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Colour balance

The colour reproduction fidelity of a photo taken through a lens compared to the original subject. Colour balance in all EF lenses is based on ISO recommended reference values and maintained within a strict tolerance range narrower than ISO's CCI allowable value ranges CCI

CCI (colour contribution index)

Colour reproduction in a colour photograph depends on three factors: the colour characteristics of the film or digital imaging system, the colour temperature of the light source illuminating the subject, and the light transmission characteristics of the lens. The colour contribution index, or CCI, is an index indicating "the amount of colour variation caused by filtering effect differences between lenses" when using a standard film and light source, and is expressed by three numbers in the form 0/5/4. These three numbers are relative values expressed as logarithms of lens transmittance at the blue-violet/green/red wavelengths corresponding to the three light sensitive emulsion layers of colour film, with larger numbers representing higher transmittance. However, since photographic lenses absorb most ultraviolet wavelengths, the blue-violet transmittance value is usually zero, so colour balance is judged by comparing the green and red values to ISO-specified reference lens values. The ISO reference lens light transmission characteristics were set according to a method proposed by Japan which involved taking the average transmittance values of 57 standard lenses

Graph-6 ISO Tolerance Range Graphed on CCI Coordinates

Graph-6 ISO Tolerance Range Graphed on CCI Coordinates

Cyan

Cyan comprising five models from representative lens manufacturers including Canon. The resulting recommended reference value of 0/5/4 is used by film manufacturers as a reference when designing the colour production characteristics of colour films. In other words, if the light transmission characteristics of a lens do not match the ISO reference values, the colour reproduction characteristics of a colour film cannot be obtained as intended by the manufacturer.

Peripheral illumination

The brightness of a lens is determined by the F number, but this value only indicates the brightness at the optical axis position, i.e., at the center of the image. The brightness (image surface illuminance) at the edge of the image is called peripheral illumination and is expressed as a percent (%) of the amount of illumination at the image center. Peripheral illumination is affected by lens vignetting and the cos4 (cosine 4) law and is inevitably lower than the center of the images Vignetting, Cos4 law

Graph-7 Image Plane Illuminance Ratio

Showing the Peripheral Illumination Characteristics

Graph-7 Image Plane Illuminance Ratio

Showing the Peripheral Illumination Characteristics

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