Using Sharpness to Measure Your Autofocus Consistency

November 15, 2016
December 16, 2016

By Ranga Burada

Autofocus plays a major role in many camera system applications with variable focus, including consumer electronic devices. Camera systems must be able to focus at a variety of distances. Optical systems on cameras only allow a certain range of distances from the camera to be in focus at once (this is often known as the depth of field, or depth of focus). The distance from the camera where objects will be most in focus, effectively the center of this range, is the focus distancethe role of the autofocus system in a camera is to set this point accurately every time.

We refer to autofocus consistency as the ability of a camera to focus on a given point correctly, repeatedly. To determine if a point is in focus, we measure the sharpness of an object (specifically, a test chart) at that distance. By taking many images of the chartand letting the autofocus system reset each time and try to focus on the chart anewwe can tell if the camera system is focusing consistently or not. By examining the MTF50 values calculated from these imagesa common objective image quality metric which correlates well with perceived sharpnesswe can tell if sharpness varied between captures, and thus if focus accuracy on the chart varied.

Imatest Autofocus Consistency Module

The Imatest Autofocus Consistency module analyzes the sharpness (specifically MTF50) results from a set of images captured at a fixed distance from an Imatest sharpness chart, such as SFRplus chart, eSFR-ISO chart, or the new AutoFocus chart.  The user can then generate MTF50 values from these images using the SFR, SFRplus, or eSFR-ISO modules in Imatest. The Autofocus Consistency module is a post-processor that runs on the outputs of these analyses and consolidates them into a more useful form. You can find a more detailed description of the test procedure here.

 

MTF50 Value

 

In the above plot, each x-axis position indicates the distance from chart to camera. The colored data marks spread vertically at each position indicate the MTF50 values calculated from the images captured at that chart distance. The consistency of the autofocus system at a given distance is indicated by the tightness of the spread of MTF50 values for images taken at that distance. The narrower this spread, the more consistent the autofocus system is. In order to determine if the system’s consistency depends on distance (perhaps it has an easy time focusing on nearby points, but tends to fail for faraway ones), this analysis is repeated at many test distances, as in the plot above.

To learn more about maintaining consistency while measuring sharpness with MTF values, visit Increasing the Repeatability of Your Sharpness Tests.

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