The Focus Level can be calculated in many ways. The common attribute of all calculations is figuring out how much Contrast is there in the photo. Although not in the scope of this article one way to calculate such a number is by running the photo through a high frequency filter – this is based on the fact that high contrast is associated with high frequencies. Active auto focus:
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Active auto focus works by measuring the distance between the camera and the object in the picture. Technically if you knew the exact distance to the object you are taking a picture of you could set the lens to the exact focus position. The active focus system shoots a beam of invisible light, usually infrared, at the object at the center of the picture and measures the distance to that object. Based on that distance the focus is set.
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Combined auto focus:
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- Some high end cameras combine both systems. The camera will pick the right system for the specific scenario or will cross check and use both at the same time. The photographer can also decide manually to use one of the two options. For example when shooting blue skies the camera can try to use the active system and measure the distance. Since the distance is infinite the camera can set the focus and skip the passive focus. In other cases when the distance is not infinite the camera can use the active system to put the lens in approximately the right position and then use the passive system for fine tuning. In dark scenarios the camera can opt to use the active system since the passive one will not work.
- So why doesn’t the auto focus work all the time?