Condenserdiffuser enlargers
Most current popular enlargers employ an optical system which includes both condenser and diffuser. For many purposes such a system offers a very practical compromise between condenser and diffuser enlargers. It allows shorter exposure times than a true diffuser enlarger, yet avoids the necessity for adjusting the position of the lamp for each variation in the degree of enlargement as is necessary in a condenser enlarger. Grain and blemishes on the negatives are subdued to a useful extent, even...
Astigmatism
The imaging situation is complicated further in that the Petzval surface represents the locus of true point images only in the absence of the aberration called astigmatism. This gives two additional curved surfaces close to the Petzval surface, which may also be thought of as surfaces of sharp focus, but in a different way. They are called astigmatic surfaces. The term 'astigmatic' comes from a Greek expression meaning 'not a point', and the two surfaces are the loci of images of points in the...
Autofocus systems Autofocus modes
For many purposes a lens that is focused visually, or even a fixed-focus lens, is adequate the various optical aids described above simply improve accuracy of focusing. But visual focusing can be slow, inaccurate and tiring, especially if continuous adjustments are necessary. Some means of obtaining or retaining focus automatically is helpful, especially when using long-focus lenses and following subjects moving obliquely across the field of view in poor light also for unattended cameras...
Lens covering power
The covering power of a lens is the diameter of the circle of good definition in the focal plane. This value is less than or rarely equal to that of the circle of illumination from the lens. Covering power is a minimum at maximum aperture and infinity focus, but increases as the lens is progressively stopped down, or focused closer Figure 10.5 . A lens designed for use with camera movements usually has extra covering power to give a circle of good definition significantly larger than the format...
The Petzval lens
Simple lenses were inconveniently slow for portraiture with the insensitive plates of the period, and active efforts were made to design a lens of large aperture the principles were already well understood, but lack of suitable optical glasses was a problem. In 1840 J. Petzval designed a lens of aperture f 3.7 using two separated, dissimilar achromatic doublets. This was the first lens to be computed specifically for photography. It had about fifteen times the transmission of other contemporary...
Contrast index
A measure of contrast devised by Kodak is the contrast index which, like GG, takes into account the toe of the negative characteristic curve. The method of determination involves the use of a rather complicated transparent scale overlaid on the characteristic curve, although an approximate method is to draw an arc of 2.0 units cutting the characteristic curve and centred on a point 0.1 units above min. The slope of the straight line joining these two points is the contrast index. A fuller and...
Instant colour processes Polacolor
A number of systems using chemical processes initiated within the camera have been designed to give 'instant pictures'. Although the chemical reactions yielding the colour images usually progress to completion in a few minutes in the hand of the photographer, such systems are often called incamera processes. The first instant-picture colour process was the Polacolor system, introduced in 1963. Polacolor prints were obtained by pressing the shutter release and pulling a film and print sandwich...
Betweenlens shutters
The ideal shutter position to control the light transmitted by a lens is near the iris diaphragm, where the beam of light is at its narrowest, and so the minimum amount of shutter blade travel is required to allow light through. The film gate area is also uniformly exposed at all stages in the operation of this type of shutter. Simple cameras may use single- or double-bladed between-lens shutters, but others use multi-bladed shutters see Figure 9.1 . Usually five blades or sectors are used, and...
Filters for darkroom use Safelight filters
Darkroom safelights use another type of colour filters. Their function is to transmit light in a restricted range of wavelengths, to give as high a level of illumination as possible that is consistent Figure 11.17 Spectral absorption curves for darkroom safelight filters. a Blue-sensitive materials. b Orthochromatic materials. c Panchromatic materials Figure 11.17 Spectral absorption curves for darkroom safelight filters. a Blue-sensitive materials. b Orthochromatic materials. c Panchromatic...
Sensitive materials and image sensors
Many light-sensitive substances, varying widely in their sensitivity, are known. Conventional photographic materials use silver halides, which are lightsensitive compounds formed by the combination of silver with members of the halogen group of the elements bromine, chlorine and iodine . Photographic materials are coated with suspensions of minute crystals with diameters from 0.03 im for high-resolution film to 1.5 im for a fast medical X-ray film of silver halide in a binding agent, nowadays...
Info Bvd
c Achromatic combination Figure 6.2 The principle of an achromatic doublet lens combination c Achromatic combination Figure 6.2 The principle of an achromatic doublet lens combination element, resulting in a combination that is positive overall, but with the positive and negative dispersions equal and cancelling out. The two elements may be separated or cemented together. A cemented achromatic doublet lens made in this way is shown in Figure 6.2. Other errors such as spherical aberration and...
Microprism grids and screens
The principle of the split-image rangefinder is used in the microprism grid array, also located in the centre of the viewfinder focusing screen. A large number of small facets in the shape of triangular or square base pyramids are embossed into the focusing screen surface Figure 9.22 . An image from the camera lens that is not precisely in focus on the screen undergoes multiple refraction by these facets so that the image appears broken up into tiny fragmentary areas with a characteristic...
Tests for permanence
It is sometimes desirable to test the completeness of the fixing and washing of negatives and prints. Two tests are required one for the presence of unwanted silver salts, the other for the presence of thiosulphate. If either of these tests is positive the permanence of the negative or print cannot be assured. A simple test for the presence of injurious residual silver compounds is to apply a drop of 0.2 per cent sodium sulphide solution to the clear margin of the negative or print after...
Special filters
There are other filters with special or distinct functions. Some of them may have no visible hue, because their absorption lies outside the visible spectrum, or may be visually opaque, because their transmittance lies outside the visible spectrum. Figure 11.10 Absorption curve of an ultraviolet UV -transmitting filter solid line , and of an UV-absorbing filter broken line Figure 11.10 Absorption curve of an ultraviolet UV -transmitting filter solid line , and of an UV-absorbing filter broken...
Colour processes
The principles of the two major types of photographic colour reproduction have been described in Chapter 14. The processes considered there illustrate the ways in which additive and subtractive colour syntheses can be carried out following an initial analysis by means of blue, green and red separation filters. It was Table 16.3 Recording of primary and complementary colours by the main types of photographic emulsion Rather light Slightly dark Slightly light Rather light Slightly dark Rather...
Elementary sensitometry
An elementary form of sensitometry, which although not of a high degree of precision can be of real practical value in testing the performance of sensitized materials or photographic solutions, can be carried out with nothing more elaborate than a step wedge and a simple visual densitometer. If a commercial step wedge is not available, a suitable substitute can be made by giving a stepped series of exposures to an ordinary film. Useful step increases in a wedge used for this purpose are density...
Colour filters for colour photography
Filtration needs for colour films differ from those for monochrome materials. In colour photography the main problem is to obtain an acceptable colour balance. Principal factors involved are the colour temperature of the illuminant used, colour film is balanced for use with a specific colour temperature, usually 3200 K or 5500 K , and the exposure duration, which for correct colour balance is usually restricted to a specified range to avoid the effects of reciprocity law failure RLF ....
Phenidone
Phenidone 1-phenyl-3-pyrazolidone , the developing properties of which were discovered in the Ilford laboratories in 1940, possesses most of the photographic properties of metol together with some unique advantages. It shares with metol the property of activating hydroquinone so that a phenidone-hydroquinone PQ mixture forms a useful and very active developer. Used alone, phenidone gives high emulsion speed but low contrast, and has a tendency to fog. Mixed with hydroquinone, however, and with...
Lf
EWA extreme wide-angle WA wide-angle SWA semi wide-angle S standard MLF medium long-focus LF long-focus VLF very long-focus ELF extreme long-focus. EWA extreme wide-angle WA wide-angle SWA semi wide-angle S standard MLF medium long-focus LF long-focus VLF very long-focus ELF extreme long-focus. Every lens projects a fuzzy edged disc of light as the base of a right circular cone whose apex is at the centre of the exit pupil of the lens. The illumination of this disc falls off towards the edges,...
Simple cameras
The term 'simple camera' means one made for ease of operation, with little choice of control settings. The archetypal simple camera was the early primitive box camera equipped with a single meniscus or doublet lens with an aperture of about f 14. Smaller apertures might be selected by 'weather' symbols on an aperture control. The lens was usually fixed-focus, set at the hyperfocal distance to give reasonably sharp focus from 2 m to infinity. A 'portrait' supplementary lens, if attached, brought...
V
Front principal Rear principal plane plane Front principal Rear principal plane plane Figure 4.9 Image formation by simple and compound lenses. a For a simple lens, distances are measured from the optical centre of the lens distance y is the focusing extension. b For a compound lens, distances are measured from the principal or nodal planes the principal planes coincide with the nodal planes when the lens is wholly in air important, as the thick lens system can be treated as if the refraction...
Camera viewpoint
A camera records a three-dimensional scene as a two-dimensional planar image in the photoplane and in the print. By placing the camera lens at the visual viewpoint, familiar photographic parameters replace the terminology of perspective as defined above. The convergent rays form a central perspective by passing through the lens, and continue diverging beyond the viewpoint to an image plane conjugate to the subject plane. The principal distance is now the image conjugate v, commonly taken as the...
Lateral chromatic aberration
Lateral or transverse chromatic aberration, sometimes also called either lateral colour or chromatic difference of magnification, is a particularly troublesome error which appears in the form of dispersed colour fringes at the edges of the image Figure 6.4 . It is an off-axis aberration, i.e. it is zero at the optical centre of the focal plane but increases as the angle of field increases. Whereas axial chromatic aberration concerns the focused distance from the lens at which the image is...
Mv
Figure 3.3 can be used for conversion from one scale to the other. Note that as colour temperature increases the mired value decreases and vice versa. The main advantage of the mired scale, apart from the smaller numbers involved, is that equal variations correspond to approximately equal visual variations in colour. Consequently, a light-balancing filter can be given a mired shift value MSV which indicates the change in colour quality given, regardless of the source being used. Yellowish or...
Circle of confusion
For a photograph viewed at Dv, any subject detail resolved and recorded optically that is smaller than 0.2 mm in general dimensions may not be perceptible to the unaided eye. Thus any detail finer than this size is not required. This leads to a practical definition of resolution. A limit is set to the diameter of an image blur circle that is not distinguishable from a true point by the unaided eye , and this is called the minimum permissible circle of confusion C . This is arrived at by...
Info Chw
Because of the conjugate relationship between u and v as given by the lens equation above, these distances are often called the object conjugate distance and image conjugate distance respectively. These terms are usually abbreviated to 'conjugates'. A summary of useful lens formulae is given in Figure 4.12, including formulae for calculating the combined focal length of two thin lenses in contact or separated by a small distance. A suitable sign convention must be used. For most elementary...
Info Nbv
Image formation by a simple positive lens Irrespective of their configuration of elements, camera lenses are similar to simple lenses in their image-forming properties. In particular, a camera lens always forms a real image if the object is at a distance of more than one focal length. The formation of the image of a point source has been discussed, now let Figure 4.8 Image formation by a positive lens. a For a distant subject F is the rear principal focal plane b for a near subject focusing...
Photometrics Square Surface
The output power of a source is an important characteristic. A source can emit energy in a wide spectral band from the ultraviolet to the infrared regions indeed, most of the output of incandescent sources is in the infrared. For most photography only the visible region is of importance. Three related photometric units are used to define light output luminous intensity, luminance and luminous flux. Luminous intensity is expressed numerically in terms of the fundamental SI unit, the candela cd ....
Info Cts
Figure 3.6 Lambert's cosine law of illumination. The illuminance E at point A distant dj from source S of intensity I is given by E I dj2, but at point B on the same surface receiving light obliquely the reduced illuminance is given by E I cos e d22 Figure 3.6 Lambert's cosine law of illumination. The illuminance E at point A distant dj from source S of intensity I is given by E I dj2, but at point B on the same surface receiving light obliquely the reduced illuminance is given by E I cos e d22...
Image sharpness
Any subject can be considered as made up of a large number of points. An ideal lens would image each of these as a point image strictly, an Airy diffraction pattern by refracting and converging the cone of light from the subject point to a focus. The purpose of focusing the camera is to adjust the image conjugate to satisfy the lens equation. The image plane is strictly correct for all object points in a conjugate plane, provided all points of the object do lie in a plane. Unfortunately,...