| How does auto-focus work? | | | | Some high end cameras combine both systems. The |
| All of us use it every time we click on the shutter | | | | camera will pick the right system for the specific |
| button. Practically all modern cameras include some | | | | scenario or will cross check and use both at the same |
| sort of an auto focus system. Thanks to the auto | | | | time. The photographer can also decide manually to |
| focus system we can enjoy an easier photography | | | | use one of the two options. For example when |
| experience and can concentrate on composing the | | | | shooting blue skies the camera can try to use the |
| right photo and capturing the moment rather than on | | | | active system and measure the distance. Since the |
| manually setting the focus. | | | | distance is infinite the camera can set the focus and |
| Automatic focus though has its limitations. For example | | | | skip the passive focus. In other cases when the |
| sometimes one might want to produce photos which | | | | distance is not infinite the camera can use the active |
| are a bit fuzzy as an artistic expression. Also the auto | | | | system to put the lens in approximately the right |
| focus implementation has its limitations and in some | | | | position and then use the passive system for fine |
| scenarios it might fail. One example is using a high end | | | | tuning. In dark scenarios the camera can opt to use the |
| SLR camera with a passive auto focus system to | | | | active system since the passive one will not work. |
| take a picture of blue skies. In most cases the camera | | | | So why doesn't the auto focus work all the time? |
| will move its motor back and forth and will eventually | | | | Even with all the electronics and computing power in |
| give up and fail to focus. | | | | the camera there will always be scenarios where the |
| To better use the auto focus system it would help to | | | | camera auto focus fails. Failure can be when the |
| understand high it actually works. Although | | | | camera can not focus and the picture is fuzzy or |
| implementations can vary we can divide them all into | | | | sometimes when the picture is actually in focus but the |
| two categories: passive and active. Most pocket | | | | camera "thinks" that it is not. |
| cameras use the cheaper passive method while high | | | | What causes such cases? The list is long but here are |
| end professional cameras use either the active or a | | | | just a few examples: |
| combination of both. | | | | - Taking low light pictures: The passive auto focus |
| Passive auto focus: | | | | system needs to "see" the picture in order to work |
| Passive auto focus can be perceived as imitating how | | | | and in low light scenarios this is not possible. Some |
| we set the focus manually. The camera defines one | | | | systems use a series of flashes to overcome this |
| or more regions in the picture (usually they are marked | | | | limitation but this solution fails many times. An active |
| as rectangles on the viewfinder or the LCD). The | | | | system can measure the distance to the object in |
| camera then analyzes the picture seen through those | | | | such scenarios but will fail if the object is not in the |
| regions and calculates a Focus Level number. The | | | | center of the picture or if there are a few objects at |
| camera then tries to move its lenses back and forth | | | | different distances. |
| as it recalculates the Focus Level. The camera looks | | | | - Active systems can fail with objects that tend to |
| for a position where the Focus Level is the highest. For | | | | absorb the infrared beam they are using. Some |
| that point if the Focus Level is above a predefined | | | | materials absorb infrared beams and will cause the |
| threshold the camera would define this region of the | | | | active system to measure the wrong distance. In |
| photo as being in focus. | | | | some scenarios other infrared sources such as |
| The Focus Level can be calculated in many ways. | | | | candles and open flame fires can render the active |
| The common attribute of all calculations is figuring out | | | | system useless. |
| how much Contrast is there in the photo. Although not | | | | - Low contrast objects such as white walls or blue |
| in the scope of this article one way to calculate such a | | | | skies. The passive auto focus relies on the fact that |
| number is by running the photo through a high | | | | the Focus Level changes significantly when moving the |
| frequency filter - this is based on the fact that high | | | | lens back and forth. This allows the camera to settle |
| contrast is associated with high frequencies. | | | | on the right focus position. The Focus Level of low |
| Active auto focus: | | | | contrast objects does not change much and fails the |
| Active auto focus works by measuring the distance | | | | passive system. |
| between the camera and the object in the picture. | | | | Knowing how the auto focus system works helps a |
| Technically if you knew the exact distance to the | | | | photographer understand why sometimes the camera |
| object you are taking a picture of you could set the | | | | can not focus. In such scenarios the photographer can |
| lens to the exact focus position. The active focus | | | | look for other solutions. Sometimes the photographer |
| system shoots a beam of invisible light, usually infrared, | | | | will have to use the manual focus. In other cases |
| at the object at the center of the picture and | | | | focusing on another object in the picture that is in the |
| measures the distance to that object. Based on that | | | | same distance but easier to focus on and locking the |
| distance the focus is set. | | | | focus on that object will solve the problem. |
| Combined auto focus: | | | | |