The central AF sensor is almost always the most accurate

Started Jun 24, 2012 | Discussions thread
SteveL54 Senior Member • Posts: 2,709
Re: The central AF sensor is almost always the most accurate

ThePaleRider wrote:

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):

Sorry my son was bugging me about running an errand so my explanation below was rushed and probably not that clear.

Go to the wikipedia page on depth of focus:

See the section on calculations. The total depth of focus is 't'.

Look at the equation in terms of image magnification. As you can see total depth of focus increases with increasing magnification when CoC and f-stop are held constant.


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.


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