DOF Question

I'm trying to understand how the depth of focus figures are arrived at.
I would assume that an absolute point (distance) is "in focus" and a region before and after that point is referred to "field of focus"...hence the depth.

I would assume that focus degrades either linearly or exponentially from the true focul point so what criteria is used to decide the boundary where it ceases to be in focus.

Obviously there is "visual perception" of what is in focus but there must also be a scientific measurement or an accepted level of distortion (blur) where the image is "out of focus".

Am I being dumb and missing something?

Bob

Not sure I understand your question but maybe this helps a lens at 2.8 will have a shallow DOF over setting the same lens at F11 If you have a stationary obj you can experiment with DOF by changing your Aperature from wide open to closed and see the difference and get a feel of what you like per the situation and then use this experiment on other objects stationary or not.

others will chime in with more knowledge

http://www.luminous-landscape.com …standing-series/dof.shtml

This might help.

James,

I understood what the term meant, just not how it was arrived at scientifically.

The link provided by Shutterfiend has answered my question. It is the circle of confusion thta is used to determine where the limits of DOF are set.

Thanks guys....Bob

For those who hate math or overly techie explanations that confuse more than they clear up...

The principle of DOF is that a perfect point is a perfect point when in focus, but the point is represented as an increasing large circle when goes more and more out of focus. The eye has a certain minimal size which it can see, where anything up to that size circle is perceived as 'equally sharp', as defined by a certain angular measurement. When the point turns into a circle where the eye can perceive it as 'out of focus', that is the Circle of Confusion which is acceptable sharpness.

So the size of the COC is dependent upon the end magnification of the image and the viewing distance of the image. If the angular measurement stays under the limit of the eye, the eye sees it as a point, even if the absolute size of the point (or COC) is actually very large. (Billboard photos are not tack sharp, they only are acceptable at the normal viewing distance!)