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Filters for colour photography.. Selection and use of materials.. Book of photography

Filters for colour photography

The saturated filters used for black and white are not suitable for colour films: filters with low density, with accurate colours, are required. There are a large number of filters of this type and indeed few of these filters are indispensable and few are useful for popular photography.

The reversible colour films of the daylight model have a colour temperature of approximately 5400 K and are suitable for sunlight at noon. When this light temperature differs (morning or evening), our mind automatically tends to correct this difference. For example, a white sheet of paper when viewed at dusk always appears white to us, in contrast, when a slide (positive image) is displayed, the mind does not apply the same correction and a picture taken in light with a different colour temperature appears as if it has a distinct colour. Some photographers believe that the charm and beauty of the dusk lies in the brilliance of warm colours and that the desire to correct them is. Contrary to what other photographers prefer some correction, even if it is partial. No, the K5400 does not seem to be a novelty in this regard except for the daylight film used in artificial light: the dominant blue tint that we get when using the daylight film without a filter in artificial light must be corrected completely. In general, it is not unpleasant for a whole series of slides to represent a similar dominant light color. Conversely, the sudden transition from a dominant blue color to a dominant yellow color (in two consecutive slides on the screen) has a bad effect on the viewer. Positive images intended for, for example, an audio-visual scene must therefore be corrected with great care.

- Color temperature:
When a piece of metal is heated, it first becomes dark red, then as the temperature increases, the light it emits passes from light red to yellow and blue. At very high temperatures, the quality of light can be expressed by taking as a basis the temperature of an imaginary substance or (black body) that emits such light. This color temperature is expressed in absolute degrees or degrees Kelvin (K). The problem is that a filter that brings a correction of K100 to K2800 light will have a value of 130 to 300 for K5000 light. The Merid method allows to mitigate this drawback and to renew each time the correct value of the correcting filter that will be used. The thermocolorimeter (thermochromatometer) allows to measure the color temperature in a continuous spectrum source.

- Using conversion filters:
Daylight film is also suitable for electronic flash and blue-capped flashes, but if this film is used in incandescent light, a blue conversion filter must be placed in front of the lens at the expense of a great loss of the actual film speed: this conversion method is not recommended. If we want to use the same type of film in incandescent light and daylight, it is better to take artificial light film (K3400: A or K3200: B) and use the orange conversion filter when working in daylight. In general, artificial light film is faster and the conversion filter only returns the actual film speed to that of daylight film.

- Anti-UV filter:
This colorless filter absorbs ultraviolet radiation and significantly reduces the atmospheric veil.

- Filters designed for colored clouds:
With negative color films, slight differences in the color temperature of the light during the taking of pictures can be corrected, as can other dominant colors caused by special conditions. Always refer to the instructions given by the film maker.

Filters for decreasing color extraction have yellow, red, and blue colors. Fingerprints can easily be imprinted on them. Additional filters: blue, red, and green are used especially under the lens and must also be flawless.

- Converting degrees Kelvin to MERID values:
To find the MERID value of a light source, divide 1 million by its color temperature expressed in degrees Kelvin. Therefore, MERID values ​​increase in the opposite direction to the degrees Kelvin. A blue filter has a negative MERID value and a yellow filter has a positive value. It is worth noting that with MERID values, the filter always gives the same correction value, whatever the color temperature of the illuminated object to which it is applied. Filter values ​​are sometimes referred to as deca-MERIDs (1 deca-MERID = 10 MERIDs). The eye is sensitive to a change of 1 deca-MERID or more.

- Converting Merid values ​​to Kelvin degrees: The Merid value of a light source is equal to 1,000,000 divided by the color temperature of that light source expressed in Kelvin degrees (K). See the conversion scale.

- Optical accessories:
In addition to filters, there are other optical accessories that can be attached to the front of the lens. Some of them affect the adjustment of the focal length of the lens and others change the image view. In most cases, the same optical accessory can be attached to different lenses for different purposes. Usually, there is an almost tangible loss in image quality.

The distortion device allows obtaining views that include a very wide field of view (in the manner of CinemaScope) on normal-size film. The image captured with this lens is compressed on both sides so that the recorded image is stretched in the vertical direction. When using a similar optical accessory during display, the image restores the natural proportions of the subject but in a panoramic view.

There are two types of focal methods: the wide-angle and the telephoto (or projective) lens. They adjust the true focal length of the lens they are attached to. They are used especially in machines with a lens that cannot be replaced. The main lens must be. calibrated at infinity.

The right angle device can be used in all 24 x 36 reflex machines and allows people to be photographed without them noticing since the photographer appears to be aiming in another direction. The image is returned from a smooth mirror from the outside.

As for the glass prism lenses, they are placed on the lens and give different effects (image explosion or multiple subjects, etc.) depending on the number of their faces and their orientation. As for the star lens, it is a small transparent glass lens containing triangular grooves that give the shape of a star to all the bright points in an image. As for the lens with blurred edges, it gives an image whose center is clear while its edges gradually become blurred. The softening lens softens the entire image, immersing it in a slight ambiguity and lack of clarity.