Archived posting to the Leica Users Group, 2001/02/06

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Subject: [Leica] DOF DOESN'T EXIST! (was: how can the 75 Lux focus go bad?)
From: Martin Howard <howard.390@osu.edu>
Date: Tue, 06 Feb 2001 23:25:46 -0500

Austin Franklin jotted down the following:

>> It is rather difficult to measure distance in a 2D print. I have a tulip,
>> photographed at f/1.4, at the closest focusing of my 75mm lens (~2'.) The DOF
>> is at best, the thickness of the surface skin (cells, protoplasm, whatever)
>> of the tulip. I would measure this as a few thousandths of an inch.
>> 
> I find that simply amazing.  As I said, this is my primary lense, and I have
> thousands of images shot at ~3' and the DOF on ones I looked at tonight show
> at least 1" or more, and the subject matter has detail enough (clothing
> texture) to easily show this detail.

There IS **NO** OBJECTIVE BASIS for depth-of-field!  You could both
argue/compare until you were both blue in the face and you would still BOTH
be right.  Not to mention the fact that you are trying to somehow
extrapolate common experiences from different subject matter (tulips vs.
people), different focusing distances (2' vs. 3'), different observers
(Austin vs. the person whose name Austin forgot to include in his message
;), and probably different films, different lighting conditions, different
developers, different papers, possibly even different mediums (paper vs.
trannies, trannies vs. computer screens, last weeks memory vs. a wall-mural)
all of which can affect the SUBJECTIVE experience of what is sharp and what
is not (which, for practical purposes, is what DOF is).

Yes, you can "calculate" DOF, but all that does is provide you with some
numerical estimation of roughly what the "average" or "normal" person will
consider to be the zone of acceptible sharpness within the subject matter
for a given print, of a given size, viewed at a given distance, made from a
negative of given film format.  It is *not* an absolute.  It does *not*
guarantee that if person A considers print X, on format Y, by lens L, at
aperture F, and focusing distance D of subject S, to exhibit Z inches of
DOF, that person B will necessarily see it the same way.  In fact, given
human variability, it is almost universally bl**dy guaranteed that they will
not and will instead spend 400 messages arguing about who's right on the
LUG, when instead we could be arguing about single-malt Scotch and if Ted's
underwear is large enough to be used on the front element of a modular 400mm
f/2.8 APO-Telyt-R lens head!


OK, time to climb off the hobby-horse... ;)

M.

- -- 
Martin Howard                     |
Visiting Scholar, CSEL, OSU       | If you have to drink and drive, make
email: howard.390@osu.edu         | it a latte...
www: http://mvhoward.i.am/        +---------------------------------------

Replies: Reply from Mark Rabiner <mark@rabiner.cncoffice.com> (Re: [Leica] DOF DOESN'T EXIST! (was: how can the 75 Lux focus go bad?))