1、BRITISH STANDARD BS 4793:1972 Recommendations for Specifying the optical performance of lenses for television camerasBS4793:1972 These recommendations, havingbeen approved by the Instrument Industry Standards Committee, were published underthe authority of the Executive Board on 24 May 1972 BSI 12-1
2、999 The following BSI references relate to the work on these recommendations: Committee reference INE/9/6 Draft for comment 70/35730 ISBN 580 06788 2 Co-operating organizations The Instrument Industry Standards Committee, under whose supervision these recommendations were prepared, consists of repre
3、sentatives from the following Government departments and scientific and industrial organizations: The Government departments and scientific and industrial organizations marked with an asterisk in the above list, together with the following were directly represented on the committee entrusted with th
4、e preparation of these recommendations: British Calibration Service Gauge and Tool Makers Association British Electrical and Allied Manufacturers HEVAC Association Association Institute of Measurement and Control* British Industrial Measuring and Control Institution of Chemical Engineers Apparatus M
5、anufacturers Association Institution of Electrical Engineers British Mechanical Engineering Institution of Heating and Ventilating Confederation Engineers British Nautical Instrument Trade Institution of Mechanical Engineers Association Institution of Production Engineers British Railways Board Mete
6、orological Office British Steel Industry Ministry of Defence, Air Force Department Council of British Manufacturers of Ministry of Defence, Army Department* Petroleum Equipment Ministry of Defence, Navy Department* Department of the Environment National Coal Board Electrical Research Association Nat
7、ional Physical Laboratory (Department Electricity Council, The Central Electricity of Trade and Industry)* Generating Board and the Area Boards in Oil Companies Materials Association* England and Wales Scientific Instrument Manufacturers Electronic Engineering Association Association* Engineering Eq
8、uipment Users Association* Sira Institute* British Photographic Manufacturers Institute of Physics and the Physical Society Association Ltd Ministry of Defence, Inspectorate of Department of Trade and Industry Armaments Federation of Manufacturing Opticians Royal Microscopical Society Flat Glass Man
9、ufacturers Association Individual experts Illuminating Engineering Society Amendments issued since publication Amd. No. Date CommentsBS4793:1972 BSI 12-1999 i Contents Page Co-operating organizations Inside front cover Foreword ii 1 Scope 1 2 Definitions 1 3 Specification of parameters relevant to o
10、ptical performance 3 3.1 Parameters relevant to optical performance 3 3.2 Parameters directly describing optical performance 3 Figure 1 Relative illumination zone 5 Figure 2 Typical spectral sensitivities 6BS4793:1972 ii BSI 12-1999 Foreword These recommendations make reference to the following Brit
11、ish Standards: BS 4779, Recommendations for measurement of the optical transfer function of optical devices. BS . . . . ., Recommendations for veiling glare measurements. These recommendations have been prepared under the authority of the Instrument Industry Standards Committee. Their principal aim
12、is to recommend objective procedures for use when specifying the performance of television lenses. One of the measures of performance adopted in these recommendations is the optical transfer function (OTF). Techniques for the accurate measurement of this function have been established in recent year
13、s, and these not only form the basis of a reliable test method but also serve as a means of expressing performance in design specifications. In addition to OTF, the complete assessment of television lenses demands the measurement of other parameters such as vignetting, veiling glare, distortion, etc
14、. It is envisaged that at some future date a generic standard for each given category of lens or optical instrument will be published, giving all those characteristics that are considered to be mandatory together with a preferred format for their presentation. The present document is to be considere
15、d as one for interim use, and its material will be incorporated in the full scheme as this develops. No previous national or international recommendations exist that use OTF in connection with the performance of lenses for television. Other standards in this series dealing with environmental, mechan
16、ical and test requirements are being prepared. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunity from legal obligat
17、ions. Summary of pages This document comprises a front cover, an inside front cover, pages i and ii, pages1 to 6, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the
18、inside front cover.BS4793:1972 BSI 12-1999 1 1 Scope These recommendations are for the specification and assessment of the optical performance of lenses for use with television cameras. These may be either zoom lenses, variable focal length lenses, or single focal length lenses, and they may be requ
19、ired to operate in either black-and-white or colour reproduction systems. A special feature of these recommendations is that they adopt the optical transfer function (OTF), or more specifically its modulus, the modulation transfer function (MTF), as a method of assessing and specifying image quality
20、. These recommendations cover the performance of the lens as a complete optical unit but do not deal with the performance of individual component parts. The condition of test must however include any additional glass paths between the lens and the image plane that are required by the optical design,
21、 such as may be introduced by colour filters, beam splitting blocks, the face plate of the camera tube, etc. 2 Definitions For the purposes of these recommendations the following definitions apply: 2.1 equivalent focal length this is the ratio of the size of a focused image of a distant object to th
22、e angle subtended by that object, when the object is near the axis and the angle subtended by it has its apex on the front nodal point of the lens and is so small that its tangent is equal to its magnitude, in radians, within the limits of the required accuracy 2.2 back focal length the back focal l
23、ength is the distance from the point of intersection of the optical axis with the back surface of the lens to the rear focal point 2.3 flange focal distance the flange focal distance is the minimum (perpendicular) distance between the plane of the flange mounting surface and the rear focal point 2.4
24、 relative aperture the relative aperture of a lens is also its “f number” and is the ratio of the equivalent focal length of the lens to the diameter or equivalent diameter (same area) of its entrance pupil. The relative aperture is normally written as f/N, where N is the above ratio the entrance pu
25、pil is specified as that for the infinite object distance 2.5 image format the image format is the shape and size of the area within which the specified image must be formed. For a rectangular image format the shape is specified by the aspect ratio of breadth to height, whereas the size is determine
26、d by the length of the diagonal 2.6 zoom ratio the zoom ratio is the ratio of the extreme values ofequivalent focal length. It is expressed as n, where n 1 2.7 lens hood the lens hood is a shield, extending beyond the lens on the object side, to exclude light from bright sources outside the object f
27、ield without obstructing light from the latter 2.8 near focusing distance the near focusing distance is the distance from the point of intersection of the optical axis with the front surface of the lens to the nearest object point that can be brought to a focus in the image plane, defined in terms o
28、f the specified flange focal distance NOTEAlthough this definition can apply to fixed focal length lenses, it is of prime importance for zoom lenses. It should also be borne in mind that, at the expense of image quality, a lens may be used at much shorter distances than those for which it is designe
29、d. 2.9 variable focal length lens a variable focal length lens is a lens whose equivalent focal length can be varied. Such a lens will normally require to be refocused when this is done 2.10 zoom lens a zoom lens is a lens whose equivalent focal length can be varied but whose image plane, for a give
30、n object plane, remains at a fixed distance from the mounting flange without the need for refocusing 2.11 line spread function the line spread function of an optical system or device is the distribution of intensity across the image of a sufficiently narrow self-luminous line object as formed by tha
31、t optical system or deviceBS4793:1972 2 BSI 12-1999 NOTEIn general the line spread function of an optical systemor device will vary with choice of image plane, wavelength, aperture, position in the image field, azimuth oftheline object, etc. 2.12 optical transfer function (OTF) this is a complex fun
32、ction of which the modulus is the modulation transfer function and the argument is the phase transfer function an alternative and mathematically rigorous definition is that the optical transfer function is the Fourier transform of the line spread function. The modulus of this function is normalized
33、to be unity at zero spatial frequency and the argument is zero at zero spatial frequency 2.13 modulation transfer factor the modulation transfer factor, at a particular spatial frequency, is defined to be the ratio of image modulation to object modulation 2.14 modulation transfer function (MTF) the
34、variation of the modulation transfer factor with spatial frequency is defined to be the modulation transfer function. This function is normalized to unity at zero spatial frequency 2.15 axial transmission factor the axial transmission factor is the ratio (normally expressed as a percentage) of the t
35、ransmitted flux to the incident flux in an axial beam of monochromatic light that passes through the lens without being obstructed by any aperture stop 2.16 relative illumination relative illumination is the ratio of the illuminance at the image plane for extra-axial field positions, to the illumina
36、nce at the centre of the field, for an object of uniform luminance 2.17 veiling glare index (bright field) the veiling glare index (bright field) is the ratio of the minimum illumination in the image of a suitable “black” patch (of specified size and shape) at the centre of the field, to the illumin
37、ation in a uniformly illuminated surrounding field filling, but not exceeding, the picture area 2.18 stray image a distribution of light in the image plane, other than the primary image, which is an identifiable image of some object 2.19 image ghost a stray image that is an identifiable image of the
38、 primary object 2.20 diaphragm ghost a stray image that is an identifiable image of the diaphragm NOTEAnti-reflection coatings greatly reduce the effects described in2.17, 2.18, 2.19 and2.20 and increase the axial transmission factor2.15. 2.21 radial distortion an object point at distance R from the
39、 optical axis should be imaged in the image plane at a distance kR from the optical axis, where k is a constant. Any departure from this simple proportionality is referred to as radial distortion If R is the distance from the optical axis of the image of an object point at distance R, then the radia
40、l distortion is defined as % R = R kR. the value of the radial distortion so defined will depend on the particular choice of value for k NOTEFor zoom lenses, k will, of course, change with focal length. 2.22 radial distortion factor this is the value of the radial distortion at a specified image dis
41、tance from the optical axis, expressed as a fraction of that distance the choice of distance to be employed varies with the condition of use but should at least include the maximum radial distortion factor obtained within the useful field of the lens 2.23 radial tracking error this is the transverse
42、 displacement of a central image point produced when the focal length is altered NOTEThe maximum value is to be quoted as a percentage of the format diagonal. 2.24 standard image plane the plane in which the modulation transfer factor of a zoom lens, at the centre of the field and for a chosen spati
43、al frequency, aperture and spectral power distribution is a maximum at each of the two extremes of the focal length rangeBS4793:1972 BSI 12-1999 3 NOTEThis plane exists for only one setting of the focusing adjustment and its position is normally established by an iterative process in which the modul
44、ation transfer factor is made a maximum at the short focal length by a choice of image plane, and at the long focal length end of the range by means of focusing adjustment provided on the lens. 2.25 axial spectral transmission characteristic this is the curve that shows the variation of the axial tr
45、ansmission factor as a function of wavelength 2.26 focusing range this is the range of object distances that can be brought into focus in the image plane of the camera by adjustment of the focusing control only NOTEThis range is not always from infinity to the near focusing distance, especially when
46、 certain attachments are used in conjunction with the basis lens. 3 Specification of parameters relevant to optical performance 3.1 Parameters relevant to optical performance 3.1.1 Equivalent focal lengths. The equivalent focal length should be within3% of the length specified by the manufacturer, b
47、ut see3.1.5. In the case of zoom lenses and variable focal length lenses the minimum and maximum focal lengths should be specified and should be within this tolerance. 3.1.2 Relative aperture. Relative aperture should be within5% of that specified by the manufacturer. For zoom lenses a constant rela
48、tive aperture should normally be held to within this tolerance over the whole zoom range for all aperture settings. In lenses with a very wide zoom range a gradual relaxation of maximum aperture may be permitted (in the interest of limiting the physical size of the lens) as the narrow angle extremit
49、y of the zoom is approached. In this case the manufacturer should specify the zoom ratio at which maximum relative aperture cannot be maintained and should also specify the maximum aperture at the narrow angle extremity of the zoom range. 3.1.3 Image format. Television images have an aspect ratio of4 : 3. The image size is determined by, but not identical with, the diagonal of the camera tube. Unless otherwise specified, the diagonal for the following common types of camera tube should be taken to be: Figure 1 illustrates this image format for television lenses and
