Introduction

The purpose of a filter is to selectively modify the light used for exposure. Filters are regularly used by professional photographers. They are an indispensable means of controlling the variations in light the photographer is likely to encounter. The range of filters used varies according to the range of situations likely to be encountered. However, when capturing Raw file images it is possible to undertake color correction in camera or in post-production. See ‘Characteristics of Light’.

Studio

In the studio the photographer is able to create images with a consistent quality using few, if any, filters. The photographer must simply ensure the film stock or image sensor white balance is carefully matched to the light source being used. The studio photographer has the option of filtering the light source, the camera lens or placing the filters between the lens and the image plane. If the photographer is using a mixture of light sources the photographer should ideally filter the light at its source. Any filter attached to the camera lens must be of premium quality - preferably glass.

Location

On location where ambient light is the primary light source or cannot be eliminated from the overall exposure the necessity to carry and use a broad range of filters increases. If choosing to purchase filters for lenses the photographer needs to be aware of the thread size on the front of each lens intending to be used. Filter sizes for fixed focal length, wide-angle and standard lenses on 35mm and DSLR cameras are usually between 48 and 55mm. Mediumformat, telephoto and zoom lenses may have thread sizes much larger. Purchasing every filter for every lens would be an expensive operation, so filters have to be selected carefully through actual rather than perceived need. Plastic filters are available from manufacturers such as ‘Cokin’ which can be adapted to fit a range of different lens diameters. The initial outlay to filter all lenses can be greatly reduced, but their working life may be far less due to the greater risk of damage.

Basic color theory

To be comfortable with filtration it helps to understand basic color theory. The broad spectrum of visible light is divided into three primary colors and three secondary colors. The primary colors of light are red, green and blue (RGB). The secondary colors are yellow, cyan and magenta. When used in the printing industry to create images black is added (CMYK or four-color printing). Each secondary color is a combination of two primary colors and is ‘complementary’ to the third primary. ~ Yellow is complementary to blue ~ Cyan is complementary to red ~ Magenta is complementary to green.