[APML]: film resolution etc

[P3D John Ohrt <johrt@xxxxxxxxxxxxxxx>]

Hi there,

I am pretty much abandoning my search for a relationship between MTR,
lpm and grain size and their contribution to the overall resolution to
the final film resolution.

But I will relate what I found out.  A good non-technical reference is
"Guide to KODAK 35 mm Films" by the Consumer Imaging Division, Eastman
Kodak Company, Rochester,NY, 1990.

In attempting to explain this in a simplified manner,  the reasons why
the relationship really isn't there becomes apparent.

Starting on p67,  Kodak starts discussion "Definition" and breaks it up
into three areas, Graininess, Resolving Power, and Sharpness.

Kodak notes that comparing the performance of film in these three
categories is only valid for films within a specific class such as black
and white negative as opposed to color negative.  Siting factors such as
diffusion, spectral sensitivity, halation, and contrast as effects that
are less of a factor among films in a class than between classes.

Kodak acknowledges that graininess does vary somewhat with the developer
when developed to the same contrast index, usually fine grain developers
reduce the film speed and vice versa and that  Graininess is more
apparent in the lighter midtones.  While the book classifies films in
terms such as microfine, very fine etc., the grain size can be measured
and is often stated by the manufacturer.

This size is inherently somewhat optomistic as it assumes the correct
exposure and development, but it is measured.

Resolving power is measured with the three parallel line test chart we
are all familiar with.  The caveat about resolving power is that the
limit is determined by the human eye and its ability to resolve
intensity in order to determine at what point the lines vanish into a
grey blurr.  The ability varies significantly person to person and the
acuity can be significantly sharpened by training.  For example it could
make a great deal of difference whether Mr. Joe Average performs this
test or an experienced radiologist.

Kodak notes this problem and also remarks that in order not to
significantly degrade film performance, the resolving power of the
optical system (lens) should be at least 3 times that of the film. 
According to Kodak, resolving power measures the ability of the film to
record fine detail.

Many manufacturers state the resolution at 1000:1 and 1.6:1 contrast. 
If only one number is stated, it will be the 1000:1 resloving power in
lines per millimeter (lpm)

Kodak considers sharpness is the subjective impression of good edge
detection between details in a photograph.

A fascinating statement when you consider that the indicator of
"sharpness" is the modulation transfer response, an objective
measurement.

Kodak does acknowledge this is their description of MTR testing.

What Kodak states next is enlightening.  They discuss a property called
degree of enlargement.  They note that under certain circumstances, any
one  of the three factors maybe the most important.

Kodak acknowledges that resolving power does not determine sharpness. 
And that when graininess and resolving power are adequate, sharpness is
the most important factor that affects definition.

To me this implies that a good MTR is more difficult to achieve than
either grain or resolving power, and that is consistant with the
doctrine optical engineers have hammered into my head.  MTR can also be
adjusted for the inclusion of each element into the system to get an
overall system MTR.  This is not true of resolving power or grain.

So I guess the optical engineers were right.  

But for practical purposes, shooting the three line resolution chart and
analyzing it by identical procedures by the same person can determine in
significant degradation or improvement has occurred with the
introduction of a new lens, or film or development technique.

That is a whole lot better than nothing.  Measuring MTR is expensive and
the effects of grain are assessable visually with modest magnification.

And for those who image 10 micron spot sizes for point sources, well
they have my admiration.

I have yet to stumble onto a film spec that quotes both resolving power
and the MTR graph for the film.  Some manufacturers quote one or the
other.  Kodak may quote either, but it seems that only the older films
have MTRs.

It would seem that there is a conspiracy to deny us the critical
information in quantitative terms, and that the only subjective data
index is the one most frequently quoted, resolving power in lpm.

This seems to be yet another case of specsmanship over knowledge.

Sure is nice to get the advice of experienced photographers.

Regards,
John
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John Ohrt,  Regina, SK, Canada
johrt@xxxxxxxxxxxxxxx
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