Re: slide vs neg

[P3D John Ohrt <johrt@xxxxxxx>]

P3D Greg Marshall wrote:
> 
> It definitely is a can of worms, which I've wallowed through before.
> First, let
> me correct a misquote:  The word I used was "accutance" not "accuracy".
> Accutance is the measure of "sharpness" as opposed to "resolution" (I
> think).

Not quite.

Accutance is measured by the modulation transfer function (response vs
lpm)

Resolving Power is measured in lpm at two different contrasts.

What I lack is an easily understood explanation of the difference.

This likely means I don't understand it as well as I think.  :-)

But I will try.

Accutance is indicator of photometric resolution (sharpness) whereas
resolving power is the indicator of spatial resolution.

Trying to state it another way, the ability to resolve gray levels is a
function of accutanace and the ability to separate lines or points is a
function of resolving power.

It is cheaper to measure resolving power than accutance to a given
accuracy,
therefore the newer media tend to only quote resolving power.  If you
look at
older specs, they may quote both, or just accutance.

Note: both resolution and accutance can be measured element by element
to closely predict system performance as well as measured at system
level.  This gives you a good cross check for workmanship.  The problem
with the Hubble is that NASA gave the assembly to a Space Flight Centre
that needed work rather than to one that had the capability to do a
system level evaluation.  Hence no cross check was possible.  This was
known and assessed prior to the decision of which center got the job. 
Once again, politics and engineering did not mix, unfortunately with the
too typical results.  But at least no one got killed this time.

A long winded way of saying periodically check your camera against a
resolution test chart.  Don't worry about if the chart is calibrated or
shot from the correct distance.  A convenient, inexpensive test is more
likely to be used, and to be useful the test need only be repeatable,
not produce absolutely accurate test results.  So get a cheap chart and
check each lens occasionally.  It is only one frame, or frame group that
is being "wasted".  If your resolution has noticeably changed, then
the accutance is likely OK as both are degraded by optical
misalignment.  Unless you misfocussed the camera, bad news awaits.  If
the resolving power has diminished, it is likely that the accutance has
been degraded.


For completeness, grain size does affect accutance and resolving power,
but that's not all.  Grain size also influences reproducability in
contact, enlarging, and reducing operations.  And in the case of 3d, can
degrade quality over and above the measurable impact of accutance and
resolving power.

If you really want to get really picky, the tests for accutance and
resolving power are idealized relative to typical camera lens focal
lengths, so that combining the data for an system accurate assessment is
more complex that it would initially appear.  But, don't bother worrying
about this.  Life is always more complicated than any superficial
explanation.


I'm wondering if a generation test (such as how many generations of
contact prints can be made before accutance and/or resolving power
degrade a measureable amount) could help assess some aspects of the
impact of grain.

OOOOPS!  Best head for T3D.  I'm away for two weeks and only monitoring
T3D, my mailbox can't cope with the volume of the other two conferences.


In the meantime, all film manufacturers that I am aware of do not quote
all of grain, resolving power, and accutance for ANY 35mm film they
make.  Without all three, you are really guessing about film
performance.

So you tell me how to evaluate two different films :-)

Regards.
--
John Ohrt,  Regina, SK, Canada
johrt@xxxxxxx



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