Hunting, auto-focus hunting ~
The Auto-focus (AF) systems on Digital SLR cameras sometimes encounter situations where they are not very good at doing the job. If the camera is unable to resolve a sharp image using the AF it will ‘Hunt’ for one. Examples where auto-focus hunting may occur would be a clear blue sky; a completely dark sky; a plain white wall; a foggy grey day; and a range of other plain, undifferentiated fields of view.
The auto function on the focus mechanism is dependent on seeing contrasting colours, shades, shadows etc. When the focus sensor sees the contrasts it compares them by measuring the differences in the contrasts. The lens can focus when the camera spots the setting where the biggest contrast is measured. If the auto sensor fails to detect a contrast it ‘hunts’ for one. If it continues failing to resolve a contrast the auto-focus hunting continues.
‘Auto-focus hunting’ occurs when the AF methods available to the camera have failed to resolve a sharp image. In this case the camera will carry on attempting to get an image by running through the full focus ranges available to the lens. If a sharp image is detected then the motor stops. If a sharp resolution is not achieved the motor will hunt until it times out or it detects a sharpness in its field of view.
Auto-focus hunting tends to drain the battery. So the camera will normally time it out to save power after a short time. In this case you will get no response to your shutter button when you push it. The camera is telling you there is nothing there to focus upon.
To cure auto-focus hunting…
To focus an image try to find something that has a strong contrast of colour or light. Put your active focus point in the viewfinder in the area where the contrast is obvious. Then push the shutter button. The camera sensor will recognise the difference across the contrasting area. It can then resolve the focus and compare sharpness data between them. When the image gets a sharp resolution the auto-focus hunting will stop.
When you have auto-focus hunting is occurring, change your focus point to a new point of contrast and try again.
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Damon is a writer-photographer and editor of this site. He has run some major websites, a computing department and a digital image library. He started out as a trained teacher and now runs training for digital photogs.
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