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Manual Focus
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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.

Ziv Haparnas is a technology veteran and writes about practical technology and science issues. This article can be reprinted and used as long as the resource box including the backlink is included. You can find more information about photo album printing and photography in general on http://www.printrates.com - a site dedicated to photo printing

Focus Modes To Use In Digital Cameras

While some of the least expensive digital cameras have only automatic focus, meaning the camera does all the work on bringing your subject into the best possible focus, most SLR digitals offer three different focus modes:  manual, single auto focus and continuous auto focus.  All three of these will be addressed here.

With manual focus, the camera stays out of the focus equation and you, the photographer, make all the decisions regarding this.  This is done by setting different buttons or actually using an attached focusing ring that rotates on the camera lens.  For those who like to have complete creative control of the finished product, this is the best focus mode.

In single auto focus mode, the camera automatically focuses when you press the shutter button either all the way down to shoot a photo or half way down to lock the focus.  This mode is useful when shooting static objects.

In continuous auto focus the camera continuously focuses on the objects in the photo.  In this mode the camera continuously corrects the focus as the objects distance from the camera changes. This mode is useful when you shoot photos of moving objects such as a race car during a race or airplanes during an air show. You can hold the shutter button half way down and continuously move the camera to follow the object. The camera will continuously keep the object in focus.

Like any other feature automatic and manual focus modes have their pros and cons. The first step to using them to your advantage is to understand how they work and what they were designed for. The next step is to experiment shoot photos using different focus modes and different types of objects and see how the camera behaves. Once you have done that you will be ready to instinctively use the best focus mode for each photo situation.

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About the Author

Oli works full time as a Market Analyst.He graduated in Management.He can help you to grow your computer consulting.
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Can you add a manual focus lens to a digital camera?

I don't know much about these kinds of things and I was wondering if you can , like, add a manual focus lens to a camera? I just got a Fujifilm S1500 as a present and it gets pretty frustrating when trying to shoot macro or moving objects.

I'm afraid not. What you need is a dslr camera. It sucks getting gifts from people who don't understand technology.

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