[Leica] Re: Depth of field... (more than you wanted to know)

["Michael D. Turner" <mike@lcl-imaging.com>]

Jim, and Paul-
"Infinity" has a more precise definition: when the lens-to-image distance is
equal to the focal length of the lens, the lens is said to be focused at
"infinity". Period. Light rays coming from an object infinitely far are
parallel and will be focused to a point. Light rays coming from nearer objects,
including alpha centauri, produce circles of confusion. If a lens manufacturer
claims infinity as a "few hundred meters", he means that objects a few hundred
meters away produce circles of confusion smaller than some given arbitrary size
(usually unspecified). The whole issue of DOF centers around what, for your
particular application, acceptable sharpness, etc. define to be the size limit
for circles of confusion for objects in the scene to be "in focus". Markings on
lens barrels are established by manufacturers to their own arbitrary standard,
which may, or may not meet your own needs.

If you visit my ftp outgoing directory ftp://lcl-imaging.com/outgoing you will
find four single page pdf's which might shed a little more light on the
subject. You might note that I didn't say much about how one might define the
circle size limits. That's an interesting subject all by itself.

- -Mike

Jim Brick wrote:

> Paul,
>
> What I recited is not only what I learned in school, 40 years ago, it is
> exactly the same thing that is written up in ALL good photographic theory
> texts. And you have to remember that GIB did not ask about "hyperfocal"
> focusing. Hyperfocal focusing still follows the optical law... acceptable
> blur (Circle of Confusion). So the hyperfocal point, and associated "in
> focus" area, will change depending upon whether you want acceptable images
> or "dead sharp" images. The hyperfocal point is special "only" because it
> references infinity. But in photographic terms, a lens manufacturer
> specifies infinity as a "few hundred meters." Not really infinity. So all
> of the DOF laws are in tact. Even when using the hyperfocal point. Which
> will probably be different for me, than for you.
>
> Jim
>
> The word "infinity", in photographic terms, is misleading.