Glossary Abbe number The reciprocal of dispersion. aberrations Optical errors in a lens which limit its capabilities ­ causing the images it forms to differ from subject appearance, usually in small details. accelerator Chemical ingredient of developer to speed up the otherwise low activity of developing agents. Normally an alkali such as sodium carbonate, borax or (high-contrast developers) sodium hydroxide. Also known as `activator' or `alkali component'. achromatic lens Corrected for chromatic aberration exactly in two separate wavelengths, usually red and blue, which are brought into the same focus plane. acid Chemical giving solutions in water with pH below 7. Because acid neutralizes an alkali, acidic solutions are often used to halt development ­ as in stop bath or fixer. active pixel sensor See CMOS. acutance Physical measure of how the transition from high density to low density reproduces in a developed film which has been exposed to a high contrast (test chart) subject.