CAF/SAF Focus Point Focus Accuracy with Telephoto Lenses (Olympus Cameras with PDAF).

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drj3 Forum Pro • Posts: 11,526
CAF/SAF Focus Point Focus Accuracy with Telephoto Lenses (Olympus Cameras with PDAF).
15

First a simple experiment using the E-M1.3 with the 300mm f4. I set the Electronic Focus Limiter maximum focus distance to prevent the camera from focusing on the distant background.

I chose the single small focus point and pointed the camera to the far right of the flower head (Image #1) and half pressed the shutter. Neither CAF nor SAF would focus or give focus confirmation. Then I moved the focus point a little closer to the flower head and repeated the half press of the shutter. I then repeatedly moved the focus point toward the flower half pressing the shutter each time, until the camera focused and confirmed focus on the flower head.

The results are shown in the first image below. CAF would only focus and confirm focus when the focus box touched the edge of the flower head. SAF would focus on the flower head and confirm focus far from the flower head indicating a focus area much larger than the indicated focus box.

A simple example of how this can affect focusing on a specific target is shown on Images 2 & 3.

The lens was pre-focused at infinity and then the small focus box was placed on the Bee Balm flower and the shutter half pressed. I repeated this test multiple times all with the same result.

The camera focused quickly and accurately on the flower head and confirmed focus when using CAF.

The camera focused on the plant leaves behind the Bee Balm flower head and confirmed focus on the center flower head, with the focus point was completely on the flower head. The camera maximized contrast on the first detailed object it found in the much larger CDAF focus area when focusing down from a more distant point.

When I pre-focused in front of the flower, SAF focused on the flower. However, if there had been any detailed object in front of the flower within the larger focus area, it would have focused in front of the flower.

This difference in CAF and SAF can also affect the precise focus location, even when the camera focuses on the target. CAF will almost always focus on the most detail within the focus box, whereas SAF will maximize contrast with its larger focus area. The difference between CAF and SAF focus is generally not very important for larger targets at a distance where the DOF is much greater as long as the target is not very close to other detailed objects. However this can be very important for smaller targets when leaves/twigs/branches are close to your desired focus point.

SAF will focus more reliably when there is little detail in the focus area or when the illumination is very low.

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