The Manual of Photography

Elizabeth Allen, Sophie Triantaphillidou

• Publisher: Routledge
• Year: 2012
• ISBN:
  9781136091094
• Edition: 10
• Topic: Medien & darstellende Kunst
• Subtopic: Digitale Medien

The tenth edition of The Manual of Photography is an indispensable textbook for anyone who is serious about photography. It is ideal if you want to gain insight into the underlying scientific principles of photography and digital imaging, whether you are a professional photographer, lab technician, researcher or student in the field, or simply an enthusiastic amateur. This comprehensive guide takes you from capture to output in both digital and film media, with sections on lens use, darkroom techniques, digital cameras and scanners, image editing techniques and processes, workflow, digital file formats and image archiving.

This iconic text was first published in 1890 and has aided many thousands of photographers in developing their own techniques and understanding of the medium. Now in full colour, The Manual of Photography still retains its clear, reader-friendly style and is filled with images and illustrations demonstrating the key principles. Not only giving you the skills and know-how to take stunning photographs, but will also allowing you to fully understand the science behind the creation of great images.

Chapter | 1 |

Introduction to the imaging process

Elizabeth Allen

All images © Elizabeth Allen unless indicated.

INTRODUCTION

The second half of the nineteenth century saw the progression from early experiments with light-sensitive compounds to the first cameras and photographic films becoming available to the general population. It developed from the minority use of the camera obscura as a painter’s tool, through the first fleeting glimpse of a photographic image in a beaker containing silver compounds after light exposure, to the permanent rendering of the image, and then to the invention of the negative–positive photographic process used today to produce an archival image. A camera using roll film, the Kodak, was available to the public in 1887 and brought photographic media to the masses. By the beginning of the twentieth century, silver halide materials were produced that were sensitive to all visible wavelengths in the electromagnetic spectrum, producing tonally acceptable images, and this paved the way for practical colour imaging processes, beginning with the Autochrome plate in 1907, to be developed.

Since this period, the ability to capture, manipulate and view accurate images of the world around us has become something that we take for granted. There are few places in the world where images are not a part of daily life. We use them to record, express, represent, manipulate and communicate ideas and information. The diverse range of applications for imaging leads to a multitude of functions for the image: as a tool of coercion in advertising, a means to convey a visual language or an aesthetic in art, a method of visualization and analysis in science, to communicate and symbolize in journalism, or simply as a means to record and capture, and sometimes enhance, the experiences of everyday life. The manufacturing industries rely on images for a multitude of purposes, from visualization during the design and development of prototypes to the inspection of manufactured components as part of industrial control processes. Photography and applied imaging techniques using visible and non-visible radiation are fundamental to some fields: in medicine for diagnosis and monitoring of the progress of disease and treatment; and in forensic science, to provide objective records in legal proceedings and for subsequent analysis.

The art and science of photography and imaging has been developed through multiple disciplines, as a result of necessity, research and practice. The imaging process results in an image that will be observed; therefore consideration of the imaging chain, in practice or theory, must include the observer. But the numerous functions of images mean that the approach to and requirements from the imaging process are various. If involved in the practice of imaging, however, whatever the function of the image, it is impossible to avoid the need to acquire technical skills and some knowledge and understanding of the theory behind the imaging process.

Knowledge of factors affecting all stages of the imaging chain allows manipulation of the final image through informed selection of materials and processes. More in-depth study of the fundamental science of imaging, as well as being interesting and diverse, serves to enhance the practical imaging process, as understanding can be gained of the mechanisms involved, and processes and systems can be characterized and controlled to produce required and predictable results. Study of imaging science encompasses the nature of light, radiometry and photometry, vision science and visual perception, optics, colour science, chemistry, psychophysics, and much more besides. It provides methodologies for the assessment of imaging systems and tackles the complex issue of the evaluation of image quality.

The greatest change in our approach to imaging since the development of the colour process has occurred in the last 25 years, with the burgeoning growth of digital imaging technologies. The first consumer electronic camera system was introduced to the public in 1981, but it took the development of the personal computer, and its leap to widespread use, before digital imaging became practical. The internet has caused and facilitated an exponential increase in image production and dissemination. Imaging has grown to embrace computer science and computer graphics in a symbiotic relationship in which each discipline uses elements of the others. Digital image processing finds application in many areas from the aesthetic enhancement of images to analysis in medical applications. The immediacy of digital imaging has raised our expectations; it is likely that this, along with the efficiency of the digital imaging process and the ease of manipulation of digital images which are, after all, just arrays of numbers, will mean that the traditional photographic process may eventually be entirely replaced.

Keeping up with the changes in technology hence requires the acquisition of different types of knowledge: technical skills in computing for example, and an understanding of the unique qualities of information represented by discrete data, in digital images, compared to the continuous representation of information, as used in analogue (silver halide) imaging. It is clear that the need for new practices and alternative approaches will continue as the technology develops further. It is important, however, in trying to understand the new technologies, not to forget where it all started. Although technology has changed the way in which we produce and view images, much of the core science upon which the foundations of photography were built remains important and relevant. Indeed, some of the science has become almost more important in our understanding of digital systems.

THE IMAGING PROCESS

The word ‘photography’ is etymologically derived from the phrase ‘to draw with light’. Modern electronic imaging techniques are commonly classed under the umbrella term ‘digital imaging’ to distinguish between them and more traditional silver halide photography; however, both encompass the same core principle: the use of light to produce a response in a light-sensitive material, which may then be rendered permanent and viewed as an image of the original scene. Detailed comparisons will be drawn between the analogue and digital processes throughout this book. Thus, an overview is presented below.

During image capture, light from a scene is refracted by a lens and focused on to an image plane containing a light-sensitive material. Refraction is the deviation of a light ray as it passes from one material to another with different optical properties, and is a result of a change in its velocity as it moves between materials of different densities (see Chapters 2 and 6). The effects of refraction can be seen in the distortion of an object when viewed from behind a glass of water. Figure 1.1 illustrates the refraction of light rays through a simple positive lens to produce an inverted image in sharp focus on the image plane.

The amount of light falling on an image sensor is controlled at exposure by a combination of aperture (the area of the lens through which light may pass) and shutter speed (the amount of time that the shutter in front of the focal plane is open). This relationship is described by the reciprocity equation: H = Et, where E is the illuminance in lux, t is the time of exposure and H is the exposure in lux-seconds. At each exposure level, a range of possible aperture (f-number) and shutter speed combinations will produce the same overall exposure. Choice of a particular combination will affect the depth of field and sharpness/motion blur in the final image. Traditionally a single increment in the scale of possible values for both shutter speed and aperture is termed a ‘stop’ (although many cameras offer half-stop intervals). Each change of a single stop in either aperture or shutter speed scale represents a halving or doubling of the amount of light falling on the sensor. The photometry of image formation is dealt with in detail in Chapter 6 and exposure estimation is the subject of Chapter 12.

Image formation occurs when the material changes or produces a response in areas where exposed, which is in some way proportional to the amount of radiation falling on it. In traditional photographic materials, exposed light-sensitive silver halide crystals (silver chloride, bromide or iodide) form a latent image (see Chapter 13). Latent in this context means ‘not yet visible’. The latent image is actually a minute change on the surface of the exposed crystal, where a small number of silver ions have been converted to silver atoms, and is not visible to the naked eye. It is also not yet permanent. In an image sensor electromagnetic radiation falls on to pixels (a contraction of ‘picture elements’), which are discrete light sensors arranged in a grid. Each pixel accumulates charge proportional to the amount of light falling on it.

Figure 1.1 Image formation using a simple lens.

After exposure, the image is processed. For simplicity we consider the monochrome process, as colour is discussed in more detail later in this chapter. In photographic chemical processing the latent image is developed (Table 1.1). During the development process silver halide crystals containing latent image are reduced to metallic silver. The silver forms tiny specks, known as photographic grains, and these appear as black in the final image. The full range of tones produced in a greyscale image is a result of different densities of clusters of grains, and the image density in any area is proportional to the amount of light that has fallen on it. The image tones at this stage will be negative compared to the original scene. After development time is completed the material is placed in a stop bath to prevent further development before the image is fixed.

In digital imaging, the processing will vary depending upon the type of sensor being used (see Chapter 9). In a charge-coupled device (CCD), the charge is transferred off the sensor (‘charge coupling’), amplified and sent to an analogue-to-digital converter. There it is sampled at discrete intervals corresponding to individual pixels and quantized. Quantization means that it is allocated a discrete integer value, later to define its pixel value. The newer complementary metal oxide semiconductor (CMOS) image sensors perform this image processing on the chip and output digital values.

Table 1.1 Monochrome photographic process

PROCESS	OUTCOME
Exposure	Formation of latent image
Processing
Development	Latent image amplified and made visible
Stop bath	Development stopped
Fixing	Unused silver halides converted into soluble compounds which dissolve in fixing agent
Washing	Soluble chemicals removed
Drying	Water removed

At this stage, the nature of the image represented by the two systems is quite different. In silver halide materials, the random arrangement of silver halide crystals in the photographic emulsion means that a continuous range of tones may be represented and silver halide methods are often referred to as analogue imaging. The digital image, however, is a grid of non-overlapping pixels, each of which is represented by an integer number that corresponds to its intensity. It cannot represent continuous tones in the same way as silver halide materials because the data are discrete. Various techniques, introduced later in this chapter, are therefore used to simulate the appearance of continuous tones.

The final step in the imaging process is image perpetuation, during which the image is rendered permanent. The silver halide image is made permanent by fixation, a process by which all remaining unexposed silver halide crystals are made soluble and washed away. A digital image is made permanent by saving it as a unique digital image file. The image is then output in some way for viewing, either as a print, a transparency or as a digital image on a computer screen. In silver halide processes this involves exposing the negative on to the print material and again processing and fixing the image. A comparison of the analogue and digital imaging processes is illustrated in Figure 1.2.

IMAGE CONTROL

By careful control of every stage of the imaging process, the photographer is able to manipulate the final image produced. Image control requires an understanding of the characteristics of imaging material and system, composition, the behaviour and manipulation of light, tone and colour, as well as the technical skill and ability to combine all of these factors for the required result. Such skills may be acquired by practice and experimentation, but having an understanding of the theory and the science behind systems and techniques allows true mastery of the process.

Control of image shape

When capturing an original scene, the photographer controls composition of the image to be projected on to the focal plane of the camera in a variety of ways. The format of the camera selected (the size of the image sensing area) will determine not only the design of the camera and therefore the capabilities of the system, but also the size, quality and aspect ratio of the final image. Large-format cameras (also known as technical or view cameras, with an image format of 5 × 4″) are designed to allow camera movements, physical manipulation of the two planes containing lens and imaging sensor separately, enabling the photographer to change the size, magnification and p...