|
Assessment of Optical Standards for Television.
Since the question has recently been raised as to the number of scanning lines required in television to produce a picture having the same standard of definition as the picture projected on the cinema screen it is worth while first to try to assess that standard and express it in terms of lines per picture.
For the purpose of calculation the dimensions of the film picture are taken as 0.6 × 0.825 inch We have to decide how far definition is affected by:-
- the film
- the camera
- photographic technique
- the taking lens
- processing
- the projector
- the projection lens.
Film
Three films are mentioned in the Kodak Data Book as suitable for cinematography, viz., a normal very high speed emulsion with a resolving power of 30 lines per mm.; i.e., 450 horizontal lines to the picture; a new very high speed emulsion of moderate contrast with a resolution of 45 lines per mm.; i.e., 675 lines per picture; and the normal high-speed emulsion of fairly high contrast which resolves 50 lines per mm. or 750 lines per picture.
Camera
No data are available as to the standard of workmanship. Perforations have a tolerance of 0.0004 in. in size and 0.0005 in. in pitch. It would thus seem that about 0.001 in. error is considered allowable in all, and probably at similar standard is aimed at in manufacture. This demands a precision of a few ten-thousandths of an inch in the individual parts of the film transport mechanism and is attainable in precision engineering. This gives 600 lines per picture.
Photographic Technique
This includes possible blurring of the image through variations in focusing and the requirements of field depth. It will be supposed that a 2 in. f /2 lens is used in the camera. Experience shows that it is impossible to be sure of focus closer than 0.002 in. even with the refined tools of the opticians testing room. It is unlikely that precision in the studio will be high. An error in focus of 0.002 in. is thus possible. At f /2 this produces a blur of 0.001 in., which is 1/600 picture height.
A guide to the limit set by depth of focus can be obtained by considering a close-up of a head nearly filling the screen. The reduction will be about 1/ 20. Now a depth of ±1 in. on either side of the part upon which attention is focused (usually the eyes) must be allowed without the image becoming perceptibly blurred. Therefore the depth of focus will be ±1/400 in. at the film; at f/2 a blur of 1/ 800 in. is produced. If this is tolerable, so is a standard of 480 lines.
The Taking Lens
No lens is perfect and generally accepted figures for the usual errors will be quoted for high quality cinema lenses.
(a) Axial: The definition can be spoiled by axial chromatic and spherical aberrations; even though these are said to be 'corrected' there are always residuals which cannot be entirely removed with the glasses at present available. In a lens for spherical aberration it will be found that if zones are isolated they will each give a slightly different focus. This variation will in a good lens amount to 0.00 4in. per inch focal length; i.e., it will be 0.008in. in a 2in. lens f /2 and will occur at a zone having a diameter of about f/3, so that a point is rendered by this zone as a ring of diameter 0.0027 in. if observation is made at the focus for central rays. Now this is the worst zone and the eye in focusing is conscious of the effect of all zones and, so to speak, integrates the effect and chooses not the focus for central rays but one where the total blurring is least. The consequence is that the resulting blur is only half that computed for the worst zone; i.e., will amount to 0.0013 in. This divided into 0.6 in. gives 450 line definition.
Chromatic aberration is smaller but will add to the size of the blur and so lower the definition slightly from this figure.
(b) Extra-axial:
Lenses have likewise residuals of astigmatism and field curvature by which the foci for points off the axis fall outside the plane through the axial focus. A 2 in. lens has a field of ±15 deg. on the film area, and within this angle departure from the focal plane may amount to 0.5% of the focal length; i.e., to 0.01 in. with a 2 in. lens. The aperture for oblique pencils is substantially less than for axial pencils, owing to cut off by the rims of the lenses, so that the aperture will be about f /3 at the edge of the field; consequently the image of a point will amount to 0.003in. This is 200 line definition, but again it is possible to choose the focal plane so as to give the best results throughout the field and an overall definition of perhaps 400 lines may be looked for.
It may be argued that the focal plane has already been chosen to get the best axial definition and that it is not legitimate to postulate a fresh choice dictated by extra-axial imagery. The answer to this is that the best lenses are so designed. that the requirements for best axial and oblique image points are met by the same choice of focus.
Processing
Film shrinkage and the effect of processing have been investigated and certainly may lead to impairment of definition, but these defects can be largely guarded against by careful treatment and storage.
The Projector
What was said about the camera applies equally to the projector, and it is probable that the standard of workmanship aims at a possible 600 line definition.
The Projection Lens
This is of an entirely different type from that of the taking lens and is of greater focal length. A 4 in. f /2 may be considered typical. Usually the axial definition is better and the oblique definition worse than that of the camera lens. The definition of axial points will therefore not suffer much on projection; the field of this type of lens is, however, far from flat and may have a divergence from flatness of as much as 0.01 in. in a good lens, giving an out-of-focus blur which is equivalent to 0.005 in. on the film. This is only 120 line definition. Even if central definition is sacrificed somewhat, that at the corners can hardly exceed a 200 line standard. At the side of the picture it may be 50% better; i.e., 300 line. With a longer focus lens definition will, of course, be somewhat better.
General Considerations
These standards may seem low, but need not cause surprise. It is generally considered that the eye accepts an image as sharp if the blur does not exceed one minute of arc at the eye. The front of the balcony maybe taken as being the best point of view in the cinema and this may be half way between the projector and screen, consequently the latter subtends twice the angle at the spectator that the film does at the projector. The standard of definition for the projection lens then should be that blur on the film does not exceed half a minute of arc (an angle of I in 7,000) and with a 4 in. lens blur should then be restricted to 4/7,000 in. The 'line' standard is thus 0.6-3-4/7,000; i.e., about 1,000. With a 7 in. projection lens a blur of 1/1,000 could be tolerated, which is 600 lines definition. This physiological tolerance is based on laboratory experiments with a stationary test object of black and white lines. In the cinema the objects on the screen are usually moving, are not geometric in shape and rarely have black and white contrast. Thus a lower standard could be tolerated in the cinema.
Conclusions
A definition represented by 600 lines is probably the highest the eye could appreciate under the most exacting conditions, and this is probably within the range of resolution of the finest grain film that is used. The conditions under which the final image is produced on the screen do not suggest that definition there ever exceeds a 400 to 500 line standard in the centre and 300 at the edges, and it may at times be lower.
Comment by J M T Evans, Wireless World, September, 1945.
The comparisons between the definition of the cinema film and the television picture made by H W Lee in the August Wireless World seem to be based upon a fundamental error. There is a vital difference between the moving images formed by the two systems. The television picture consists of a fixed mosaic (raster) which remains identical from frame to frame, while the cinema film consists of a random mosaic (the grain of the film), the grain structure of one frame not being the same as the grain structure of the preceding or succeeding frame.
Because of the persistence of vision of the human eye, a cinema picture therefore gives a much better definition than would be supposed from an analysis of the picture elements. This is not so with the television picture where the same 'grain structure' persists from frame to frame and is thus much more noticeable to the eye.
The comparison between the definitions of the cinema and the television picture is, therefore, only valid if we compare the television picture with one frame (a still) from the film. When this is done the shortcomings of the film are apparent.
This means that the definition of the television image to give results as good as the cinema film in motion will have to be considerably higher than the definition of one frame of the film.
| 