why micro focus adjustment ?

Started Feb 25, 2013 | Questions thread
Allan Olesen
Senior MemberPosts: 2,613
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Re: why micro focus adjustment not in the camera ?
In reply to Chimere, Feb 25, 2013

Chimere wrote:

However, people in this forum talk about the lens getting calibrated.

There are some lens specific probable causes for focus error.

First there is focus shift when changing aperture. There is a discussion elsewhere in this thread about that topic.

Then there is chromatic aberration. If the red, green and blue light is not focused in the same plane, and the PDAF sensors are more sensitive to one wavelength, there may be a focus error on lenses with heavy CA. This is purely my own theory, and it may be entirely wrong. But it is well known that the necessary micro adjust tends to change with the color temperature of the scene, so that somewhat supports my theory.

And then there is a third possible cause which I have been told about and don't know if I should believe. Apparently, most focusing with AF is one try only: The camera PDAF sensors measures the exact focus error in the image, calculates how much the focus mechanism should rotate, rotates the mechanism exactly this amount. And then it takes the photo, no matter if best focus was achieved. So if the prediction of necessary turning of the mechanism was wrong, you will end up with a focus error.  The relationship between the focus error measured by the PDAF sensors and the necessary rotation of the focus mechanism is of course lens specific, and this relationship is described in data tables which are stored electronically in the lens. So if the prediction was wrong, you will have to change the contents of these tables.

Regarding the last explanation, I think there are several weak points. For example, one should expect that if a lens has front focus when it is first focused far away and then focused on the subject, then it should have back focus if it was first focused near and then focused on the subject. And the idea that the focusing is not at all an iterative process also amazes me.

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