For infinitely distant objects the radius Of the chromatic disk of confusion is proportional to the linear aperture, and independent of the focal length ( " vide supra ", " Monochromatic Aberration of the Axis Point " ); and since this disk becomes the less harmful with an increasing image of a given object, or with increasing focal length, it follows that the deterioration of the image is proportional to the ratio of the aperture to the focal length, i . e . the " relative aperture . " ( This explains the gigantic focal lengths in vogue before the discovery of achromatism .)
