This article is part of the online digital photography course.
When your camera focuses on the target have you ever wondered how it works?
When you rotate the focusing ring (even when you use the auto focus) you are just regulating the distance between the lens elements and the sensor, a subject appears sharp only when this distance is correct.
The light enters the lens and is collected towards the sensor creating a cone, if the vertex hits the sensor’s plane the image is focused, otherwise it will result blurred.
In the auto focus you have just to press until half the shutter release and automatically an electric engine will move the groups of elements until the correct focus, on the viewfinder should appear a light to inform you that you can shoot (some cameras also utter a buzzer).
SLR usually have 4 focusing modes.
If the subject is decentralized you may frame and focus it by positioning it in the center of the frame, and later recompose the frame without releasing the shutter release in way to keep the setting, finally you have just to shoot!
When your camera focuses on the target have you ever wondered how it works?
When you rotate the focusing ring (even when you use the auto focus) you are just regulating the distance between the lens elements and the sensor, a subject appears sharp only when this distance is correct.
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| Focusing ring of the Rolley B35 (the more external, the internal is the aperture), on the old cameras on the focusing ring the distance appeared in both feet and meters. |
The light enters the lens and is collected towards the sensor creating a cone, if the vertex hits the sensor’s plane the image is focused, otherwise it will result blurred.
In the auto focus you have just to press until half the shutter release and automatically an electric engine will move the groups of elements until the correct focus, on the viewfinder should appear a light to inform you that you can shoot (some cameras also utter a buzzer).
SLR usually have 4 focusing modes.
- One shot AF (Canon) or also AF-S (Auto Focus Single on Nikon): it works as I just explained, you have to press halfway the shutter release to obtain the subject correctly on focus, it’s better to use this AF mode with still subjects.
- Continuous AF, AI Servo (Canon) or AF-C (Nikon): This mode continuously update the focus for all the time we keep the shutter release pushed, useful when we have to shoot moving subjects.
- Intelligent AF, AI Focus (Canon) o AF-A (Nikon): The camera decides automatically, interpreting the scene, if it has to use the AF-S mode or the AF-C (continuous) mode, this kind of focusing is also called hybrid for this peculiarity.
- Predictive AF: This also has to be used with moving subject, the camera corrects the focus trying to foresee the point of focus esteeming the subject’s movement during the brief time of the shot.
- Manual: this kind of focus was pretty used in the times of analogic photography, when the auto focus systems were still pretty inaccurate, to use it you have to disable the auto focus and rotate the ring of the lens until the image appears sharp. Nowadays is still used because in some occasions the auto focus doesn't work, for example if another object obstructs the line of sight, the auto focus could not focus on the wanted plane, or another very common case, with a monochromatic subject (as a colored wall) the autofocus may jam trying to value the distance.
If the subject is decentralized you may frame and focus it by positioning it in the center of the frame, and later recompose the frame without releasing the shutter release in way to keep the setting, finally you have just to shoot!
Translation by Nina Kozul SHARE
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