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Subject: Microfilm resolution

Microfilm resolution

From: Charles Stewart <cstewart>
Date: Wednesday, August 11, 1999
There is a lot of confusion in the matters of "resolving power",
"resolution" and "quality index".  All these methods of
determination involve a subjective element: the perception/judgement
of the evaluator. However, I shall try to clarify a little:

    1.  Mr. Saretzky is correct in distinguishing between the
        resolving power of a system and simple pattern recognition.
        The first of these takes reduction into account to arrive at
        the overall resolving power of the imaging system--leaving
        character size out of the equation.  The second may be used
        to determine adequacy of imaging of the source
        document--irrespective of reduction.

    2.  I also take "flow camera" and "rotary camera" to be
        interchangeable terms.

    3.  When you refer to "a resolution of...", I assume you are
        referring to the line-pair patterns, which express frequency
        per mm.  In the quality index system these patterns are said
        to correspond to certain character heights--again, without
        respect to reduction.  For example, to achieve the "highest
        quality" level, you are required to resolve a minimum
        frequency of 8.0 line pairs, for characters of 1.0 mm. in
        height.  The frequency required increases with each
        generation reproduced, so that you must resolve the 9.0
        pattern if you intend to draw one generation more from the
        negative (a reading copy, for example), and the 10.0 pattern
        if you are creating a third generation of film.

    4.  Theoretically, the resolving power of the system does not
        change with reduction, however the pattern recognition does
        become lower with greater reduction, hence worsening
        resolution in real terms, and in terms of "quality index".
        Therefore--to use Mr. Saretzky's example--you do get a
        resolving power of greater than 120 line pairs/mm. by
        resolving 6.1 (the nearest pattern on ISO #2 for planetary
        filming is 6.3) at 24X but you do not resolve the frequency
        necessary to achieve "highest quality" for small characters.
        In other words, optical resolution in absolute terms
        declines with greater reduction, and this is inherent in
        optical systems, including our own biological one: it's the
        reason you can read a road sign better close up than from
        far away.  Your 20/20 vision (if you're a lucky one) is your
        system resolving power.  It's said to be 20/20 regardless of
        "reduction", but it doesn't do a good job on the freeway
        sign from half a mile away; it's not "highest quality"
        imaging in that instance.

    5.  Different standards tend to apply for more demanding
        applications: at very high reductions, lower pattern
        resolution may be acceptable, since it is a more reasonable
        expectation. The "medium quality" reproduction may be what
        is strived for.  It's one of those tradeoffs we're so
        familiar with life (more comfort/lower gas mileage; higher
        dividends/greater risk, etc.)

I apologize for not answering the query per se, but I was glad to
seize the opportunity to discuss this, since I sometimes get
frustrated by the confusion that surrounds the subject.

Chas. Stewart
Sr. Photographic Technician,
head, microfilm, LPS, UC, Berkeley

                  Conservation DistList Instance 13:14
                  Distributed: Monday, August 16, 1999
                       Message Id: cdl-13-14-003
Received on Wednesday, 11 August, 1999

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