Zoom - SpeedyLook encyclopedia

Etymology

zoom

the verb to zoom appears in English at the end of the 19th century, creation in form of Onomatopée to express a buzz, a humming. Then it was applied to the Avion S of the beginning of the century which carried out piqués and candles, from there it took a picturesque direction of “fast displacement”.
About 1935, it is employed to express a fast displacement of a broad plan towards a close-up using an objective zoom lens . It is only towards 1974 that the simple substantive zoom indicating this objective is employed.
Passed in French, the word is employed towards the end of the year 1950 like the verb zoomer, to modify framing by changing the focal distance of the zoom (often to increase the central zone of the image). Now, the zooming term is also used like a male common noun in the French language, whereas it is a verb conjugated with the English participle.
In 1958 a mark of objectives French, Angénieux, deposits the name Zoom . Since, the word zoom indicates an objective with variable focal distance génériquement.

Sources: TLFi

Designation of the characteristics of a zoom

Whereas for an objective with fixed focal distance, one specifies the sometimes followed focal distance opening, for example: 14 mm f/2,8 or 50 mm f/1,4 or 600 mm f/5,6, for a zoom, one specify the range of variation of focal distance of smallest to largest in the form of 2 numbers separated by an indent. One can make follow this indication of the opening of the zoom to the extreme focal distances according to a close syntax.

For example: 28-70 mm f/2,8 for a zoom whose full opening is constant, 70-300 mm f/4-5,6 for a zoom of full opening f/4 to 70 mm and f/5,6 to 300 mm, etc

The numerical relationship between longest and the shortest focal distance are called “amplitude” or “power” of the zoom, sometimes one uses also the English term “arranges”.

For example, a zoom 35-105 mm is known as “zoom 3×” or of amplitude 3.

Various categories of zooms

As for the fixed focal distances where one distinguishes the objectives great angle, the standard focal distances and the teleobjectives, it is current to classify the zooms according to the focal distances which they cover.

This classification depends on the positioning of the extreme focal distances of the zoom compared to the standard focal distance corresponding to the diagonal of the image. The standard focal distance is theoretically of 43 mm for the images 24x36 mm, but one often takes 50 mm, approximately 30 mm for the numerical sensors of cameras reflex camera, even less in cinema.

Standard zooms

The shortest focal distance of these zooms is lower than the standard focal distance, whereas the longest focal distance exceeds it sometimes much.

A zoom transtandard will thus cover in the worst case the focal distances active of the semi-large angle to the small teleobjective: the 35-70 mm was current with beginning of the year 90. But now one will go from the great angle to the more or less powerful teleobjective.

Currently for the format 24 X 36, the ranges of focal distances of the most current zooms transtandards are:

28-70 mm, 28-85 mm, 28-105 mm, 28-135 mm, 28-200 mm, even 28-300 Misters
24-70 mm, 24-85 mm, 24-135 mm, 24-200 mm.

but there is also of the 35-105 Misters.

The zooms transtandards is the most used zooms, with the increase in their amplitude, it is often the only objective which many photo hobbyists have. The zooms transtandards of strong amplitude are the subject of optical compromises. Their quality is often perfectible with the extreme focal distances, the problem being above all to limit the price of them.

There exist all the same zooms transtandards top-of-the-range (and thus expensive) like the 35-350 mm of Canon or to a lesser extent the 24-120 mm of Nikon. The zooms luminous transtandards: 28-70 mm f/2,8 and 24-70 mm f/2,8 are them also of better quality but more expensive than the 28-200 mm, 28-300 mm and 24-135 mm of the same marks, but their amplitude remains limited.

Appearance of numerical cameras with " small capteur" led the manufacturers to propose adapted zooms transtandards. The standard focal distance of these apparatuses being of approximately 30 mm, the transtandards proposed are of the 18-50 mm, 18-70 mm, 18-105 mm, 18-200 mm and since autumn 2006,18-250 Misters.

There still, the zooms luminous transtandards are of low amplitude: 17-50 and 18-50 mm f/2,8, 17-70 mm f/2,8-4,5.

To Olympus which uses sensors even smaller, the transtandards go from 14-42 mm to 14-56 Misters.

Zooms great angle

It is zooms for which the longest focal distance remains lower or equal to the standard focal distance.

They are zooms exceeding very seldom amplitude 2 and offers it manufacturers evolved/moved only very slowly.

If the 25-50 mm Tokina of beginning of the year 80 were a zoom great angle (nearer to one 25-45 mm on the level of the real focal distances), with beginning of the year 90, the 20-35 mm was what was done best for the short focal distance.

In the middle of the Nineties, Sigma was the first manufacturer to propose a zoom of lower focal distance with one 18-35 Misters Depuis, much of manufacturers proposed of the 17-35 mm, but Sigma remains recordman of the world in the short focal distances with its 15-30 mm and 12-24 mm (covering both the format 24 X 36 mm).

For the numerical cameras with " small capteur" , there exist some zooms great angle: 11-18 mm Tamron, 10-20 mm Sigma, 10-22 mm Canon, but only 12-24 mm at Nikon. For Olympus which uses smaller sensors, the zoom great angle is one 7-14 mm (equivalent with one 14-28 mm in the format 24x36).

Whatever the manufacturer, these zooms great angle are regarded as top-of-the-range material and are thus more expensive than the zooms transtandards.

Télézooms

The shortest focal distance of these zooms is higher or equal to the standard focal distance.

In the Eighties, the télézooms of 80-200 mm were among the marketed first. Since, several manufacturers propose télézooms of 70-300 mm f/4-5,6 (or 75-300 mm), which often constitute the 2nd zoom bought by photo hobbyists in complement of a zoom transtandard of amplitude 2,5 to 4.

The ranges of focal distances covered by the télézooms are varied and different from one manufacturer to another. One finds in particular:

of the luminous télézooms but of low amplitude: 70-200 mm f/2,8 and 120-300 mm f/2,8
many télézooms of amplitude 3: 100-300 mm, 135-400 mm, 170-500 mm, 200-600 mm
some télézooms equipped with an optical stabilizer: 80-400 mm, 100-400 mm
and of the more extreme télézooms: 50-500 mm, 300-800 mm

but this list is far from being exhaustive.

For the numerical cameras with " small capteur" , the offer as regards télézooms until the summer 2006 remained limited to 55-200 mm (or 40-150 mm at Olympus) which offers the same possibilities of framing that of the 80-300 mm in the format 24 X 36. In any event, to conceive télézooms intended to function only on cameras numerical with small sensor, whereas one proposes already a range of télézooms which also functions in 24x36, has only one limited interest.

The difference in aspect of the same object photographed with an Objective great angle according to whether it is in the center or the periphery of the image, would be awkward for animated objects. Also, in the field of the Video and the Cinema, the objectives used are télézooms. Their focal distance starts with the standard focal distance and of strong amplitudes about 12x are frequent.

Mechanical engineering of a zoom

The majority of the zooms have 2 rotary setting rings. The ring with the back of the objective makes it possible to regulate the focal distance. The ring located at before makes it possible to regulate (or to improve if one uses the Autofocus) the distance from development. These zooms are qualified zooms " 2 bagues".

There exist also zooms monobagues. The rotation of the ring makes it possible to regulate or to improve the distance from development, whereas to modify the focal distance, it is necessary to make slide the ring of before behind. Most of the time, by pushing the ring forwards, one will lengthen the focal distance of the objective. The zooms monobagues are also called zooms " with pompe". It is especially about télézooms, even if in the Eighties, the transtandard Vivitar 35-80 mm f/2,8 were a pump zoom.

Optical construction of a zoom

The zoom is technically an objective comprising several groups of lenses, of which some are mobile: what makes it possible to vary some the focal Distance between two extreme values.

If one makes a parallel with the explanations given in this encyclopedia about the optical construction of the objectives Grand angle and the Téléobjectif S, one can deduce that if a strongly convergent light unit or strongly diverge moves of before behind, in such a way that in an extreme position the block before objective is overall divergent and the block postpones overall convergent, whereas in the other extreme position the block before objective is convergent and the divergent back block, one will have in the first case a great angle, and in 2nd teleobjective.

In practice, it is also necessary to preserve the clearness of the image during the variation of focal distance. So several groups of lenses can move in a different way, during a change of focal distance.

The objective can see its overall length evolving/moving during the variation of focal distance. In the case of the télézooms, the objective is lengthened at the same time as its focal distance. Some of these télézooms have a button of locking to prevent this lengthening when the objective is transported frontal lens downwards.

This extension with the increase in the focal distance is the general rule. But there exist objectives transtandards for which the frontal lens advances at the same time in position great angle and position teleobjective, the minimal length being reached for an intermediate focal distance. Other objectives do not change dimensions when the focal distance evolves/moves, only the internal lenses move. For others finally, it is by shortening the focal distance that the frontal light unit will advance.

Optical defects of the zooms

One suspects it, a zoom is at least prone to the same optical defects as the fixed focal distances that it replaces.

The distortion

It is a defect which all the zooms because of dissymmetries on the level of the provision of the lenses have.

A zoom transtandard usually displays a strong distorsion out of barrel to its shorter focal distance. This distortion quickly decreases when the focal distance of a few millimetres is increased, then, after being last by a focal distance without distortion, one obtains a distortion while growing which, according to the objective can remain moderate or become definitely visible with the longest focal distance.

A télézoom of moderate amplitude (3x), can also have one to him (light) distortion out of barrel with its shorter focal distance, but will have a distortion while growing with the higher focal distances. In general, the distortion remains moderate on a télézoom. On certain models it is unperceivable in the majority of the cases. The télézooms of great amplitude have proportionally more distortion.

A zoom great angle displays a distorsion out of barrel to its shorter focal distance. This distortion decreases regularly with the increase in the focal distance, but it can still remain a light distortion (in barrilet or growing) with the longest focal distance. In theory, in fact the zooms great angle should display the most important distorsions. In practice, if the manufacturer chooses to correct this defect well, in particular by means of aspheric lenses, one can reach great extreme angles with a very moderate distortion. The 12-24 mm of Sigma does not have more distortion than of good more banals 35-70 Misters.

The Ornamentation with vignettes

It is especially present at full opening in position great angle, which one uses a zoom great angle or a zoom transtandard. By increasing the focal distance of a zoom, the ornamentation with vignettes becomes more moderate even unperceivable. The télézooms practically do not have ornamentation with vignettes.

To note that the luminous objectives, which have a constant opening of f/2,8 have less ornamentation with vignettes with the shortest focal distance than of the equivalent objectives but of more modest luminosity. That comes owing to the fact that, to keep a constant opening to all the focal distances, the Diaphragme closes in position great angle, thus reducing the ornamentation with vignettes to the focal distance where it would be most important.

Advantages and disadvantages of the zooms

Luminosity

If luminous objectives are sought, an opening of f/1,8 can be found with fixed focal distances for all the focal distances from 20 to 85 Misters For the 50 mm, the opening of f/1,4 is current. In comparison, the zooms luminous transtandards open only with f/2,8 and thus receive 2,5 times less light than the equivalent fixed focal distances, or even 4 times less than one opening objective with f/1,4.

Between a teleobjective of 200 mm (opening easily with f/4) and the focal distance 200 mm of a zoom transtandard 28-200 or 24-200, or 18-200 for a digital device, whose full opening is of f/6,3 there is the same variation of luminosity. And in this case, it will be able to make the autofocus inoperative on the zoom, whereas there is no problem with the focal distance fixe.
One can however choose an intermediate possibility by using to 200 mm one 70-300 mm f/4-5,6 which should not make patauger the autofocus.

Obstruction

A zoom luminous transtandard exceeds 10 cm length and 7 cm in diameter. In proportion, a fixed focal distance from 20 to 85 mm will take much less place and will have on the apparatus a more discrete aspect. But if one must buy 3 objectives or to replace a zoom more, the unit space saver will not be found on the final level of the obstruction.

A zoom transtandard of amplitude 3 and average luminosity will have as for him encombremant the longest focal distance which it replaces. And if one chooses a transtandard strong amplitude, one will certainly have a more cumbersome objective, but it will be able to replace 5 or 6 fixed focal distances.

In the field of the télézooms, with the shortest focal distance, the objective remains rather long. For example the 50-500 mm of Sigma measurement 22 cm with its shortest focal distance and weighs 1,8 kg, that is to say around 7 times more in length and weight that one 50 mm of fixed focal distance. But there too, considering it replaces several teleobjectives including one 500 mm, the choice of the zoom is interesting.

Purchase price

The purchase price of an optical instrument is related not only on the difficulties of manufacture but also to the number of specimens which the manufacturer thinks of selling. A modern fixed focal distance comprises 10 to 15 lenses, whereas a zoom has of them 15 to 20. Finally the variation is weak.

In practice, the zooms being now the objectives which are sold best, they are hardly more expensive than fixed focal distances. And more and more, the manufacturers propose a majority of zooms in their range, the fixed focal distances being reserved for the special uses (very luminous objectives, focal extremes, special objectives).

Use

A zoom makes it possible to avoid too frequently changing objective. It is its main advantage. But that can be also a disadvantage at the artistic level. A zoom makes it possible to quickly obtain the broad or tight framing which one wishes from the place where one is. However, sometimes, it can be preferable to move compared to the subject to tally it differently. Without preventing this more creative behavior, the use of a zoom will encourage there less than the possession of a limited number of fixed focal distances.

In conclusion

For the photographic uses most current, the zoom advantageously replaces a series of objectives to fixed focal distance (handling, obstruction and purchase price of the complete equipment). The majority of the objectives assembled on the cameras are zooms today. But when, for a precise use, often professional, one seeks optical quality or the maximum luminosity, of the objectives of fixed focal distance make it possible to do still better than with a zoom.

See too

Objective great Objective angle
of long focal distance
Distortion (optical)
Ornamentation with vignettes

:Image: Zoom.gif: for an animated example of zoom

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