Understanding Pentax AF

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ozdean
ozdean Forum Pro • Posts: 24,111
Understanding Pentax AF
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The AF system generally defaults to focusing towards the centre of the frame and the closest element until told otherwise.

AFS

Think of the S in AFS to not only stand for Single Shot but also Set and Stationary/Slow.

In this mode with a shutter half press (or rear button AF) the camera sets the focal plane from the camera and locks it until the shutter has finished firing or the shutter button is released. The focal distance never changes in AFS so this is why it should only be used for stationary/slow subjects; the exception being for Catch in Focus where the subject moves into the focal plane and the shutter fires with a manual lens.

You can get a sharp shot ie when a model is walking down the catwalk, if you have enough dof but the subject can be OOF because the model has moved out of the set focal plane between the time of half press and full shutter press when shallower dofs are required and used.

This also becomes very evident when trying to photograph kids & pets while in AFS, when the subject has moved too far from the set focal plane.

Because of the set focal plane it is not wise to use Continuous Shutter unless the subjects are stationary ie a family portrait shoot to get 1 shot out of say 5 without a blink or funny face.

In AFS Shutter Release/Focus Priority and Hold do not apply as Focus Priority and Hold are the defaults for AFS. SR should always be set on when in AFS because you are not following a moving subject and want the camera and sensor to be steady.

AFS & AFC FOCUSING AREAS

Centre Point is always the most accurate and fastest because it is the highest contrast and resolution point of any lens - always try to use it as it is very accurate.

Select Point allows you to set your focal point within the frame - ie Portraiture (Closest Eye).

Zone Select Camera finds points it can focus on - think of it as Zone Averaging.

Auto Zone Self selects points across the frame and averages them to form what it thinks should be the focal plane after consulting algorithms etc.

AFC

For moving subjects. Focus is continuously at work within the parameters you set.

SR and Horizon Correction IMO should be off in this mode so that they are not fighting to correct your intentional camera movement (Panning etc). Keep Shutter Speeds and ISO up as needed. The focal plane is continually changing to match the subject. Keep Hold off unless there are elements popping up particularly in front of your subject to distract AFC ie trees, people, other birds flying etc. Hold can be applied progressively. Focus Priority is the ultimate but if you have dof you can go Release Priority but expect some duds.

AFC EXCLUSIVE FOCUSING AREAS

Expanded Area AF comes in 3 zone area sizes and can be thought of as a centre point with semi tracking around it, if the central zone point loses its focus the points around it look for what it had and take over focus. The zone can be moved around the frame.

If you were photographing a formula 1 car that was moving left to right you could set your zone to the left so the car had lead space on the right, position the central point on the driver's head and the peripheral points will pick up the driver's head if you lost it in your panning.

NB I have not tried this as yet but am looking forward to using it.

The AFC modes I have always favoured have been centre point or zone 9 point if I need leeway. I have found them to work well.

LIVE VIEW

I seldom use Live View so feel unqualified to comment but basically the camera becomes like a mirror less with Face Detection (Locks on peoples faces) and Tracking (you can set the camera on a tripod and it tracks movement across the screen) plus the usual Multipoint Zones, Select Point and Centre Point.

A simple old trick before AF came out on the MEF was to manually prefocus ie focus on the finish line in a race and snap as much as you like at the racers as they cross it and every shot will be in perfect focus.

I trust this is helpful in understanding Pentax AF. If there are things left out please fell free to chime in.

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Regards Dean - Capturing Creation
N.B. All my Images are Protected by Copyright

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