Performances of the eye

Note:

It is highly advised to also consult the following articles:

* Diaphragm

* Performances of the eye

* Iris and diaphragm

* orthoscopic Distance

* Fuzzy, clearness and contrast

* Depth of hearth

* Depth of field

which is necessary to the comprehension of this last article.

Note: This article is initially resulting from that entitled Depth of field which was the subject of a cutting to form or supplement the articles mentioned above.

The human eye is at the same time an optical instrument and a sensor connected to the brain. It receives the light emitted or considered by the objects and transforms it into a " image" mental which constitutes what one calls " the vue".

The eyes are also the bodies + used them after the brain, except if the person is nonconspicuous or blind man or very badly conspicuous. One estimates at several hundreds of thousands the number of movements which the eyes in one day make, from where tiredness at the people who make use of it very often and or perhaps make use of it badly.

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Optical aspects

The objective

Compared with the modern objectives, the human eye is a rather poor optical instrument but it is nevertheless able, in certain connections, of rather astonishing performances.
Il is despite everything one of the astonishing results of the evolution (see the other shapes of the eye in the animal). As for its performances, they are also the proof of the successive adaptations to the human needs in their basic environment. His cultural and scientific needs having progressed, the man built the technical solutions to compensate for his limits in the fields which it invested: astronomy, microbiology, the examination of nature, and all the techniques which rose from it.

The light crosses the cornea, the crystalline lens, the vitreous humor for finally reaching the retina.

It is the presence of the crystalline lens, rather similar to a lens, which gives the resemblance to a photo objective. This crystalline lens with the single property to become deformed and thus to change curve. It is what allows " the accommodation" , kind of development allowing to see Net of the objects located at variable distances.

The sensor

The visual cells, which form the retina, approximately 500.000, are of two types: sticks particularly sensitive to the weak lights and cones which allow the discrimination of the colors. Contrary to films and the sensors which have the same properties on all their surface, the retina is populated densément in the center and less and less while going towards the edges, where one does not find any more but of rare sticks.

The central zone is occupied by the mackled or spot yellow , of approximately 2 mm ² and richly equipped in cones and sticks. The medium of the Macula itself is occupied by a small depression 0,4 mms in diameter, the fovea , where are only extremely tight cones. It is there that the vision is maximum, at least in good light. The night, for lack of sticks, the Fovea is blind and to perceive the star light low magnitude, it is necessary to look at a little at side, by using the cells of mackled.

The place of the retina, from which leave the nervous fibers towards the optical chiasma being deprived of sensitive cells, is called ''' spot plugs ''': the brain compensates for the lack of information not only by the stereoscopic vision of the two eyes but also by filling information of only one eye by a cerebral process. Certain visual experiments highlight this spot and the attitude of the brain ( the cat and the mouse ).

Vision

The fovea corresponds to an angle of vision of approximately 1,5° and it is on it which we bring instinctively the image of the point which we are fixing.

Far from the fovea, the vision decreases quickly until becoming very poor, but the eye compensates for this sufficiency by a very great mobility thanks to which it can explore the various parts of the subject which is in front of him.

The brain, whose retina is only one prolongation, memorizes the results and adapts to the various situations to constitute, with information which it receives, a “mental image” usable.

It is imagined easily that if the images of two points distinct from a subject are formed on the same visual cell, these two points will not be distinguished but on the contrary not perceived like a single point.

Research showed that a normal eye could separate from the distant points luminous angularly from approximately 1 ' of angle (one minute), which is equivalent to approximately 1/3000eme of radian. We will call e (epsilon) this minimal angle.

Let us retain the value of the angular resolving power maximum of the eye: e=1/3000 radian . At best, the eye separates from the details of 0,33 mm to 1 m, or of 3,3 mm to 10 m, or of 3,3 cm to 100 m, etc,

Physiological aspects

Still is needed, to reach this limit, that the points is very contrasted compared to the bottom (two close stars, for example), which is almost never the case for the objects of the daily newspaper. In addition, this resolving power decreases with tiredness and possible pathologies.

For the current applications, one will adopt e=1/1500 radian . This value, nearer to the conditions usual and generally sufficient, will be discussed further.

With the age, the crystalline lens loses its flexibility and, last 40 years, each one is afflicted with " presbytie" who prevents accommodation.

Binocular vision

Stereoscopy

It should not be forgotten that we have two eyes. That gives us a vision of the relief known under the name of stereoscopy.

Binocular correction and directing eye

The brain is able to correct the defects of vision of an eye by using the other.

It is noticed rather easily that when one looks in front of oneself, one does not see in the same way of the two eyes.

The experiment is easy to make: look at a remote object and place a finger raised at end of arm in your mink field (made the development on the object and not on your finger). You will distinguish two images from your finger, fuzzy. Close an eye, then close the other. After some tests, you will notice that when you close one of the eyes, the finger does not seem to move. When you close the other, you have the impression that the finger moves.

When the brain saw two images, it knew with certainty which there was one finger. It thus tried to give a priority to one of the two images on the other: the eye which provides this " meilleure" image is called directing eye . A droitier can have the left eye like directing eye, and vice versa . That can prove a little awkward to draw with rifle, or more still with the arc (because a droitier who has his directing eye on the left will have to draw with an arc for left-handed person; he will thus have less power in his left arm compared to a left-handed person).

Application to the photograph

With a little drive, and with the proviso of not using extreme focal distances, one can photograph and keep the two open eyes. It is particularly interesting with a reflex camera because the aiming disappears during the catch from sight.

The eye " libre" also allows to anticipate the photograph because he sees beyond the framework.

At the end of a few seconds, even if image of one of eyes is fuzzy (case of the myope without glasses which corrects the eye of aiming with the eyepiece of the camera), one ends up more not seeing but one image, with the Collimateur S of the objective superimposed.

This situation is enough tiring, it is not recommended to prolong it too a long time.

Application to the reading

According to a study, it would seem during a reading " normale" that the eyes sweep the letters of a word several times: initially the recognition of each letter by the eyes then each eye rebalaie the letters and the brain reconstitutes then the mot.

The study concerned of the English reading of English. But that it in other languages is, using the same characters (the characters used to write this page, that is called Latin characters?) or of the different characters (e.g.: characters Arab, Indian, Chinese, Japanese, Khmer and other languages of the world) or the different directions of reading (left-right-hand side, left right-hand side, haut-bas, bas-haut, circulars, various directions)? Studies probably in progress.

There thus exist various manners of reading, the manner (to write (characters) and thus) of reading and the manner of thinking (to process the data and to interpret them) are dependant. There is which " photographient" the words as a whole, it is enough for them just to sweep the page for reading. Others do not photograph the words, but read that certain words (verbs and nouns).

Each one has its or its rates/rhythms of reading, according to the form (if tired or not, concentrate or not), and or according to the object: a page of a novel (of the same letters cuts) is not read like a page of newspaper (quite apparent headlines compared to the texts to attract the eyes), or a magazine of charm (it does not have there to read, but to observe and appreciate: -))

See too

wikilivre of photography - chapter 14: Clearness of the photographic images