The Difference Between SAF (PDAF + CDAF) and CAF (PDAF) Focusing with the E-M1.2 and E-M1.3.

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drj3 Forum Pro • Posts: 10,832
The Difference Between SAF (PDAF + CDAF) and CAF (PDAF) Focusing with the E-M1.2 and E-M1.3.
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There is a good reason that one should generally choose CAF focus for the E-M1s when you want the most accurate focus for a detailed target. CAF confirms focus based on information and distinct detail within the focus box. The focus area for confirming SAF focus is much larger than the indicated box size which allows it to focus more easily with greater amount of information in the focus area when there is little detail. Unfortunately this can result in greater focus variability with a small bias toward front focus which can be very important when you want maximum detail for a specific target.

This is difference is easy to demonstrate. In the first image below using the small single focus box, CAF will only focus when the focus box includes at least some part of the branch. SAF will focus on the branch when the focus box is located completely outside the branch as it is in the first image. The actual focus area for the small single focus box in SAF is somewhat larger than the large single focus box in CAF.

In CAF mode the camera will focus on the most distinct detail in the focus box. SAF will focus to maximize contrast in a somewhat larger area. If you use a large focus area, CAF will still focus on the most distinct part of the frame (closer things tend to be more distinct) whereas SAF will try to maximize contrast in the frame. Maximizing contrast can lead to unpredictable focus.

In the second image and third images using all focus points, SAF sometimes focuses on the window decals (image #2), or on the window pane or on the much more distant evergreen tree(image #3). In my test SAF was more likely to focus on the evergreen when the last focus was more distant and focus on the window pane or decals when the previous focus distance was very close. However, it would also randomly jump from the pre-focused decal or evergreen to the other one even when pre-focused precisely. CAF using all focus points always focused on the edge of one of the decals even when pre-focused on the distant evergreen as long as any part of the decal was in the frame.

In a later post I will give detailed results on focusing accuracy for CAF/SAF with high and low illumination when using the E-M1.3+MC20+300mm with the butterfly decal.

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drj3

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