question: Why is something blurry if it’s only a couple of cm’s away from my eyes?
comment: Because the lens in your eye can't change shape in such a way that makes focusing on such a close object possible. It just hasn't evolved to do it.
passages: Only one plane can be in precise focus. Objects not in the plane of focus are blurred; the blurring increases with distance from the plane of focus. But blurring less than a certain amount is imperceptible under normal viewing conditions; objects for which blurring is imperceptible are within the "depth of field"
  
  (DoF).
  
  DoF decreases as magnification increases;
   When a camera lens is focused to project an object some distance away onto the film or detector, the objects that are closer in distance, relative to the distant object, are also "approximately" in focus. The range of distances that are nearly in focus is called the depth of field. Depth of field generally increases with decreasing aperture diameter (increasing f-number). The unfocused blur outside the depth of field is sometimes used for artistic effect in photography. The subjective appearance of this blur is known as bokeh.
   BULLET::::- The sharpness gradients are at an angle to the line of sight. When the plane of focus is almost perpendicular to the image plane, the sharpness gradients are almost perpendicular to the line of sight.
  
  BULLET::::- When the plane of focus is at a substantial angle to the image plane, objects at the same distance from the camera are rendered with unequal sharpness, depending on their positions in the scene.
  
  BULLET::::- Objects at greatly different distances from the camera are rendered sharp if they are within the DoF wedge.
   Section::::Classification.
  
  An eye that has no refractive error when viewing distant objects is said to have "emmetropia" or be "emmetropic" meaning the eye is in a state in which it can focus parallel rays of light (light from distant objects) on the retina, without using any accommodation. A distant object in this case is defined as an object located beyond 6 meters, or 20 feet, from the eye, since the light from those objects arrives as essentially parallel rays when considering the limitations of human perception.
   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.
   BULLET::::- The DoF extends between two parallel planes on either side of the plane of focus; the DoF is finite in depth but infinite in height and width.
  
  BULLET::::- The sharpness gradients on each side of the DoF are along the line of sight.
  
  BULLET::::- Objects at the same distance from the camera are rendered equally sharp.
  
  BULLET::::- Objects at significantly different distances from the camera are rendered with unequal sharpness.
  
  In a photograph using tilt:
   An image, or image point or region, is in focus if light from object points is converged almost as much as possible in the image, and out of focus if light is not well converged. The border between these is sometimes defined using a "circle of confusion" criterion.
  
  A principal focus or focal point is a special focus:
   Section::::Mechanism.
   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.
   While optical resolution, as commonly used with reference to camera systems, describes only the number of pixels in an image, and hence the potential to show fine detail, the transfer function describes the ability of adjacent pixels to change from black to white in response to patterns of varying spatial frequency, and hence the actual capability to show fine detail, whether with full or reduced contrast. An image reproduced with an optical transfer function that 'rolls off' at high spatial frequencies will appear 'blurred' in everyday language.
   BULLET::::- eye relief: For optics such as binoculars or a rifle scope, eye relief is the distance from the eyepiece to the viewer's eye that matches the eyepiece exit pupil to the eye's entrance pupil. Short eye relief requires the observer to press their eye close to the eyepiece in order to see an un-vignetted image. For a shooter, eye relief is an important safety consideration. An optic with too short an eye relief can cut skin at the contact point between the optic and the shooter's eyebrow due to recoil.
   Section::::Focus on near objects.
  
  The refractive index of the eye’s lens system allows the eye to produce sharply focused images. For example, geometrical optics show that as a distant object is brought closer to the eye, the focus of the object becomes blurrier in the plane behind the retina; however, as a result of the increase in the refractive power of the eye, this image becomes clear. The refractive power mainly resides in the cornea, but the overall refractive power is achieved by the actual lens changing its shape.
   A near object (for example, a computer screen) appears large in the field of vision, and the eye receives light from wide angles. When moving focus from a distant to a near object, the eyes converge. The ciliary muscle constricts making the lens thicker, shortening its focal length. The pupil constricts in order to prevent strongly diverging light rays hitting the periphery of the cornea and the lens from entering the eye and creating a blurred image.
  
  Section::::Pathway.
   Many motion blur factors have existed for a long time in film and video (e.g. slow camera shutter speed). The emergence of digital video, and HDTV display technologies, introduced many additional factors that now contribute to motion blur. The following factors are generally the primary or secondary causes of perceived motion blur in video. In many cases, multiple factors can occur at the same time within the entire chain, from the original media or broadcast, all the way to the receiver end.
  
  BULLET::::- Pixel response time on LCD displays (motion blur caused by slow pixel response time)
   1. For describing the largest blur spot that is indistinguishable from a point. A lens can precisely focus objects at only one distance; objects at other distances are "defocused". Defocused object points are imaged as "blur spots" rather than points; the greater the distance an object is from the plane of focus, the greater the size of the blur spot. Such a blur spot has the same shape as the lens aperture, but for simplicity, is usually treated as if it were circular. In practice, objects at considerably different distances from the camera can still appear sharp (Ray 2000, 50); the range of object distances over which objects appear sharp is the depth of field (“DoF”). The common criterion for “acceptable sharpness” in the final image (e.g., print, projection screen, or electronic display) is that the blur spot be indistinguishable from a point." 
   Because the effect is caused by the relative motion between the camera, and the objects and scene, motion blur may be avoided by panning the camera to track those moving objects. In this case, even with long exposure times, the objects will appear sharper, and the background more blurred.
  
  Section::::Applications of motion blur.:Animation.
  
  In computer animation this effect must be simulated as a virtual camera actually does capture a discrete moment in time. This simulated motion blur is typically applied when either the camera or objects in the scene move rapidly.
   The blur circle, of diameter "C", in the focused object plane at distance "S", is an unfocused virtual image of the object at distance "S" as shown in the diagram. It depends only on these distances and the aperture diameter "A", via similar triangles, independent of the lens focal length:
  
  The circle of confusion in the image plane is obtained by multiplying by magnification "m":
  
  where the magnification "m" is given by the ratio of focus distances:
  
  Using the lens equation we can solve for the auxiliary variable "f":
  
  which yields
   BULLET::::- The DoF extends between two planes on either side of the plane of focus that intersect at a point beneath the lens (see Depth of field in the Scheimpflug principle article for an illustration).
  
  BULLET::::- The DoF is wedge shaped, with the apex of the wedge near the camera, and the height of the wedge increasing with distance from the camera.
  
  BULLET::::- When the plane of focus is at a substantial angle to the image plane, the DoF can be small in height but infinite in width and depth.
   BULLET::::- Diffraction
  
  BULLET::::- In a real imaging environment, the depths of objects comparing to the aperture are usually not enough to lead to serious diffraction.
  
  BULLET::::- However, a long enough depth of the object can truly blurs the image.
  
  BULLET::::- Effective Focus
  
  BULLET::::- Small aperture, small blurring radius, few wave information.
  
  BULLET::::- Loses details in comparing to a large aperture.
  
  In conclusion, diffraction explains a micro behavior whereas DOF shows a macro behavior. Both of them are related to aperture size.
  
  Section::::Linear canonical transform.
   If the camera lens is focused at or beyond its hyperfocal distance, then the depth of field becomes large, covering everything from half the hyperfocal distance to infinity. This effect is used to make "focus free" or fixed-focus cameras.
  
  Section::::Motion blur.
  
  Motion blur is caused when either the camera or the subject moves during the exposure. This causes a distinctive streaky appearance to the moving object or the entire picture (in the case of camera shake).
   The term "" is sometimes heard (by analogy with critical focus) for "obtaining maximal optical resolution", as limited by the sensor/film and lens, and in practice means minimizing camera shake – using a tripod or alternative support, mirror lock-up, a cable release or timer, image stabilizing lenses – and optimal aperture for the lens and scene, usually 2–3 stops down from wide-open (more for deeper scenes: balances off diffraction blur with defocus blur or lens limits at wide-open).
  
  Section::::See also.
  
  BULLET::::- Contrast (vision)
  
  BULLET::::- Cornsweet illusion
  
  BULLET::::- Edge enhancement
  
  BULLET::::- Mach bands
  
  BULLET::::- Ringing artifact
  
  Section::::Further reading.
   BULLET::::- The Micro Four Thirds system cameras and lenses perform automatic distortion correction using correction parameters that are stored in each lens's firmware, and are applied automatically by the camera and raw converter software. The optics of most of these lenses feature substantially more distortion than their counterparts in systems that don't offer such automatic corrections, but the software-corrected final images show noticeably less distortion than competing designs.
  
  Section::::Software correction.:Manual.
  
  Manual systems allow manual adjustment of distortion parameters:
   BULLET::::- Lower camera shutter speeds common in Hollywood production films (blur in the content of the film), and common in miniaturized camera sensors that require more light.
  
  BULLET::::- Blur from eye tracking fast-moving objects on sample-and-hold LCD, plasma, or microdisplay.
  
  BULLET::::- Resolution resampling (blur due to resizing image to fit the native resolution of the HDTV); not a motion blur.
  
  BULLET::::- Deinterlacing by the display, and processing by studios. These processes can soften images, and/or introduce motion-speed irregularities.
  
  BULLET::::- Compression artifacts, present in digital video streams, can contribute additional blur during fast motion.
   A primary aspect in regards to the human visual system is blur perception. It plays a vital role in ocular focusing in order for one to attain clarity central to retinal-imagery.
  
  Visual perception is a complex system in which blur perception plays a key role in focusing on near or far objects. Retinal focus patterns are critical in blur perception. These patterns are composed of distal and proximal retinal defocus. Depending on the object’s distance and motion from the individual viewing it, these patterns contain a balance and an imbalance of focus in both directions.
   The blur disk diameter formula_9 of a detail at distance formula_10 from the subject can be expressed as a function of the subject magnification formula_11, focal length formula_12, f-number formula_13, or alternatively the aperture formula_14, according to
  
  The minus sign applies to a foreground object, and the plus sign applies to a background object.
  
  The blur increases with the distance from the subject; when formula_9 is less than the circule of confusion, the detail is within the depth of field.
  
  Section::::Notes and references.
  
  Section::::Notes and references.:Sources.
  
  BULLET::::- Available as GIF images on the Large Format page.
  
  Section::::Further reading.
  