The central AF sensor is almost always the most accurate

Started Jun 24, 2012 | Discussions thread
ThePaleRider Regular Member • Posts: 251
Re: The central AF sensor is almost always the most accurate

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.

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.

SteveL54 wrote:

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):

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

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:

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

Steve

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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