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When the focal film distance (FFD), also known as source-to-image distance (SID), is halved, the exposure factor must be adjusted to maintain the desired image quality. The relationship between FFD and exposure is governed by the inverse square law, which states that the intensity of radiation is inversely proportional to the square of the distance from the source.

If you reduce the distance between the x-ray source and the film to half, the intensity of radiation reaching the film will increase by a factor of four (since (1/2)² = 1/4). This means that for the same amount of radiation exposure at the original distance, you would need only one-quarter of the exposure factor to achieve a similar exposure level on the film.

Thus, to compensate for the halving of the distance and to prevent overexposure or to maintain consistent image quality, the exposure factor should be reduced to one-quarter of the original. This adjustment ensures that the image remains properly processed and maintains optimal quality.