actinic focus

Definition: The point in space at which an optical system focuses the most chemically effective part of the electro-magnetic spectrum for a given actinic reaction. * E.g. with blue-sensitive black & white printing papers, the actinic focus of the enlarging lens is the focal point for blue light - hence the practice of focusing the baseboard image with a blue filter when using lenses with undercorrected chromatic aberration. * Also known as chemical focus.
Through the glass, blue-ly
With blue-sensitive black & white printing papers, the actinic focus of the enlarging lens is the focal point for blue light - from which arose the now largely obsolete practice of focusing the enlarger baseboard image with a blue filter when using lenses with undercorrected chromatic aberration. Modern lenses are very well corrected. Besides, with increased use of multigrade papers, the actinic focus is spread over more wavelengths.

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