Archived posting to the Leica Users Group, 2001/02/09
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[...follow on from part 1]
Infront of the plane of focus and behind the same, you will have a transfer
function that is not a steep cut-off, but which describes a curve instead.
Take the smallest imaginable increment further away from the plane of focus
towards the camera lens, and there will be a difference. This is true of
both sides of the plane of focus. Everything infront of the plane of focus
that is going to be progressively more fuzzy, as is everything behind it (at
some point. There is no objective DOF. We cannot measure any area which is
independent of magnification where we do not have this gradual decrease in
contrast between the grid lines and the background (up to the limits of our
measuring instrumentation: theoretically this holds true for any
infinitessemally small step away from the plane of focus). The only way we
can see DOF is if we are viewing it at a magnification where details in the
film fall below the level of human acuity. This will be true regardless of
what apperture you're using, although the change in contrast change will be
different for different apertures.
Now, with a physical film, what you will have succeeded in doing is showing
(a) where the plane of focus is located and (b) that the physical
manifestation of the mathematical concept of a plane of focus is not
infinitely thin, because film is not infinitely thin. The latter of which,
BTW, have any bearing on DOF (apparent zone of sharpness).
>> Note, however, that depth-of-field is NOT the same as resolving power of the
>> film.
>>
> Agreed, but they can inter-relate.
>
Again, an unsubstantiated statement. How and when do they interrelate?
Specifically, for the case which you are claiming exists (objective DOF) how
do they interrelate?
>> <snip> Their answers will change, although nothing execpt magnification has.
>>
> Of course, that has never been disputed, and it isn't relevent to what I have
> been saying.
>
Of course it is! It's central! The standard definition of DOF is the field
of acceptible sharpness in the object space around the focal plane, as
manifested in a photograph. You're claiming that it somehow exists
independently of an observer and can be measured. Yet I showed that it
doesn't. Simply dismissing it is not sufficient, you need to either show
how my explanation is incorrect, or how what you are talking about is
conceptually different from what I illustrated.
>> 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.
>>
> No, the DOF will not be different. Only the calculated DOF will be different,
> since you changed one of the values of the equation. The ability of the same
> person to 'resolve' parts of the same image ('measure' actually) from the same
> distance will not change because you changed a variable in your calculations.
>
Again, you have failed to show that there is any kind of DOF other than
subjective DOF. You presented a method for measuring the resolving ability
of the film, but that was all. You have not shown the separate existance of
the "other" DOF. You have not shown how it relates to the standard accepted
definition of DOF. You have not shown how it is an objective property of
the world.
In fact, you have given a very strong argument for the fact that DOF is a
subjective property: "the ability of the same person to 'resolve' parts of
the same image from the same distance will not change." If we imagined
human acuity to be 10 times better, the DOF would change as a result of
this. If we got two subjects, one with eyesight twice as good as the other,
their reports of what the DOF is would be different.
This is indeed the very definition of DOF. This is what "subjective" means:
dependent upon the presence of and perceptual ability of an observer. DOF
does not exist independent of an observer. In fact, we only experience it
because at many (typical) magnifications, the eye's ability to resolve
detail is less than the film's. If it was not, DOF would not exist at all,
subjective or not. Ever. The equations just reflect this fact by the
chosen size of the CoCs for different levels of image magnification and the
standardization of viewing distance.
>> If DOF is an objective property, how do you explain the two examples above?
>>
> I don't know what else to explain to try to get you to get it. I don't know
> what your background is, but I can only guess it isn't in engineering or
> science. That's not meant rudely at all, BTW.
>
Well, it certainly comes across as rude: The innuendo in your two first
senteces above is that I am either too stupid to understand what you
consider to be crystal clear explanations, or I'm too uneducated to
understand them. Neither implication is particularly flattering, especially
for someone whose job is within research and education.
And no, you're not going to pull me into a schlong comparison contest.
Suffice to say that I have a suffient background in technology,
science, and philosophy to participate in this discussion. I don't know
everything about optics, I don't know everything about photography, but I do
have a desire to understand and learn more. When I come across someone who
emphatically states that which apperas to fly in the face of accepted
knowledge, it sparks my interest. However, until you can provide something
more substantial than blanket claims, new undefined terminology, or
innuendo, I remain unconvinced that your understanding is greater than mine
in this matter.
As for how to explain so that I "get it", I asked you to substatiate your
claims by providing evidence and explanation for two simple points, which
you have not yet done. I will repeat them for your convenience:
(1) How the concept of depth-of-field can exist independent of an observer.
This, in turn, means showing that:
a) the size of the CoC is independent of the human perceptual system;
b) how depth-of-field can be objectively measured. Not calculated, but
measured, and how the *value* of that measurement is obtained independent of
the peculiarities of the human perceptual system.
c) how the concept of depth-of-field would be unaltered if the human
perceptual system were, say, 100x better than it actually is.
Note: (c) will follow from (b).
(2) How what you call "optical" depth-of-field relates to, and is
conceptually different from, what you call "consciousness of DOF". This
would require you to show:
a) that there is a difference between CoCs on the film resulting from
depth-of-field and those resulting from a misaligned film plane.
b) how these two are independent of each other.
c) that the "optical" component of these is independent of viewing
magnification, but the "consciousness" component is not.
If you answer these points, I will be perfectly satisfied. I'll happily
revise my understanding of DOF if indeed it turns out to be incomplete or
incorrect, but I will not do so on unsubstatiated grounds.
Respectfully,
M.
- --
Martin Howard | "It is undesirable to believe a proposition
Visiting Scholar, CSEL, OSU | when there is no ground whatever for
email: howard.390@osu.edu | supposing it true" -- Bertrand Russell
www: http://mvhoward.i.am/ +--------------------------------------------
- --
Martin Howard |
Visiting Scholar, CSEL, OSU | It's not who you are, it's who
email: howard.390@osu.edu | you know that counts.
www: http://mvhoward.i.am/ +---------------------------------------