Monday, 5 November 2012

One Shot Vs AI Servo - Choosing the right AF Mode for your Shooting Situation

One question frequently asked by beginners to photography is - One Shot Vs AI Servo (AF – S Vs AF – C for Nikon users) which one is right for me?


Before answering the question let us first take a look at the basic difference between these focusing modes.

One Shot or AF - S

In the one shot focus mode, when you press the shutter button half way, the camera will focus once. When the camera achieves focus the focus confirmation light for the focus point which achieved focus will flash in red. If the beep is set to on the camera will beep once when focus is achieved.

If your camera is set to evaluative metering mode the camera will also set the exposure when the focus is achieved. If you keep the shutter button half pressed, the focus remains locked and you are free to recompose the shot if needed
without change of focus.

If in case the camera fails to achieve focus, the focus confirmation light in the viewfinder will keep blinking and even if you press the shutter button completely a picture will not be taken. In such situations you need to re-compose your shot and try to focus again.

AI Servo or AF – C

In AI Servo focus mode the camera will keep tracking the subject and focus continuously while you keep the shutter button half pressed.

Even though the camera continuously focuses on the subject, the exposure is set at the time of releasing the shutter.

In AI Servo focus mode the beeper will not beep and the focus confirmation light in the viewfinder will not light even if the focus is achieved.

The camera takes a shot if you press the shutter release button completely irrespective of the fact that whether focus was achieved or not.

Which Focusing Mode to Use?

One-Shot or AF – S is the standard setting for digital cameras and so is the most widely used focusing mode. This mode is well suited for subjects that are stationary but is susceptible to focus errors when shooting fast moving subjects.

AI Servo or AF – C is the preferred focusing mode for shooting moving subjects as the camera will constantly update focus information when the distance between the subject and the camera changes. Most modern digital cameras feature advanced functions like predictive function in AI Servo mode in which the camera observes the movement of the subject and sets the focus predicting where the subject will be at the time of the exposure (practically it is compensating for the shutter lag). 

In case of Canon cameras, One thing to remember while choosing AF modes is that when you are shooting in low light conditions where you would like a little help from the external flash units AF assist infra red beam, this feature will only work when the camera is set to One shot focus mode.

So we have now come to the conclusion that one shot focus mode or still subjects and AI Servo focus mode for moving subjects. But in real life situations especially when shooting action shots (mainly in sports and wildlife photography) photographers would often require the convenience of both the shooting modes. For e.g. They would want to continuously keep the subject in focus, but they’ll require to recompose the shot for better composition at the same time. But sadly Continuously focusing the subject is not possible in One shot mode and Re composing the frame is not possible in AI Servo mode. And as it is difficult to frequently shift from one mode to another there are other ways to achieve the best of both worlds. We will discuss about Setting Back Button Focus and the advantage of using it in the next article.

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