Micro AF Adjustment -- Would this work?

Started Feb 13, 2009 | Discussions thread
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Jerry Gardner
Regular MemberPosts: 125
Micro AF Adjustment -- Would this work?
Feb 13, 2009

Reading through the owners manual of my 1Ds3, I was thinking of how to quickly set the AF micro adjustment for each of my lenses. I came up with the following procedure, which I haven't tried yet, but which should work and should be faster and just as accurate as other methods such as LensAlign, and doesn't require even looking at the images to determine best focus.

What I'm thinking of relies on how JPEG compression works. It uses discrete cosine transformation on 8x8 blocks of pixels, and this process tends to preserve high-frequency spatial information in the compressed file. As a result of this, the file size of images in sharp focus should be larger than files out of focus.

So the procedure would work as follows:

1. Set the camera to the maximum quality JPEG setting. Set manual exposure, any white balance setting other than auto, mirror up mode.

2. Mount the camera on a sturdy tripod.

3. Point it at a flat, perpendicular target with lots of high-contrast detail (a resolution chart would work well) that fills the frame. Set the lens aperture to maximum (e.g 2.8). Light the target with a constant light source--you want as little variability here as possible. Adjust shutter speed to achieve proper exposure.

4. Set single-shot AF and defocus the lens before each shot (e.g. by putting your hand between the lens and the target and letting it focus on that) to force the camera to focus each shot from scratch.

5. Take a series of shots varying the micro AF microadjustment from -20 to +20. Trip the camera with a cable release or use the self timer to prevent camera movement.

6. Inspect the file sizes of the images using the shooting info display in the camera's playback mode. The image with the largest file size will be the sharpest--set the AF microadjustment to the value used to create this file.

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