Autofocus Error – MultiTest
(See Chart Features for general chart features - changing scales, copying to clipboard etc)
The Autofocus Error chart shows the error – in focus adjustment units (AF Microadjustment, AF Fine Tune etc) – from the expected focus position for any particular focus adjustment value tested.
The data is shown in a classic box-plot format, with the expected focus adjustment value along the bottom, and the error from this point shown vertically.
- The bars at the top and bottom of each point represent the full extremes of the focus error.
- The box in the middle shows the extent of the middle 50% of data points (the interquartile range)
- The thicker bar somewhere within the box shows the median value.
A perfect lens and camera autofocus system would have all lines along the vertical zero axis, half way up the chart. But every focus operation will exhibit some sort of error from the perfect position.
In the above example, the data at AF Microadjustment -20 (the far left point) shows an error range of from -5.5 to around +1, but most of the points are within the very small (almost invisible) box near the top. This means that the point with an error of -5.5 is an usually point, and mostly the error at this point is between 0 and +1 focus adjustment units – which is very good performance.
By comaprison, at the +20 end – the far right – the box section (which represents typical values – the middle 50% of measurements) is spread between around -5 and +2, with the extremes moving out even further.
In reality, from looking at the majority of points on the chart we can conclude that there is a typical AF error of between about 0 and -5 units for any shot taken.
Remember, this is not the focus adjustment value that you need to put into the camera, but the error of the camera/lens system focusing on any particular point. It can only be determined when there is a complete set of data to allow us to know where the focus should be.