Re: [Leica] Re: DOF -Optical vs Apparent

[Martin Howard <howard.390@osu.edu>]

Austin Franklin jotted down the following:

> DOF is basically a limit of resolution.  Whether it is lense resolution,
> film resolution, paper resolution or the resolution of the eye.

Specifically, the eye, as it relates to the viewed image.

> One way to measure the DOF, is to take an image of a (very fine) grid on a
> slant.  I would believe you need to choose the grid based on which
> resolution you are trying to measure, film, paper (which is only one form of
> replication that one can choose), or eye.

And how would you choose it independent of the eye?

> I would suggest using a very coarse grain film for this example, since you
> will easily reach the limits of the film with most any Leica lense...and it
> will make the 'measurement' easier.  The finer grained the film, the less
> the DOF will be on the film.

So, lets assume that we have a theoretically perfect film.  It can resolve
infinitely well, which means -- by your token -- that there will be no DOF
on the film.  Yet, clearly, there are going to be areas which to the viewer
appear to be in equally in focus, unless we look at the film as such high
magnification that it becomes meaningless to talk about an image any longer.

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.

> 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.  Which requires something to be
viewed at a particular magnification and physical distance from the eye.
Which leads us right back into CoCs, which leads us directly to the issue
that DOF is subjective and cannot exist independently of the observer.

Note, however, that depth-of-field is NOT the same as resolving power of the
film.  If your making this claim, then all you are doing is taking the old
concept of "resolving power of the optical system" and calling it "depth of
field", which not only is not helpful, but it is contrary to the established
definition of depth of field and contrary to just about everyone else's use
of the term in the photographic field.

Example one: take a picture of some objects, with some in the foreground out
of focus and some in the background out of focus around one or more objects
in the plane of focus.  Use any film, lens, film format you like.  Make a
print of a given size.  Display this print on a wall and let an observer
stand, say, 1m away from it.  Ask them which objects are in focus and which
are not.  Then let them move 0.5m closer to the print and ask them again.
Their answers will change, although nothing execpt magnification has.

Example two: Take exactly the same negative and calculate the DOF using the
established formulas.  Let's assume it was taken with a 35mm camera and
therefore we use a CoC size of 0.033mm.  Note what the near and far ranges
of the DOF are.  Now, substitue a CoC size of 0.020mm in the formulas.  You
will find that you now have a completely different DOF range (narrower) yet
absolutely nothing else has changed.  We can measure every damn thing we
choose about the negative, the print, the camera, the focal length,
aperture, film resolution, MTF resolving power, distance to subject,
lighting conditions, or anything else and they will be identical for the two
cases: yet the DOF will be different.

If DOF is an objective property, how do you explain the two examples above?


M.

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Martin Howard                     | iCon          iDole       iRate
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