Autofocus

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Active systems

The active system equips the compact cameras like the reflex cameras. It is about a system of Mise at the telemetric point: a transmitting cell sends on the subject one or more infra-red beams and a receiving cell receives the beam returned by the subject. The distance between the apparatus and the subject can thus be given; the development is then ordered by a Servomoteur.

Active autofocus by Ultrason S also existed. The principle was to measure the time taken by the sound to make an outward journey return between the apparatus and the subject.

Passive systems

The passive system equips certain cameras reflex camera. It uses a telemetric system made up of sensors CCC placed at the bottom of the darkroom. It downwards receives a fraction of reflected light by the mirror reflex camera. Its principle is based on the correlation of phase or contrast. Nevertheless, in order to improve the behavior of it, certain apparatuses comprise also a red transmitting cell or infra-red which makes it possible to project a test card on the subject in order to facilitate the work of the passive AF.

The majority of the marks of cases reflex camera chose to preserve mechanical couplings between their cases and the objectives. The engine which makes turn the ring of settling is located either inside the case or in optics. The first solution made it possible to the manufacturers to maintain a compatibility between the objectives not autofocus and the cases autofocus, and reciprocally. Other manufacturers, after an unfruitful test of engine in the case, Canon in particular, chose to create a new mounting for its cases autofocus where the coupling with the case is entirely electric. There are thus an engine in the objective for the development and another for the adjustment of the diaphragm. Sigma, which started to manufacture cases reflex camera in the Nineties also followed this way. Since, other manufacturers, Nikon in particular, chose to put engines in their top-of-the-range teleobjectives to accelerate the speed of development.

Advantages and disadvantages

The development automatic (or autofocus) was quickly perceived by the public like a significant progress by reducing the number of photographs missed all while not requiring any effort by the photographer himself.

Autofocus credits

This type of autofocus functions of course of the little contrasted subjects: snows, white wall in the center of the image. Its principal disadvantage is that if a very clean pane even is intercalated between the apparatus and the subject, the apparatus will try to make the development on the pane, which will give a very fuzzy photograph! To cure this problem, much of compact cameras lay out of a mode “infinite landscape” which it should not be forgotten to activate if one makes for example photographs since a vehicle.

Considering its operating process, the active autofocus is especially ready to measure short distances, which is sufficient for the objective S standards and Grand angle but makes it not very compatible with the use of the powerful Téléobjectif S.

Autofocus passive

The first limitation is due at the speed of development which can be limited by the mechanics of the apparatus or optics. The autofocus then tends to introduce a time between the pressure of the release and the imagery. This time was particularly sensitive on bottom-of-the-range apparatuses in the Nineties. Currently, the large manufacturers of reflex cameras equip with the same system autofocus it quasi totality of their cases. Only the cases of professional level (with more than 1200 € into silver, 2500 € numerically) have more powerful autofocus.

Secondly, the system of the autofocus is limited by the contrast or the luminosity of the subject photographed. In the case of the reflex cameras, one generally says whom an objective which does not open with a diaphragm more important than f/5.6 does not allow the good performance of the autofocus (the exact value depends nevertheless exact characteristics on the complete system). In the case of the Zoom S transtandards of strong amplitude, the autofocus which functions well with the majority of the Focale S can have missed during an adjustment on the maximum focal distance. Lastly, has less to have at the beginning a very luminous objective (f/2.8 or better) the use of a Multiplicateur of focal distance is incompatible with the autofocus. This is why more and more of reflex camera resort to the active autofocus which can function in the complete absence of light. It is for example the case of Nikon D40 left at the end of 2006.

Then, the autofocus must especially determine the position of the subject and one knows that this choice is not easy (similar problems exist for the calculation of the exposure according to the subject). Following the example apparatuses having an active autofocus which consider that the subject is always in the center of the image, in the Nineties the reflex cameras autofocus of line entry had only one sensor autofocus. This led many photographers has to tally their subject in the central zone of the sight. To solve this problem, the solution consisted in memorizing the distance to the subject while pressing on the release with semi-race. One then can recadrer the subject differently before taking the photograph. But of course this procedure puts up badly with catches of sights on the sharp one: one was to then return to the manual method.

Currently, even the reflex cameras of line entry have several sensors autofocus (5 or more) divided into various places of the image. The system enables you to select one or the other zone. The apparatuses of medium and high-end include options to detect automatically certain attitudes of the subject starting from algorithms taking again the characteristics of thousands of real images. In the middle of the Nineties, Canon developed for its top-of-the-range cases (EOS 5,50E, 3 and 30) a system of selection of the sensor autofocus by the movements of the eye of the photographer. A laser beam makes it possible to detect the position of the Pupille of the photographer in the sight. This system requires as a preliminary a calibration by the photographer. On a case EOS 3 (apparatus semi-pro comprising 45 sensors autofocus, number ever exceeded), with a luminous teleobjective, this system makes it possible at the time of photographs of circus to make the development on the deer and not on the bars of the cage. But on the other cases the selection of the sensor by the eye is more random. It would be besides incompatible with the wearing of glasses. And then to think of looking in the sight the element of the most important subject is not always an instinctive reflex.

Another solution which with the favor of the professionals and the enlightened amateurs consists in authorizing the final improvement of the point . It is possible when the objective is been driven by a top-of-the-range engine (envisaged for that), or if there is an automatic system of unlocking of the engine of the case. In the contrary case, it will be necessary to force on the ring of development, which is not advised.

Lastly, the subjects moving starting from 50 km/h (14 m/sec is a reaction time of 0.072 dryness) constitute targets difficult to follow as regards autofocus because their reaction time reaches sometimes a good fraction of a second. One for that developed of many technologies (whose predictive autofocus which tries to anticipate the position of a mobile subject within the framework) but generally, the photographer decontaminates his autofocus and works as a manual mode (sporting photography, macrophotograhie, etc)

In the extreme cases, a not very powerful autofocus, used with a not very luminous objective can prevent to start or put out of order the development of a photograph which, without autofocus, would have been almost clear. On the other hand, with top-of-the-range material, it is possible to make clear images in gust of objects moving, whereas a manual development would not be fast enough. At all events, the contribution of the autofocus is such as the vast majority of the cameras is equipped today with such a device which was not been sulky a long time by the professionals who learned how to use it with his forces and his weaknesses.

References

Technology Camera: The Dark Side off the Lens , Norman Goldberg.
Applied Photographic Optics , Sidney Ray.
Manual off Photography: Photographic and DIGITAL Imaging , Ralph Jacobson, Sidney Ray, Geoffrey G Attridge, Norman Axford.

See too

External bonds does

the autofocus with contrast of phase
Comment function the autofocus?
detailed Explanation of the method of detection of contrast

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