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# The central AF sensor is almost always the most accurate

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

Sorry to continue side-tracking this, but are you sure about this?

My understanding of field versus focus depths are thus (Wiki provides a nice diagram):

http://en.wikipedia.org/wiki/Depth_of_field

A little before half way down, under the section "Derivation of the DOF formulas", there is a simple diagram with Df, Dn, vf and vn annotations. My understanding is that (Df - Dn) is the depth of field and (vf - vn) is the depth of focus and that there is a linear relationship between the two: the magnification factor of the system. If the scene behind the lens is smaller than the scene in front of it (as it normally is for an SLR) then the magnification factor will be less than one (obviously) and the depth of focus will be less than the depth of field, but it will be proportional to it, so as depth of focus increases, so does depth of field .

I do not believe that depth of focus decreases as depth of field increases , quite the opposite in fact. My understanding is that depth of focus increases as depth of field increases , and depth of field may be demonstrated to increase as focal length decreases.

Depth of focus/field both shrink as focal distance decreases, as they do when aperture size increases. This is obvious from the diagram.

SteveL54 wrote:

ThePaleRider wrote:

As an aside for a moment, I am not sure that I understand your distinction between depth of field and depth of focus . They both represent the same thing in the two very differently sized scenes; one in front of the lens (depth of field) and one behind it (depth of focus). Is there not a linear relationship betwen the two, the same as all other dimensions in both scenes, which renders any distinction pointless other then in terms of semantics?

Depth of focus has a reciprical relationship to depth of field as it relates to image magnification. Depth of focus actually decreases with decreasing focal length for a subject at constant distance. The decreased depth of focus normally encountered with ultra wide angle lenses actually makes lens positioning relative to the sensor more critical than with telephotos.

Steve

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