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[OM] [OT] What (exactly) is depth of field?

Subject: [OM] [OT] What (exactly) is depth of field?
From: "Higgins, Michael (Ex AS01)" <MHiggins@xxxxxxxxxxxxxxxx>
Date: Thu, 11 Mar 1999 17:52:06 +1000
Sorry for asking a fairly general lens question on the Oly list, but you all
seem like such a nice bunch I didn't think you'd mind.  And know there are a
lot of experts out there, so I'm sure to get some valuable answers.

I understand that depth of field is the range of distances that are in focus
for a lens / aperture combination, but don't understand the nature of how
focus varies in this range and the range extending of either side.  Is there
only one distance from the lens that is EXACTLY in focus with degrees of
being out of focus on either side? and that at a particular, arbitrary point
this degree of out-of-focuses is considered unacceptable? and this is
defined as the boundary of the dept of field?  And this limit is rather
arbitrary? - who decides it?  
Or is there some characteristic of lenses that starts to put objects badly
out of focus at these well defined points, while between these points the
focus remains very good?

Also, is depth of field a constant for a particular focal length, aperture
combination regardless of lens design?  Or do different lens of the same
focal length exhibit different depth of field characteristics, and if so is
this feature ever tested in lens tests?  (previous discussions of boche have
led me to believe that some out-of-focus blurs are better than other
out-of-focus blurs)

Or maybe I should just go find a stuffed bird and decide for myself the
answers to these questions. [almost OM content]

Mike

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