How Does The Camera Auto Focus Work And Why Sometimes It Fails

How Does The Camera Auto Focus Work And Why Sometimes It Fails



How Does the​ Camera Auto Focus Work and​ Why Sometimes It Fails?
How does auto-focus work?
All of​ us use it​ every time we click on the​ shutter button .​
Practically all modern cameras include some sort of​ an​ auto focus system .​
Thanks to​ the​ auto focus system we can enjoy an​ easier photography experience and​ can concentrate on composing the​ right photo and​ capturing the​ moment rather than on manually setting the​ focus .​
Automatic focus though has its limitations .​
For example sometimes one might want to​ produce photos which are a​ bit fuzzy as​ an​ artistic expression .​
Also the​ auto focus implementation has its limitations and​ in​ some scenarios it​ might fail .​
One example is​ using a​ high end SLR camera with a​ passive auto focus system to​ take a​ picture of​ blue skies .​
In most cases the​ camera will move its motor back and​ forth and​ will eventually give up and​ fail to​ focus.
To better use the​ auto focus system it​ would help to​ understand high it​ actually works .​
Although implementations can vary we can divide them all into two categories: passive and​ active .​
Most pocket cameras use the​ cheaper passive method while high end professional cameras use either the​ active or​ a​ combination of​ both.
Passive auto focus:
Passive auto focus can be perceived as​ imitating how we set the​ focus manually .​
The camera defines one or​ more regions in​ the​ picture (usually they are marked as​ rectangles on the​ viewfinder or​ the​ LCD) .​
The camera then analyzes the​ picture seen through those regions and​ calculates a​ Focus Level number .​
The camera then tries to​ move its lenses back and​ forth as​ it​ recalculates the​ Focus Level .​
The camera looks for​ a​ position where the​ Focus Level is​ the​ highest .​
For that point if​ the​ Focus Level is​ above a​ predefined threshold the​ camera would define this region of​ the​ photo as​ being in​ focus .​
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:
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.
Combined auto focus:
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?
Even with all the​ electronics and​ computing power in​ the​ camera there will always be scenarios where the​ camera auto focus fails .​
Failure can be when the​ camera can not focus and​ the​ picture is​ fuzzy or​ sometimes when the​ picture is​ actually in​ focus but the​ camera thinks that it​ is​ not.
What causes such cases? the​ list is​ long but here are just a​ few examples:
- Taking low light pictures: the​ passive auto focus system needs to​ see the​ picture in​ order to​ work and​ in​ low light scenarios this is​ not possible .​
Some systems use a​ series of​ flashes to​ overcome this limitation but this solution fails many times .​
An active system can measure the​ distance to​ the​ object in​ such scenarios but will fail if​ the​ object is​ not in​ the​ center of​ the​ picture or​ if​ there are a​ few objects at​ different distances.
- Active systems can fail with objects that tend to​ absorb the​ infrared beam they are using .​
Some materials absorb infrared beams and​ will cause the​ active system to​ measure the​ wrong distance .​
In some scenarios other infrared sources such as​ candles and​ open flame fires can render the​ active system useless.
- Low contrast objects such as​ white walls or​ blue skies .​
The passive auto focus relies on the​ fact that the​ Focus Level changes significantly when moving the​ lens back and​ forth .​
This allows the​ camera to​ settle on the​ right focus position .​
The Focus Level of​ low contrast objects does not change much and​ fails the​ passive system.

Knowing how the​ auto focus system works helps a​ photographer understand why sometimes the​ camera can not focus .​
In such scenarios the​ photographer can look for​ other solutions .​
Sometimes the​ photographer will have to​ use the​ manual focus .​
In other cases focusing on another object in​ the​ picture that is​ in​ the​ same distance but easier to​ focus on and​ locking the​ focus on that object will solve the​ problem.




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