Archived posting to the Leica Users Group, 2001/02/09
[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]
At 8:41 PM -0600 2/9/01, Henry Ambrose wrote:
>http://www.ida.liu.se/~marho/dof/dof.html
>
>**********quote from above article***********
>The presence of aberrations, especially spherical, does increase depth of
>focus, and hence depth of field as well. The reason is that the rays from
>the rim of the aperture of a spherically aberrant lens come to a focus at
>a different distance from the lens than rays from the center, so that
>there will be an image of sorts all the way from the marginal focus to
>the axial focus, although at no point
>will the image be clean and sharp. Incidentally, this explains why simple
>lenses can be used on box cameras with no focusing mechanisms; a well
>corrected could not be used this way because it would have too little
>depth of focus.
>******end quote****************
>
>I think this is the best explanation for why the newest lenses and films
>seem to not work well using old values for DOF. Its why when you follow
>the marks on the lens for DOF at a given aperture and distance they might
>not suffice.
>The easy answer is use one stop more than indicated.
>
>I'm happy to have lenses that good.
>
>Henry
Hey, we can have a thread that includes bokeh and DOF at the same
time! This should make almost everyone happy :-).
Undercorrected spherical aberration = (usually) good bokeh.
Overcorrected spherical aberration = (usually) bad bokeh.
While calculated DOF depends on a perfect lens, and perceived DOF is
affected by lens defects (and in view of recent arguments: these are
the only variations on DOF allowed by the definition of DOF)
aberrations have to be huge to be of real use on box cameras per the
explanation in the Leica Manual. The reason that the 'simple lenses'
work is not because of aberrations so much as that enlargements are
not anticipated beyond 2x.
- --
* Henning J. Wulff
/|\ Wulff Photography & Design
/###\ mailto:henningw@archiphoto.com
|[ ]| http://www.archiphoto.com