because the depth of focus of the human eye is limited. In addition, there are several depth estimation algorithms based on defocus and blurring. Some jumping spiders are known to use image defocus to judge depth.
   For adaption to occur at very low levels, the human eye needs to have a large sample of light across the signal in order to get a reliable image. This leads to the human eye being unable to resolve high spatial frequencies in low light since the observer is spatially averaging the light signal.
   The depth that the water appears to be when viewed from above is known as the "apparent depth". This is an important consideration for spearfishing from the surface because it will make the target fish appear to be in a different place, and the fisher must aim lower to catch the fish. Conversely, an object above the water has a higher "apparent height" when viewed from below the water. The opposite correction must be made by an archer fish.
   Such different structures are found in the polychaete "Torrea candida". Its eyes have a main and two accessory retinae. The accessory retinae sense UV-light ("λ" = 400 nm) and the main retina senses blue-green light ("λ" = 560 nm). If the light sensed from all retinae is compared, the depth can be estimated, and so for "Torrea candida" such a ratio-chromatic depth gauge has been proposed.
   BULLET::::- Phytoplankton loss rate is independent of depth and of growth rate.
  
  BULLET::::- Nutrients in the mixed layer are high enough that production is not nutrient limited
  
  BULLET::::- Daily photosynthetic production of the phytoplankton community at any depth is proportional to the mean daily light energy at that depth.
  
  In other words, light is assumed to be the only factor that limits the growth of phytoplankton during pre-bloom months and the light a phytoplankton community is subject to is determined by the incident irradiance and the coefficient of light extinction.
  
  Section::::Critical Depth Hypothesis.:Mechanism.
   Section::::Interaction of stereopsis with other depth cues.
   BULLET::::- Accommodation : This is an oculomotor cue for depth perception. When we try to focus on far away objects, the ciliary muscles stretch the eye lens, making it thinner, and hence changing the focal length. The kinesthetic sensations of the contracting and relaxing ciliary muscles (intraocular muscles) is sent to the visual cortex where it is used for interpreting distance/depth. Accommodation is only effective for distances less than 2 meters.
   When visibility is poor, as at night during rainstorms or fog, the eye tends to relax and focus on its best distance, technically known as "empty field" or "dark focus". This distance is usually just under one meter (one yard), but varies considerably among people. The tendency is aggravated by objects close to the eye, drawing focus closer.
   Mesopic vision occurs in intermediate lighting conditions (luminance level 10 to 10 cd/m) and is effectively a combination of scotopic and photopic vision. This gives inaccurate visual acuity and color discrimination.
  
  In normal light (luminance level 10 to 10 cd/m), the vision of cone cells dominates and is photopic vision. There is good visual acuity (VA) and color discrimination.
  
  In scientific literature, one occasionally encounters the term "scotopic lux" which corresponds to photopic lux, but uses instead the scotopic visibility weighting function.
  
  Section::::Wavelength sensitivity.
   Scotopic vision
  
  Scotopic vision is the vision of the eye under low-light levels. The term comes from Greek "skotos", meaning "darkness", and "-opia", meaning "a condition of sight". In the human eye, cone cells are nonfunctional in low visible light. Scotopic vision is produced exclusively through rod cells, which are most sensitive to wavelengths of around 498 nm (green–blue) and are insensitive to wavelengths longer than about 640 nm (reddish orange). This condition is called the Purkinje effect.
  
  Section::::Amacrine cells.
   Certain lensed lighting instruments (e.g. ellipsoidal reflector spotlights) have a good deal of "throw" and do not lose much intensity as distance increases. These light sources tend to be more effective at large distances than soft light sources. At large distances, an effective soft light source would have to be very large. The (mostly) parallel rays of such instruments tends to cast hard shadows, unlike soft light sources.
  
  Section::::Softness/hardness of various light sources.
   Light and color can influence how people perceive the area around them. Different light sources affect how the colors of walls and other objects are seen. Specific hues of colors seen under natural sunlight may vary when seen under the light from an incandescent (tungsten) light-bulb: lighter colors may appear to be more orange or "brownish" and darker colors may appear even darker. Light and the color of an object can affect how one perceives its positioning. If light or shadow, or the color of the object, masks an object's true contour (outline of a figure) it can appear to be shaped differently from reality. Objects under a uniform light-source will promote better impression of three-dimensional shape. The color of an object may affect whether or not it seems to be in motion. In particular, the trajectories of objects under a light source whose intensity varies with space are more difficult to determine than identical objects under a uniform light source. This could possibly be interpreted as interference between motion and color perception, both of which are more difficult under variable lighting.
   BULLET::::- Elevation : When an object is visible relative to the horizon, we tend to perceive objects which are closer to the horizon as being farther away from us, and objects which are farther from the horizon as being closer to us. In addition, if an object moves from a position close the horizon to a position higher or lower than the horizon, it will appear to move closer to the viewer.
  
  Section::::Binocular cues.
  
  Binocular cues provide depth information when viewing a scene with both eyes.
   At the extreme, a plenoptic camera captures 4D light field information about a scene, so the focus and depth of field can be altered after the photo is taken.
  
  Section::::Diffraction and DOF.
   With Olivo Barbieri's method, which also tricks the eye into seeing a real landscape as miniature via the shift-tilt method, certain objects in the frame betray the illusion. For example: a telephone pole extending vertically from the bottom to the top of frame will appear soft focus at the bottom, sharp focus in the middle and return to soft focus at the top. In reality, because the telephone pole is within a single plane in 3D space, the degree of focus should remain constant from top to bottom - this is what Smallgantics attempts to accurately simulate.
  
  Section::::Process.
   Rodinal increases perceived edge sharpness dramatically: in the case of standing development ( no agitation for long developing time, an hour or more) the developer is used up more quickly in dark than in light areas, development of light areas next to dark areas (the edges of the image) is reduced, increasing contrast at edges. Very dilute Rodinal (1 part to 100 or more) is often used to maximise this effect.
   Gloom
  
  Gloom is a low level of light which is so dim that there are physiological and psychological effects. Human vision at this level becomes monochrome and has lessened clarity.
  
  Section::::Optical and psychological effects.
  
  Light conditions may be considered gloomy when the level of light in an environment is too low for the proper function of cone cells, and colour vision is lost. In a study by Rothwell and Campbell, light levels described as "gloomy" fell between 28 and 3.6 cd/m^-2.
   Section::::Mood.
   Parallax scanning
  
  Parallax scanning depth enhancing imaging methods rely on discrete parallax differences between depth planes in a scene. The differences are caused by a parallax scan. When properly balanced (tuned) and displayed, the discrete parallax differences are perceived by the brain as depth.
   As with other depth cues, the kinetic depth effect is almost always produced in combination with other effects, most notably the motion parallax effect. For instance, the rotating circles illusion and the rotating dots visualization (which is similar in principle to the projected wireframe demonstration mentioned above) rely strongly on the previous knowledge that objects (or parts thereof) further from the observer appear to move more slowly than those that are closer.
   In addition, recent studies also proved that tungsten spectral energy influences the convergence which can also show an impact on reading and near visual tasks because of its brightness and miosis. Prolonged reading and working under this lighting can cause convergence disorders and visual fatigue
  
  Section::::Types.:Divergence.
   The smallest detectable visual angle produced by a single fine dark line against a uniformly illuminated background is also much less than foveal cone size or regular visual acuity. In this case, under optimal conditions, the limit is about 0.5 arc seconds or only about 2% of the diameter of a foveal cone. This produces a contrast of about 1% with the illumination of surrounding cones. The mechanism of detection is the ability to detect such small differences in contrast or illumination, and does not depend on the angular width of the bar, which cannot be discerned. Thus as the line gets finer, it appears to get fainter but not thinner.
  