D300 Fine Focus Adjustment and Depth of Field

Started Dec 8, 2007 | Discussions thread
SteveL54 Senior Member • Posts: 2,709
Re: D300 Fine Focus Adjustment and Depth of Field

Bmark wrote:

I have read several threads and have not yet seen this discussed. As
I understand depth of field, it extends 1/3 in front of perfect focus
and 2/3 behind. Shouldn't that be considered when making a fine
focus adjustment for any given lens? Of course, then there is the
issue of making that measurement.

Yeah, as mentioned by bagnon previously, for close distances the sharpest focus will be near the middle of DOF.

I also have seen some people mention that the 45 degree target
approach is not an accurate way to measure whether or not fine focus
is needed. I suppose that is because of dept of field? If not,
could someone explain?

The AF sensors are sensitive to detail across a strip that extends outside the marked brackets in the viewfinder. I did a quick check and found the D300 central sensor can respond to an edge positioned about half the height of the marked bracket above or below it. So the overall height is about twice as indicated by the bracket.

If one focuses on a page of text running at an angle to the sensor it can pick up mutiple rows of text. Each of these rows will be a different distance from the camera's AF sensor plane. So the AF system may not 'pick' the same row of text that you think it targets.

However, when I've tried this the AF system generally picks the middle, but sometimes seems to choose a line one up or down from the middle. If a line is sharp that is well outside the AF zone then something is not right.

It's best to avoid any such ambiguity in a 45 degree test by designing a target zone on the sheet that has only one horizontal line running across it. Here's an example.

http://focustestchart.com/chart.html

Steve

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