The central AF sensor is almost always the most accurate

Started Jun 24, 2012 | Discussions thread
ThePaleRider
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Re: The central AF sensor is almost always the most accurate
In reply to SteveL54, Jun 24, 2012

Thank you. I appreciate that info.

SteveL54 wrote:

ThePaleRider wrote:

But with focal length (f) on the bottom like that, as focal length decreases (eg 35mm down to 24mm), total depth of focus must   increase unless image distance decreases by proportionately more.

Yes, I understand the image distance does decrease at a greater rate for a given decrease in focal length. Otherwise the two formulas would disagree and it's not stated that they employ different sets of simplifying conditions (e.g. one for macro and one for normal)

Also note the comment in that article pertaining to the use of rear filters with wide angle lenses. They are more likely to disturb focus accuracy with wide angle lenses.

From that article, it seems that your assertion pertains more to macro photography, where magnification is greatest, but I certainly see where you are coming from.

Well, depth of focus is discussed more in macro and microscopic subjects, but that equation isn't simplyfied for macro. Of course they do mention just below that the m term goes to zero for many distant subjects.

However for subjects at near distance the impact on focusing precision seems significant.

Another more lengthy reference can be found here:

http://www.trenholm.org/

The link there permits you to download the document, "The Ins and Outs of Focus"

It has a nice example on document page 55 (reader page 60) illustrating how a slight film position error affects the focus accuracy of an image for given focal lengths used.

There also happens to be a recent post by Joe W about the topic associated with the question as to why wide angle lenses are so prone to focus errors despite depth of field being so large.

http://forums.dpreview.com/forums/read.asp?forum=1021&message=41768731

I can't confirm the numbers, but it helps illustrate the issue.

Steve

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