Re: [OM] Hyperfocal focussing?

[John A. Lind]

At 11:15 7/1/02, Sean Davis wrote:
> (de-lurking)
>
> Could someone do me the favor of explaining the concept of
> "hyperfocal
> focussing" (don't know if i have the term right).  I have a
> small idea
> of what it's about, but don't really get it.  Does it have anything
> to
> do with the depth-of-field scale on a prime lens?  Every time i try
> to
> use that, i simply end up with a small focussed part in the middle,
> and everything else out of whack.  Confused!!
>
> Sean D.

Took me a while to understand it completely too.  The principle of
"hyperfocal focusing" is based upon depth of field.  For a
given focal length and a given critical focusing distance, as a lens is
stopped down from wide open, the depth of field increases in front of and
behind the critical focus distance.  However, it grows faster behind
the critical focus distance than in front of it.  Stop down the lens
sufficiently and the depth of field behind the critical focus distance
extends to "infinity."  For relatively close focusing
distances, one might have to stop the lens down much farther than it's
designed for (e.g., f/128).

For each focal length and f-stop combination, there is a critical focus
distance at which the depth of field behind it just barely extends to
infinity.  This is the setting at which the depth of field has its
greatest.  Focus closer and the rear boundary for DoF is less than
infinity and it shrinks faster than the front one moves.  Focus
farther and the rear boundary doesn't get any farther than
"infinity" but the front one moves farther from the camera
position.

One other item that hyperfocal distance depends on:  Maximum
Allowable Circle of Confusion Diameter.  The only things truly in
focus are in a plane parallel to the film plane at the lens' critical
focus distance.  Everything else varies from not quite in focus to
grossly out of focus.  The human eye has an acuity limit (about 1
arcsecond).  Therefore, something must be sufficiently out of focus
for the human eye (and brain) to discern that it is out of focus. 
If this is reduced to discerning the difference between an
infinitessimally small pinpoint or a spot with finite diameter, this is
what defines the maximum allowable circle of confusion diameter
_on_the_film_, and the exact location for the front and rear DoF
boundaries.

The reason for some discussion about maximum allowable circle of
confusion diameter is the danger in blindly using published hyperfocal
distances for focal lengths and apertures, or one given by some software
utility (on-line or otherwise).  All of these make assumptions about
what the maximum allowable circle of confusion diameter is.  The
value used for it is a *subjective* choice based on intended image
enlargement (from the film) and its intended viewing distance.  If I
make a 4x6 print from a 35mm negative (about 4X enlargement), and view it
from 14 inches, the DoF will appear to be greater in it than it will in
an 8x12 print from the same negative (about 8X enlargement), and view it
from the same distance.  This is a _very_important_ consideration
when deciding to use hyperfocal focusing to maximize depth of
field!  I have seen 4x6 proofs of landscapes look fine and 8x12 or
11x16 prints "fall apart" with noticeable lack of sharpness in
detail with very distant objects as a result.  Hyperfocal focusing
is *not* a panacea, but a tool to be used carefully with an understanding
of its pitfalls.  My "serious" 35mm photography is
intended for large prints (11x16; about 12X magnification) and/or for
projection to even greater magnification (compared to viewing a 4x6 at 14
inches).  I very rarely use hyperfocal focusing for landscapes,
scenics  or architectural photographs.  On those occasions when
it is used, it is done with great care, very conservatively (very small
CoC diameter), and "safety" shots are made with focus set to
infinity.

I have more on hyperfocal distance here:
 
http://johnlind.tripod.com/science/sciencelens.html#Hyperfocal
Distance

-- John


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