Reikan FoCal - Dual-Pixel Raw Focus Offset – Aperture Sharpness

Dual-Pixel Raw Focus Offset – Aperture Sharpness

(See Chart Features for general chart features - changing scales, copying to clipboard etc)

When the test mode is set to Raw and the Dual Pixel Raw option is enabled in both FoCal and on the camera, further information can be obtained about the real focus position.

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Key:

  • Red circles – calculated measured perfect focus offset from the current focus position
  • Pink area – indication of confidence of the measurement (the real result is somewhere within this area)
  • Blue line – 0 offset (perfect focus) indicator line

Basic Interpretation

The DPR Focus Offset (Dual Pixel Raw Focus Offset) chart indicates the amount of separation between the A and B subpixel images of the dual-pixel sensor – this is directly proportional to the difference between the actual focus point and the “perfect” focus point. An image separation value of 0 microns means that the image is perfectly in focus.

The image separation metric specifically indicates what is happening at the sensor. How this relates to real-world focus offset depends on focal length, aperture and subject distance, so it's important to understand that this is NOT a direct indication of focus offset at the target. However, for a given focal length, aperture and focus distance, the values can be directly compared.

This chart is immediately useful for a couple of reasons:

  1. How well you initially focused can be seen by how close the wide value (f/2.8 in this case) is to 0. In the example above, you can see that the f/2.8 value is very close to the zero line so this is a well-focused lens (at f/2.8) and we can be confident of all the results from this test (from the point of view of correct initial focus at least).
  2. Bearing in mind the lens is perfectly focused at f/2.8, it’s immediately interesting to see a fairly large focus shift at f/4-f/4.5. This indicates that the lens may benefit from manually focusing if you want the sharpest image possible at these apertures.

This is a new metric and we are currently collecting data to be able to process into more immediately meaningful information in a future update.

Corner Brightness Profile – Aperture Sharpness | This Page