ISO 6846:1992 写真—白黒連続調紙—印刷用のISO感度とISO範囲の決定 | ページ 2

Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards bodies (ISO member bodies). The work of preparing International Standards is normally carried out through ISO technical committees. Each member body interested in a subject for which a technical committee has been established has the right to be represented on that committee. International organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.

Draft International Standards adopted by the technical committees are circulated to the member bodies for voting. Publication as an International Standard requires approval by at least 75 % of the member bodies casting a vote.

International Standard ISO 6846 was prepared by Technical Committee ISO/TC 42, Photography.

This second edition cancels and replaces the first edition (ISO 6846:1983), of which it constitutes a technical revision.

Annexes A, B, C and D of this International Standard are for information only.

Introduction

This International Standard establishes a method for measuring those photographic characteristics of papers used for printing from negative images which will aid users to select the appropriate products for their applications. ISO speed and ISO range are two measurements considered important for this purpose.

Studies have shown that highly acceptable prints are generally obtained if the log exposure range (LER) of a photographic paper is equal to the effective density range of the negative. ¹⁾ Therefore, ISO range, which is directly related to LER provides a useful criterion for grading papers. Some diversion from this criterion can be required to obtain optimum prints because of subject matter, individual preferences, and paper characteristics such as D_max, surface and curve shape.

Each manufacturer has established a unique system for grading of papers and designations which makes it extremely difficult for users to identify products which are comparable for printing characteristics. The ISO range number is introduced to reduce this source of confusion.

ISO range is not a measure of image contrast, but a useful guide for selecting the proper paper (or the proper filter for variable contrast paper) for a given effective negative density range, and for comparing products from various manufacturers. Print image contrast can be specified by three different types of objective measurements in relation to two specified points on the paper characteristic curve

• a) difference in density between the two points (density range);
• b) the slope of a straight line connecting the points (average gradient); and
• c) the difference in log exposure for the two points (log exposure range).

Of the three, log exposure range is most closely associated with the concept of paper contrast grades (see annexes B and C).

The diffuse density range of a negative is not an accurate measure of the illuminance range at the printing plane when printers are employed that have specular type optical systems. Since conditions which satisfy the diffuse density criteria normally exist only in contact printing, the term"effective density range" has been introduced (see annex A).

It is the effective density range of the negative which should be matched with the log exposure range of the printing material in the usual projection printing mode for optimum tone reproduction.

The effective density range of a negative image can be calculated from the measured ISO diffuse transmission density range by applying the proper correction factors for specularity and flare to the values. Alternatively, the illuminance range can be measured in the printing plane with a suitable photometer (see annex A).

Most manufacturers have classified photographic papers by using a grade number ranging from 00 to 6. The larger the grade number, the greater is the contrast within a specific line of papers.

ISO range numbers will generally be lower as contrast increases. The concept of ISO range was adopted recognizing that users generally evaluate the density range of a negative (visually) in selecting the paper grade to use. If the density range is small, a paper with a small ISO range number should be used. It will take time to adapt to this new concept, but establishing an internationally accepted method of classifying papers in this manner will have significant long-term benefits for users.

It is not feasible to include provisions in this International Standard for the wide range of exposing conditions used in the trade. For this reason, the interpretation of the speed and range numbers obtained by following the procedures herein are possibly not directly applicable when products are exposed using sources other than a tungsten lamp operating at 2 856 K.

Since a particular paper is designed to provide optimum results in particular processes, this International Standard does not specify one single process. To do so would be considered unduly restrictive and could result in yielding ISO speed and ISO range numbers which were not typical of those obtained in processes specified by the manufacturer.

1 Scope

This International Standard specifies the method for determining the ISO speed and ISO range of black-and-white photographic papers used to make positive reflection prints of continuous-tone subjects from black-and-white film negatives. It pertains to all conventional silver-halide contact and enlarging papers used for continuous-tone or pictorial photography. This includes papers with variable contrast. The papers may be processed in conventional chemicals and equipment, but also using special procedures such as those involving activators or heat for development.

This International Standard does not apply to:

• a) non-silver papers;
• b) high contrast silver papers, such as those used in the graphic arts, or other non-pictorial applications;
• c) silver papers used to obtain a direct positive or reversal image.

2 Normative references

The following standards contain provisions which, through reference in this text, constitute provisions of this International Standard. At the time of publication, the editions indicated were valid. All standards are subject to revision, and parties to agreements based on this International Standard are encouraged to investigate the possibility of applying the most recent editions of the standards indicated below. Members of IEC and ISO maintain registers of currently valid International Standards.

3 Definitions

For the purposes of this International Standard, the following definitions apply.