Introduction

There is an overwhelming range of image capture mediums available. These range from digital image sensors to various colour and black and white films. Choosing the appropriate medium is an essential skill for every photographer.

Capture mediums

Light sensitive surfaces can be divided into four main types:

ISO

All film has an ISO (International Standards Organisation) or ASA (American Standards Association) rating. This rating is a measure of its effective speed (susceptibility to light and contrast). Digital image sensors are not interchangeable but it is possible to alter their degree of sensitivity to light (ISO).

Tungsten and daylight

Tungsten film (3200K) is only available as professional film. Daylight film (5500K) is readily available. Colour film achieves correct colour balance when used with the appropriate light source. Correct colour for digital images is achieved by choosing auto white balance or by matching the light source to the corresponding white balance setting in the camera’s menu. The types of colour film suitable for this subject are listed below.

www.kodak.com

Choosing a capture medium

There has been a progressive move towards digital image capture due to its ability to produce an image requiring no processing. Instead it creates image files, downloaded to a computer, suitable for printing and desktop publishing. A limiting factor has been the high cost of digital cameras capable of providing the image quality suitable for commercial illustration, but for those photographers who require lower resolution images coupled with quick turnaround and output digital offers an economically viable alternative (see ‘Essential Skills: Digital Imaging’). Commercial photography reproduced in magazines has traditionally been produced using positive or reversal (transparency) film because it is a one-step process to achieve a positive image. However this will undoubtedly change as digital technology creates sensors with more and more pixels recording more image data. Used commercially to a lesser extent negative film creates, within its limitations, an image the opposite of what is viewed through the camera. Only when it is printed does it become a positive image. The advantage of using negative film is its greater ‘latitude’ and ability to handle higher subject contrast levels.

Image processing

Digital image processing is carried out on a computer platform. The commercial illustration industry favours Apple Macintosh computers using the software ‘Adobe Photoshop’. Colour positive film is processed using the E-6 process. This is a Kodak processing system and will process nearly all colour positive films. Colour negative film is processed using the C-41 process. This is also a Kodak processing system and will process nearly all colour negative films. Colour processing should be undertaken by a professional laboratory. Although it is possible to purchase chemicals to process colour film the money saved may well be a false economy when considering the experience and equipment required to produce consistent and accurate colour. The number of black and white processing systems is as varied as the number of films available. Although not covered in this book it is recommended all photographers develop a thorough understanding of black and white processing. It is relatively simple technology (it has changed little since its introduction) and easy to learn.

Appropriate exposure

Assessing correct image exposure can be achieved by viewing the film on a light box or by examining a histogram of the digital image file. It takes practice to be precise about the subtleties of underexposure and overexposure but there is a simple starting point. If there is little or no image detail something is wrong.

If when viewing a digital image a large peak is apparent at one or both ends of the histogram then detail (recorded data) due to incorrect exposure or excessive subject contrast may be missing.

If negative film appears ‘dense’ (transmits very little light) with no visible detail it is probably overexposed. If positive film appears ‘dense’ it is probably underexposed.

If negative film appears ‘thin’ (transmits nearly all light) with no visible detail it is probably underexposed. If positive film appears ‘thin’ it is probably overexposed.

Limitations

Expiry date

All film products have an expiry date printed on their packaging. Do not use film past this date as the manufacturer will not guarantee correct colour. Store unexposed film at a constant temperature, preferably in a refrigerator, but do not freeze.

Colour temperature

It is not important to understand fully the theory of colour temperature other than to know that capturing colour images requires the correct match between capture medium and light source to avoid excessive colour casts. Black and white film is relatively unaffected by colour temperature although a small increase in exposure (as indicated by the MIE reading) is often required when using tungsten lights.

To render correct colour, the use of a ‘white balance’ or filtration can be used to balance any image sensor or film to any lighting situation. The filtration required for film is listed in the manufacturer’s specifications packaged with the film. See ‘Light’.

Kata Bayer

Reciprocity

Reciprocity, more correctly referred to as reciprocity failure, is a measure of the film’s ability or inability to handle extreme exposure times. Reciprocity takes effect when shutter speeds are greater than 1 second when using daylight colour film, greater than 30 seconds when using tungsten colour film and 1 second when using black and white. Without going into the causes of reciprocity the remedy is to reduce shutter speed (time) and compensate by increasing aperture (intensity). Increasing exposure by increasing time will only compound the problem. The results of not compensating for reciprocity is an underexposed image, varying shifts in colour rendition and unpredictable results.

Latitude

Latitude is a measure of the ability or inability to record detail in subjects with extreme contrast and variation from correct exposure. It is accepted most modern films and image sensors have an approximately five to seven stops latitude, although this will increase as manufacturers develop technology. This means if you underexposed an 18% grey card by three stops it would appear black on the processed transparency film or digital file. If you overexposed it by three stops it would appear white. The human eye has almost limitless latitude because of its ability to compensate for changes in contrast and light levels. Film or image sensors are incapable of doing this due to their limited latitude. Black and white and colour negative film have a latitude of seven stops and can handle a contrast ratio of 128:1. Colour transparency with five stops latitude can handle a contrast ratio of 32:1. The human eye is capable of adjusting to a ratio in excess of 1000:1.

Ricky Bond

Push and pull

A safety net all photographers can use is the manipulation of film processing after exposure. Despite the precision of the camera and metering systems used, human and equipment error can still occur when taking a photograph. If the situation allows, bracketing (exposure one and two stops either side of and including normal) is a way of ensuring correct exposure. When there is not the opportunity to bracket and all exposures are meter indicated exposure (MIE) it is advisable to clip test the film.

Clip test

Clip testing is a method of removing the first few frames from an exposed roll of film and processing as normal (i.e. to manufacturer’s specifications). If these frames appear underexposed, a push process (over processing the film) may improve or compensate for any error in exposure. If overexposure is evident a pull process (under processing) may correct the result. The amount of pushing or pulling required to produce an acceptable result is generally quantified in stops. If an image appears underexposed by one stop push the film one stop. If an image appears overexposed one stop pull the film one stop.

Push processing colour transparency having correct exposure is also an option. It has the affect of cleaning up the highlights and giving an appearance of a slight increase in contrast. Pushing in excess of what would be required to achieve this can be an interesting exercise. The results can be unpredictably dramatic. Most professional film processing laboratories offer this service.

Normal process

One stop push - Fabio Sarraff

Cross processing

Similar to over- and underprocessing is the practice of processing a film in chemicals different to that suggested by the manufacturer.

If a transparency film normally processed E-6 is instead processed C-41 (colour negative) the result is quite different to what would be expected. The result is a transparency with negative colours and tones.

Matching a film to an incorrect process can be done in any combination but the results can vary from amazing to very disappointing, but well worth the experimentation.

It is important to note film speed changes when cross processing. As a general rule transparency film should be underexposed by one stop when processing in C-41, and negative film overexposed by one stop. This is only a guide and variations in film speed and processing should be tested to obtain the result you want.

Digital cross processing

Using the software Adobe Photoshop it is possible to create a visual equivalent of cross processing using digital editing techniques (see ‘Essential Skills: Photoshop CS’).

Normal process E-6

Cross process C-41 - Itti Karuson

Image preview

Polaroid

Since its introduction in 1946 Polaroid materials have become a common tool in the assessment of exposure, contrast, composition and design. To most people, whether it be you, a lecturer, an art director or someone wanting a family portrait, it is the first evidence of the photographic process and an indicator of where improvements can be made. Polaroid has become ‘the rough drawings’ on the way to the final photograph. All Polaroid materials, colour or black and white, will give a positive image. In some cases a negative, as well as a positive, will be produced that can be printed at a later date.

When using Polaroid materials closely follow the instructions relating to film speed (ISO or ASA) and observe the processing times relating to room temperature. It is important processing times be followed carefully. When assessing a Polaroid image for exposure relative to another film type it should be realised more detail will be seen in a correctly exposed colour transparency than will be seen in the Polaroid’s positive reflective print. This is because transparencies are viewed by transmitted light and prints by reflective light. The Polaroid image will appear ‘thin’ if it is overexposed and ‘dense’ if it is underexposed. Types of Polaroid film suitable for this subject are listed below.

Digital display

Digital capture supercedes the use of Polaroid as the image can be seen immediately on the camera’s LCD screen or downloaded to a suitable computer. The computer must have a monitor and software capable of displaying the digital file at high resolution together with a histogram of the image levels.

Digital capture

The image sensor of a digital camera turns the image viewed into 12 bits of memory dedicated to each of the three colour channels (RGB). Most digital cameras save in the following file formats: JPEG, TIFF and RAW (see ‘Essential Skills: Photoshop CS’).

JPEG files are created when the image data is processed in-camera. This data, at 8 bits per channel (256 levels per channel), is the smallest file size of the three formats and by compression enables many images to be stored on the camera’s memory card. This format is suitable if minimum post-production is anticipated, the output is to monitor display, the print size no greater than A4 and a large number of images are required. Its limitation is that the higher the compression rate, the greater the loss of quality of the final image.

TIFF format uses ‘lossless’ compression to process the data to 8 bits per channel (256 levels per channel) and is the accepted standard for high quality images and desk top publication (dtp). At present this format creates the largest processed file size the camera is able to output. However fewer images can be stored on the memory card.

RAW format is the unprocessed image data (usually 12 bits per channel) recorded by the camera’s image sensor. This is referred to as a ‘digital negative’ and offers the potential for the highest quality image when processed using designated imaging software. In many ways a RAW file contains similar information to an exposed but unprocessed frame of film. It not only enables the photographer to save an unprocessed version of the image but unlike film can be processed and manipulated as many times and in as many ways as the software allows.

Stuart Wilson

Spiro Alexopolous