Deceptively simple pieces of colored glass or plastic, filters are extremely versatile photographic tools, allowing us to change the tones of an image very subtly or by extreme amounts. To understand them fully, it is worth taking a moment to consider just what an optical filter does. Certain wavelengths of light are absorbed, while others pass through freely, allowing the photographer to use just the type of light to suit the subject and film and if necessary shift the emphasis.
The rule is that a filter passes light matching its own color, and absorbs (darkens) other colors – particularly those farthest from it in the spectrum. To check out filter effects, first find yourself a bold, multi-colored design – a book jacket or cornflake packet perhaps. View it through a strong red filter (even a sweet wrapping will do). The whole subject appears red, but check how light or dark the original colors now appear. Strong blue and green subject areas are relatively darker in tone, almost indistinguishable from black areas. This is because very few of their colors make it through the filter.
Red parts of the subject, however, look much paler than before, practically the same as white areas since they reflect as much red light as the white areas do (the other colors that white reflects are absorbed by the red filter). The same tonal changes occur when the light source illuminating your subject is filtered instead of your eye or camera lens, so a red filter over your studio lights or flashgun will have the same effect.
Colored filters help you to alter the relationship between the grey tones made by monochrome film. Used with color film they allow you to shoot using light sources for which the film is not balanced (e.g. daylight film in tungsten light) without getting an overall color cast. By using paler filters you can gently ‘warm up’ or ‘cool down’ color images when the subject or lighting color is not ideal.
Filter types. Photographic filters are made in three main types. The simplest are very thin flexible squares of dyed gelatin or polyester which you either hold over the lens or fit into a filter holder. These are relatively cheap but soon pick up finger marks and scratches if used often. The majority of dyed filters are now manufactured in optical (CR 39) resin, square in shape to slip into a holder. Unlike the gelatin type, these are not flexible and will crack if dropped but
they resist scratching much better. The third type are made in glass and sold in circular mounts which screw into the front rim of the lens. These tend to be the most expensive as they are usually made from optical glass and frequently treated with anti-reflection coatings like camera lenses.
Remember that the filter is the first thing the light passes through and therefore it needs to be as clean and free from damage as possible. A glass or resin filter may slightly alter the position of sharp focus, so always focus with your filter in place, especially if shooting at a wide aperture. It’s difficult to fit a filter over the front of a wide-angle lens without the corners of the picture darkening (‘cut-off ’). Some extreme wide-angles therefore have 3 or 4 internal color filters which a dial on the lens barrel will bring into use.
Because a colored filter absorbs part of the light from the image, it reduces its brightness. This may be taken care of by a ‘through-the-lens’ exposure meter such as in SLR cameras because it reads through the filter. If using a separate hand-held meter you will have to increase exposure by a ‘filter factor’.
