Multiplier of focal distance

A multiplying of focal distance is an accessory which one intercalates between the objective and the case and which makes it possible to obtain framing that an objective of would get focal longer than that one uses.

Various models

The multiplier of focal distance more the current is the lapping machine of focal distance (called lapping machine more simply). As its name indicates it, it makes it possible to obtain the framing which an objective would provide having 2 times the focal distance of the objective that one uses.

Same manner, there existed tripleurs of focal distance, to obtain the framing of an objective which would have a focal distance triples that used.

The multipliers of focal distance having disadvantages, multiplying by 1,4 were marketed to reduce those.

Among the special models, at the beginning of the Autofocus, there were multipliers by 1,7 with mobile lenses, of which the principal goal was to allow the use of the autofocus with old objectives Canon functioning in manual settling.

On certain lapping machines of focal distance, the light unit is removable, which makes it possible to transform the lapping machine into ring lengthens.

Optical construction

A multiplier of focal distance consists of a whole of lenses mainly divergent (in general 7 lenses on a good lapping machine). Positioned between the objective and the case, it draws aside the luminous rays, so that the central part of the initial image is projected increased on significant surface. It thus acts like the divergent block of a teleobjective.

Advantages and disadvantages

A multiplier of focal distance makes it possible to have the framing which would provide of the powerful Téléobjectif S, without the disadvantage of having to buy other expensive, heavy and cumbersome objectives. It is thus an economic solution which also makes it possible to be equipped slightly.

On the other hand, by multiplying dimensions of the central part of the image, one multiplies squared his surface, and its luminosity is divided in the same proportions.

Concretely:

With a lapping machine of focal distance, the luminosity is divided by 4. 2 diaphragms are thus lost.
With a triplor of focal distance, the luminosity is divided by 9. One thus loses more than 3 diaphragms.
With a multiplier by 1,4 the luminosity is still divided by 2. One thus loses 1 diaphragm.

Another consequence relates to the autofocus. If one doubles the focal distance of an objective open to f/2,8 (for example a luminous zoom), one obtains another objective certainly compact for his focal distance, but opened with f/5,6 is the maximum limit so that the autofocus still functions.

With less luminous objectives, the use of a lapping machine (or a triplor) forces to pass in manual settling. And in this case there, one will have of a not very luminous aiming and a ring of settling too little geared down. One sees here the interest of a multiplier by 1,4 which, in addition to making lose less light, will allow the autofocus with objectives open to f/4 or if not a development easier manual.

By increasing an image, one increases also all his defects. To have thus still good images with a lapping machine (even triplor), it is necessary that the clearness and the definition of the image without this accessory are already excellent, which is seldom the case for the télézooms used with their maximum focal distance.

It should be noted that concerning this problem, certain manufacturers propose multipliers by 1,4 and of the specific lapping machines for some their teleobjectives, in order to avoid a too strong deterioration of the quality of the final image.

See too

On Wikipédia

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