SAF Front Focus and CAF/SAF Focus Comparisons with the 300mm f4 – E-M1s.

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SAF Front Focus and CAF/SAF Focus Comparisons with the 300mm f4 – E-M1s.
7

The target was a photograph of a butterfly decal (Image #1) displayed on my computer monitor. The monitor allows very precise location of the focus point by viewing the clarity of the individual pixels.

The monitor was photographed at an angle (Image #2) so that actual focus location could be easily determined with a high degree of accuracy (the center of the DOF) and the focus errors computed based on the EXIF focus steps.

The camera settings were - Small single focus point, AF Scanner = Mode 3, C-AF Sensitivity = +2, with Aperture Priority.

I evaluated focus accuracy and variability using two different photographic techniques.

In the first condition I started with an unfocused image, half pressed the shutter button to focus, and then fully pressed the shutter button after focus confirmation (with a 2 second delay). This is labeled Normal Focus on the images and tables.

The second condition was to Pre-Focus on the image and then refocus before exposure. This is my typical photographic technique for stationary birds. It the same as Normal Focus, but I stop after the half press of the shutter button with focus confirmation. I release the shutter button and check the focus point position and initial focus. I then half press the shutter button again and with focus confirmation, I expose the image. The camera is Pre-focused on the target before I focus for exposure.

Image #3 shows the location of the focus box on the target.

Image #4 is a crop of the images with a focus distance equal to the median for the CAF – Normal Focus, SAF Pre-Focus, and SAF Normal Focus conditions. (The CAF –Pre-Focus median was essentially identical to the CAF-Normal Focus with only ≈ .012 inches difference in focus distance).

Image #5 is a crop of the center of image #4. You may need to click on Original at the bottom of the image to see the difference in sharpness with the DPR jpegs. A well focused image would have the pixels in the two vertical lines in the center of approximately equal in sharpness with the actual focus distance between the two lines (CAF – NF with FS = 8488). The two SAF images are both front focused. The line on the left is sharp on the SAF images and the line on the right is unfocused. The detail on the left line is sharper than the detail on the right line in these two images since the DOF is shifted to the left, closer to the camera.

The results shown in the table in Image #6 and are typical. The Median CAF focus for both Normal and Pre-Focus is always very close to the best focus for all evaluations I have conducted. The Median SAF focus is almost always front focused, though the amount of front focus will vary with the particular target, the focus point and the amount of illumination.

The Focus variability (SD) with Pre-Focus CAF is typically considerably less than the other three conditions, which are often similar in size. Simply maintaining the half press of the shutter for a longer period of time after focus confirmation in the CAF Normal condition, giving the camera more time to determine correct focus, might result in focus variation similar to that of the Pre-focus CAF condition. However, I have not tested that possibility. This would probably require the AF scanner to be set to mode 3 and C-AF Sensitivity to +2 to keep the camera continually checking focus.

Image #7 is one of the images from a similar test with the MC20+300mm where I photographed one of the decals on the window with both high and low illumination. The angle was not as great so you see less difference in sharpness at the focus point, but can more easily see the front focus shift producing greater sharpness on the closer (left) side of the image.

The table in Image #8 summarizes the results of the MC20+300mm test. The low illumination/contrast condition, summarized in the lower half of the table, often required multiple half presses of the shutter button to achieve focus confirmation with CAF when the image was unfocused. CAF was still more accurate even with this focus difficulty. If this was a real target, I would have switched to the larger single focus point for easier focus, but that would probably result in more focus variability.

Image #1

Image #2

Image # 3

Image #4

Image #5

Image #6

Image #7

Image #8

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drj3

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