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Thread: Microadjustment, what is happening?

  1. #1
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    Default Microadjustment, what is happening?

    After reading Artie's excellent tutorial on this subject I am wondering what exactly is being "adjusted" during this process of microadjustment. Does anyone know exactly what it is inside the camera and / or lens that we are adjusting?

  2. #2
    Flavio Rose
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    A number is written in nonvolatile storage of the camera, the number being specific to a particular lens. The manufacturers don't say much else. Whenever the camera focuses, it tells the lens in some way where to place the lens' focusing element -- that's the meaning of the AF/MF switch on many lenses, i.e. should the camera say where to place the focusing element or not. This indication the camera gives of where to place the focusing element is presumably adjusted in some way based on the recorded microadjustment number for that lens.

    I seem to remember reading in Chuck Westfall's now discontinued "Tech Tips" that the range of microadjustment is a few times the depth of focus, but I can't put my finger on that statement now.

  3. #3
    Flavio Rose
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    It's possible that what I said is wrong.

    Here is a statement by Chuck Westfall in the July 2009 Tech Tips:

    "One important concept to understand about the EOS autofocus system is that focus confirmation is based on successful completion of the lens drive command. This command originates in the camera body as raw data about the degree of defocus at the focal plane, but it is modified by the CPU of the lens that's mounted to the camera, based on a number of different factors. One consequence of the EOS design is that the resulting image may be out of focus if the lens is not operating completely within its design tolerances. Recent EOS models including the 1Ds Mark III, 1D Mark III, 5D Mark II and 50D have AF Microadjustment settings that allow users to calibrate their camera according to the lens in use."

    I don't quite understand what is meant by "raw data about the degree of defocus at the focal plane."

    The guys at Sigma, Tamron, and Tokina have figured this out (presumably by creating a piece of electronics that snoops on the traffic between lens and camera), but I bet that isn't very easy to do.
    Last edited by Flavio Rose; 01-12-2011 at 05:18 PM.

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    Here's a very simplified explanation of phase detect autofocus that leads to some easy guesses about how the focus tuning works. Phase detect autofocus works like binocular vision, looking the image from a section on the left and right sides of the lens. And a very important point for Nikon users - I'm not sure about Canon.

    If you imagine light rays from the subject going to each side of the lens then back to be focused on the sensor you get an elongated diamond shape. If the focus is behind the sensor then the light from the left is shifted to the left and the light from the right is shifted to the right on the sensor because the rays have not come together yet. If the focus is in front of the sensor then the light from the left is shifted to the right and the light from the right is shifted to the left because the rays have crossed and are diverging at the sensor.

    The phase detect focus sensors take a horizontal slice of the image from the left and right light rays - the same part of the subject but different light from the subject. They look for patterns to see if they are aligned, and if not which way they are shifted. Then they know which way to move the focus and how much - both of which are key to why phase detect autofocus is so fast. The body tells the lens how far out of focus it is and the lens decides how far to turn the focus motor.

    For the tuning the body simply lies to the lens by the amount of the tuning.

    The important point for Nikon users is to not use tripod mode live view when doing focus tuning. That uses a completely separate contrast detect focusing system. Probably best to avoid live view altogether for safety.

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