Astigmatism Factor – Focus Consistency
(See Chart Features for general chart features - changing scales, copying to clipboard etc)
The Astigmatism Factor chart shows the image quality ratio between the horizontal and vertical analysis directions.
Key:
- Red diamond: a point where the horizontal axis is sharper than the vertical axis
- Blue diamond: a point where the vertical axis is sharper than the horizontal axis
- Blue Line: median (average) astigmatism factor
Where FoCal Comparison Data is available, the horizontal coloured bands indicate how this lens compares to other lenses of the same type on the same camera model:
- Green area - above average performance
- Blue area - average performance
- Red area - below average performance
Basic Interpretation
The Astigmatism Factor is an absolute indication of the difference between the horizontal and vertical sharpness. This means that a difference in either direction will increase the Astigmatism Factor value (height vertically on the chart). In order to determine the sharpest axis, the points are coloured as shown in the key above.
Vibration
Astigmatism Factor can also indicate that the camera is experiencing high levels of vibration during a test. This is typically shown by a high value (>5%) of Astigmatism Factor at some points but not necessarily all of them. Typically, the points will show as red diamonds if vibration is the cause - this means the blurring is in the vertical axis which is what you would usually expect from a vibrating setup.
Further Details
FoCal Comparison Data Interpretation
The horizontal red, blue and green bands of the FoCal Comparison Data allow you to see how your lens is performing compared to the average values of other users of the same camera/lens combination.
The example above shows a well calibrated lens producing results in the green area – i.e. that are better than the majority of other users.
Results Progression – Focus Consistency | This Page | Red-Blue Ratio – Focus Consistency