Archived posting to the Leica Users Group, 2001/02/09

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Subject: Re: [Leica] Re: DOF -Optical vs Apparent -- Part 1
From: Martin Howard <howard.390@osu.edu>
Date: Fri, 09 Feb 2001 19:39:07 -0500

The LUG server limites messages over 10,000 characters -- maybe it's trying
to tell me something ;) -- so this is posted in two parts.  This is part
one.

austin@darkroom.com jotted down the following:

>> Also, the films of today are much better at resolving fine detail than the
>> films of 30 years ago -- yet our perception of DOF has not changed much,
>> which would run counter to your argument.
>> 
> How does that counter my argument?
> 
Because you're making the claim that DOF is an objective property.  If the
materials have changed, then DOF should have changed too.  Yet I can shoot
the same scene on new film, printed on new paper, shot with a new lens, at
the same aperture, printed at the same size, and viewed from the same
distance.  I will not see any difference in DOF.  Contrast will probably
change, meaning the prints will undoubtedly look a little different, but in
terms of DOF, they will be the same.

As for the claim that APO/ASPH lenses and new films require recalculation of
the DOF, it doesn't hold up.  If something is below the limits of human
actuity, then it is below the limits of human actuity.  It doesn't matter
whether it's just under or 100x under, we will still not be able to see it.

Which means that people are in all likelihood misunderstanding what DOF is
and making larger prints or looking more closely at prints of the same size,
which violates the assumptions behind the calculation of DOF (which were
used to mark the lenses).  Magnification has changed, which means that you
need to compensate when taking the picture by, say, moving the infinity mark
to the f/8 index when actually shooting at f/11.

The *only* time that it will appear to the viewer that the DOF is different
in these two prints is if in the original old print there were details that
were not being resolved by the film, which *are* being resolved by the new
film.  In other words, if the limits of the materials are below that of the
human perceptual system FOR THE MAGNIFICATION IN USE!!!  Again, change the
print size, DOF changes.  However, as we shall see below, this does NOT mean
that DOF exists independently, or is an objective property of either
negative, or prints.

>>> To determine the DOF of the film, simply view the film and see where the
>>> grid runs together.
>>> 
>> Ah -- and therein lies the crux, n'est pas?  "see where the grid runs
>> together".  Which requires perception.
>> 
> Er, in reality, no.  You are going to argue that everything requires
> perception or it doesn't exist or something like that.
> 
No, I'm not going to push this into the subjective/objective philosophical
debate.  That would be a cop out.  But, if it doesn't require perception,
what measuring device would you use that is independent of magnification?

> It is simply a matter of the impact the 'area of confusion' has on your
> result.
> 
The what!?  What is an "area of confusion"?  This is the first time the term
has entered this discussion and it has, AFAIK, no definition.

> That does not  negate the ability to measure it, it only adds error to your
> measurement.  I believe the 'area of confusion' will be negligible to the
> overall measurement that can be achieved.
> 
OK, first off, you're going to have to define "area and confusion" and how
it relates to DOF.  Do you mean the CoCs, or something else?

Second, what you may or may not believe is frankly irrelevant.  If you want
to convince others that your understanding of something is correct, you need
to provide convincing arguments for it.  You need to be able to present how
it relates to other known knowledge, what the implications are, and also
account for existing phenomena.  Simply stating that you believe something
doesn't cut it.  I, personally, believe that there is no such thing as "area
of confusion" and that is has absolutely not relevance to this discussion at
all -- but I'm willing to bet a rather large sum of money that my belief is
not sufficient to convince you that I'm correct.

>> Which requires something to be viewed at a particular magnification and
>> physical distance from the eye.
>> 
> No.  It converges, whether YOU view it or not.
> 

Well, we really have two types of "convergence" going on here.  First, the
grid pattern will converge continuously, regardless of where you've focused
the lens.  This is the result of the projection of a receeding textured
object onto a two-dimensional plane.  You will be able to see this
convergence up to the limits of film resolution, after which it will just
register as an even tone.

Second, the "convergence" which I assume you are referring to isn't actually
convergence.  What it is is the effects of focus.  Let's assume we have a
black grid on a white background.  At the exact point of focus, the drop off
in intensity between the white and the black will be very sharp.  Plotting
intensity against space (orthogonal to the direction of the grid) we'd get a
perfect shift from 100% black to 100% white with a theoretically perfect
lens and film.

[Part two to follow...]

- -- 
Martin Howard                     |
Visiting Scholar, CSEL, OSU       | "Close doesn't shoot any rabbit"
email: howard.390@osu.edu         |                     -- Swedish idiom
www: http://mvhoward.i.am/        +---------------------------------------

Replies: Reply from "Henning J. Wulff" <henningw@archiphoto.com> (Re: [Leica] Re: DOF -Optical vs Apparent -- Part 1)
Reply from "R. Saylor" <rlsaylor@ix.netcom.com> (Re: [Leica] Re: DOF -Optical vs Apparent -- Part 1)