Archived posting to the Leica Users Group, 1998/05/24

[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]

Subject: Re: [Leica] Fast M6 Focusing - Was Something Else!
From: Paul and Paula Butzi <>
Date: Sun, 24 May 1998 10:22:21 -0700

At 05:48 AM 5/24/98 , you wrote:
> Even if you have a 25 foot zone of apparent focus, there is still only a
> sliver  of that zone that is in perfect, super sharp focus, right? >>
>Correct - There is only one plane that is in perfect focus.  In front and
>behind that plane is an area of "accceptable focus" - which means that it is
>out of focus but by an amount that is acceptable.  Of course the question is
>what is acceptable. 

If you're talking geometric ray tracing, this is true.  In the world of real
photos, optical resolution (which is different from sharpness, see below)
is the convolution of the effect you describe, and the effects of
diffraction.  This resolution is convolved with the resolution of the
medium as well.  In practice, this means there is a zone of non-zero depth
which for all intents and purposes has more or less constant sharpness
throughout.  The plane of 'perfect' focus does not appear in the resulting
photograph as infinite resolution, right?

Beware of equating such resolution with 'sharpness'.  The sensation of
'sharpness' occurs in your nervous system.  It is affected by
many factors, including overall image contrast, color, the viewing light,
the texture of the surface on which the image is presented, your health,
etc. etc. etc.

- -Paul