Adjust camera focus in ARKit

As stated by Alexander, iOS 11.3 brings autofocus to ARKit.
The corresponding documentation site shows how it is declared:

var isAutoFocusEnabled: Bool { get set }

You can access it this way:

var configuration = ARWorldTrackingConfiguration()
configuration.isAutoFocusEnabled = true // or false

However, as it is true by default, you should not even have to set it manually, unless you chose to opt out.

UPDATE: Starting from iOS 11.3 ARKit supports autofocusing, and it's enabled by default (more info). Manual focusing still aren't available.

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Prior to iOS 11.3 ARKit did not supported neither manual focus adjust nor autofocusing.

Here is Apple's reply on the subject (Oct 2017):

ARKit does not run with autofocus enabled as it may adversely affect plane detection. There is an existing feature request to support autofocus and no need to file additional requests. Any other focus discrepancies should be filed as bug reports. Be sure to include the device model and OS version. (source)

There is another thread on Apple forums where a developer claims he was able to adjust autofocus by calling AVCaptureDevice.setFocusModeLocked(lensPosition:completionHandler:) method on private AVCaptureDevice used by ARKit and it appears it's not affecting tracking. Though the method itself is public, the ARKit's AVCaptureDevice is not, so using this hack in production would most likely result in App Store rejection.