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Old December 26th, 2007, 10:37 AM
Doug Kerr Doug Kerr is offline
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Join Date: May 2006
Location: Alamogordo, New Mexico, USA
Posts: 8,625

Hi, Bart,

Originally Posted by Bart_van_der_Wolf View Post
What happens as one manually adjusts the focus is that at the exact optimal focus setting the background will change from uniform gray into larger (colored) aliased dots when viewed on the camera's LCD. The circles and cross hair will allow to acquire AF easily, and when calibration is optimal, moiré will be maximized on the camera's LCD.
How neat!

. . . otherwise the defocus will act as a low-pass filter and prevent the aliasing.
Of course! What the moiré does is allow us to visualize the higher-frequency components in the image (just as many so-called "contrast detection" AF systems actually do), and of course they increase as the "low pass" effect of the defocus spread function declines.

This of course makes perfect sense as moiré is a visible manifestation of aliasing, which occurs from high-frequency components beyond the Nyquist limit for the system. Less HF content - less moiré.

The flat computer screen will prevent misinterpretation of the focus distance, because the AF system cannot react to phase effects from subjects at other distances. That makes it quite easy to get repeatable results. And because the computer LCD emits light, it can be easily done indoors, at common shooting distances for the lens to be calibrated for.
Very clever!

Brilliant, my friend.

Best regards,

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