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>http://johnlind.tripod.com/science/sciencelens.html#Hyperfocal
Distance
-- John
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