Objective great angle

An objective great angle is an objective with short focal. The effect of this short focal distance is to allow a broad framing of brought closer objects which one cannot move away. For example, it can be used to as a whole photograph a landscape in the form of a Panorama. One can also choose to use these objectives to accentuate the prospects.

The effect of prospect for a great angle tends to make appear various plans of the same image more distant from/to each other than actually, contrary to the teleobjectives which tend rather to " resserrer" subjects in a single plan.

This phenomenon of incidence the prospect is not due to the focal distance of the intrinsic objective of manner but by the distance from use to the object photographed (see also, will infra: " The defects optiques" : distortion).

There is no precise definition of the minimal focal distance from which one must speak about great angle , but one generally leaves the principle that in this category all the objectives are included which have an angle of vision broader than the natural perception of the human eye.

It is considered that the standard focal distance corresponds to the diagonal of the image on film or the sensor. For a support of the type film 35 mm (format of image 24x36 mm), this diagonal make 43 Misters In practice, the objectives " standard" cases reflex camera have focal distances of 50 mm, whereas with a compact case, the focal distance of 35 mm is more frequent. An objective great angle will have a shorter focal distance.

Semi great angle with the super great angle

For a manufacturer of objectives, one 35 mm is already a great angle, and one 20 mm a hyper great angle. It is always commercially interesting to use superlatives. Let us examine the possibilities of the various focal distances available for a support of the type film 35 mm (format of image 24x36 mm):

the 35 mm should be regarded only as one semi great angle. One away meter, it makes it possible to make return an object of one meter on the largest edge of the image. For the photographs containing of the characters in edge of image, the sometimes awkward effects of prospects with objectives great angle or very great angle are almost invisible with one 35 Misters

the 28 mm was the focal distance great angle most current used in the Seventies. It is also the shortest focal distance of the majority of the current Zoom S transtandards. This focal distance is already interesting for landscapes or effects of prospects. The 28 mm and the 35 mm are also regarded as focal " classiques" for the photo-report.

the 24 mm can be regarded as a truth great angle. One away meter, it makes it possible to make return an object of 1,5 m on the largest edge of the image, (1 m on the small edge). It is the shortest focal distance covered by zooms transtandards (for the format of image 24x36 mm). This focal distance is interesting for the landscapes and the photographs of high buildings. It is the minimal focal distance usable to avoid a caricatural effect on the people who would be in periphery of the image (see further).

the 20 or 21 mm (according to the marks) was the shortest focal distance available in the Seventies. The zooms great angle of beginning of the year 90 were of the 20-35 Misters With one 21 mm, the coverage in the diagonal is of 90°. These objectives start to deserve to be described as " very large angle".

the 18 or 17 mm deserves to be called angle very great even super great angle. One away meter, one 18 mm makes it possible to make return an object of 2 m on the largest edge of the image and the coverage on this edge is of 90°. The angle is of 100° in the diagonal, 104° for one 17 Misters These focal distances are interesting for the overall plans of landscapes, the photographs of raised buildings, and especially to accentuate the prospects. They do not agree to photograph in a realistic way of the groups of people. In the middle of the Nineties, the 18-35 mm Sigma was the zooms great angle having the shortest focal distance. Since, various manufacturers marketed of the 17-35 mm, but 17 mm are the shortest focal distance which they propose on a Zoom covering the format of image 24x36 Misters

the 14 mm is the fixed focal distance shortest available in several marks. These objectives deserve to be called angle super great even of hyper great angle, or great extreme angle. One away meter, they make it possible to tally an object of 2,5 X 1,7 Mr. the field angle is of 104° on the large edge of the photograph and 114° in the diagonal.

the 12 mm : Only one model of objective exists with this focal distance, the zoom 12-24 mm of Sigma. One away meter, the coverage is of 2 m out of 3! It is for the moment the focal distance great angle shortest available in the format 24x36 and it is not impossible that it remains it.

Great angles and numerical cases

The advent of the apparatuses digital reflexes gave birth to a problem in the use of the objectives. For reasons of manufacturing costs, the sensors of the majority of the digital reflexes are smaller than the impressionable surface of the film of a reflex camera 24x36. They thus recover only the central part of the image obtained with the silver case . In the majority of the marks, it is necessary to apply a multiplying coefficient of 1,5 to find the focal distance of an objective which would offer same framing in 24x36. Only Canon and Nikon sells cases " large capteur" with more than 2500 € (autumn 2007), but the multiplying coefficient is of 1,6 on the remainder from the Canon range and 1,5 for Nikon. At Sigma, they is 1,7. The smallest sensors of the digital reflexes are those of Olympus with a multiplying coefficient of 2 and one image with format 4/3.

To cure this problem, the manufacturers conceived objectives of shorter focal distances only intended for the apparatuses with " small capteurs". Whereas the zooms transtandard 28-70 mm or 28-105 mm are current in format 24x36, the basic transtandards for numerical cases are of the 18-50 mm or 18-70mm. In the same way, for the zooms great angle, the 10-20 mm, 11-18 mm, 10-22 mm, even more modestly 12-24 mm, plays the part of the 17-35 mm in the format 24x36. At Olympus whose sensors are smaller, the transtandards are from the 14-42 to 14-50 mm, and the zoom great angle is one 7-14 Misters.

Calculation of the field angle

The field angle covered by an objective can be calculated by means of the mathematical function arc tangent according to the formula:
angle = 2 X arctan (0,5 L/F), where L is the length of an edge of image or its diagonal, and F the focal distance of the objective.

For a support of the type film 35 mm (format of image 24x36 mm), the field angle are thus equal to:

2 X arctan (18/F) for the large edge of the image,
2 X arctan (12/F) for the small edge of the image,
2 X arctan (21,6/F) for the diagonal of the image.

Great angle and fish-eye

The objectives great angle evoked in this page provide an orthoscopic prospect. I.e. an objective without defect, used to photograph a plane object, like a geographical map, will provide a not deformed image of it if one took the precaution to put the camera (and thus film or the sensor) quite parallel with the chart. Under these conditions, one 12 mm does not offer (for the format 24x36) only one field angle of 122°. To cover a field angle of 180°, would be needed a null focal distance!

There however exist objectives making it possible to photograph on 180°. These objectives are called Fish-eye S and provide a circular prospect, i.e. the image which they record without deformation is that of the interior of a sphere. With these objectives, the images of plane objects are strongly deformed, the straight lines being curved towards outside. N the other hand, the majority of the fish-eyes offer an angle of sight of 180° in the diagonal of the image, and approximately 150 ° on the large edge, whereas others provide a circular image covering 180° in all the directions.

The focal distance used for the fisheye is generally of 15-16mm. It can go down to 8 even 6 Misters.

The fisheye 6mm Nikon as for him can reach 220° of diagonal, it is extremely rare and weighs several kilos. It is very difficult to find even on the Web of the images carried out starting from this objective.

Optical construction of a great angle

The majority of the objectives great angle are built according to the model of the " Teleobjective inversé" or " Rétrofocus ". If the objective consisted of a single lens, its Focale would be the distance between the center of this lens and the plan of film or the sensor making it possible to give a clear image of a distant object. However, on a camera reflex camera, there is a distance of approximately 4 cm between the plan of film or the sensor and the ring of fixing of the objective. For objective of 35 mm or less, it would be necessary thus that lenses return inside case, which is incompatible with the presence of the tilted mirror which is used for the aiming, then swivels quickly to the top at the moment of the photograph.

The principle of construction of the assembly rétrofocus is that of the application of the Formule of Gullstrand. It in fact consists in placing a optical Lentille divergent before and a convergent back, contrary to the assembly " Teleobjective " , used for the objectives of Long focal distance.

In the mark Voigtländer an objective super great angle (12 mm) built starting from a strongly convergent group of lenses exists. But it is intended for a compact case 24 X 36, which makes it possible the lenses to penetrate inside the case.

On the contrary, the only solution for a case reflex camera is to place the block of lenses of short focal distance more far from the film plan or of the sensor which it would owe the being to provide a clear image. Under such conditions, the objective will provide a clear image only for brought closer or very brought closer objects. To correct the clearness of the objects moved away, one will add one or more divergent lenses in front of the objective.

To some extent, an objective great angle can be regarded as a short-sighted objective at which one puts glasses.

Optical defects of the great angles

The objectives great angle from their nature, are the optics most prone to the aberrations of sphericity (inter alia, difficulty to have a clear image at the same time in the center and on the edges of the image). But it is probably the optical defect which the originators of objectives great angle seek to correct in first.

The very dissymmetrical construction (convergent lenses with the back, divergent with before) of an objective great angle of the rétrofocus type supports the appearance of a distortion out of barrel: the central part of the image being less reduced than the peripheral part, the straight lines of the subject photographed appear curved towards outside. To correct this defect, the divergent lenses of an objective great angle are divergent meniscuses: seen outside of the objective, they are bent, sometimes very strongly. They could thus make think of a convergent lens. But they are even more curved towards the interior on the inner face than they are it on the external face.

The use of aspheric lenses in the objectives great angle also makes it possible to reduce the aberrations of sphericity and the distortion.

The objectives very great angle (21 mm and in on this side in the format 24 X 36) give very typified images in which the characters located in periphery of the image appear lengthened towards the outside of the photograph, whereas in the center, the proportions width/height are normal. Of an optical defect of the objectives great angle it is not a question there, but of the consequence of rules of Géométrie. While being very close to the subject, the objective great angle will photograph of face the objects located at the center of the photograph, but of profile those (in relief) located in periphery. But when the photograph is looked at, under an angle more closed than with the catch of sight, one will expect that all the objects of the photograph were photographed of face. As it is not the case, one will obtain deformations sometimes surprising.

Another defect running with the objectives great angle is the Vignetage (edges of the image darker than the center). Construction Rétrofocus of the objectives great angle limits this defect partly. On the other hand, on the 12 mm Voigtländer, the ornamentation with vignettes is so important that it is equipped with a gray filter degraded to obscure the center of the image. But this great angle not rétrofocus displays on the other hand a very low distorsion.

Finally, the Chromatic aberration is the only optical defect to which the objectives great angle are less prone than the other categories of objectives.

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