What is the passive autofocus of an SLR, description of the operation for measuring the distance from the subject, how the lens is automatically set in motion for optimal focus.
Autofocus
Passive autofocuses are mounted on reflex cameras and high-class compact cameras.
Their operation, which can be well described by thinking of an electronic rangefinder, is based on a sensor that has the task of assessing the contrast of the image before emitting an electrical signal proportionate to it.
The lens is automatically moved, in search of the perfect focus position capable of generating an electrical output signal, dependent on the contrast and of maximum intensity.
The signal sent is processed by an electronic control unit which has the task of driving the electric motor which is responsible for moving the objective lenses.
Camera manufacturers have mainly traveled two paths: some have preferred a unique autofocus motor incorporated in the camera itself, to be connected by means of special transmissions to the lenses, others have instead focused on an engine in each lens, connected via electrical contacts to the commanders located on the body machine.
For the reason mentioned above, universal optical manufacturers have developed objectives of both types.
In the viewfinder of the cameras, one or more boxes indicate the areas within which the autofocus is read.
If the measuring point is unique in the center, you point it at the subject, first pressing the shutter button halfway, to activate the autofocus, then pressing it fully to take the picture.
Sometimes the autofocus could mis-focus because it is deceived by various factors that could adversely affect.
This is the typical case of group photos in which the subjects, being on different planes, are not all perfectly focused: to avoid this inconvenience, it is advisable to point the central area of the viewfinder on an intermediate subject and try to exploit narrower apertures of the lens to compensate for this particular situation with the greater depth of field allowed by the lens.
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Another case in which the autofocus may not work properly is when the lighting is poor, in this case it may be necessary to intervene manually to be on the safe side or to take several shots of which to keep only the successful ones.
