 |
|
| A B C D E F G H I J K L M N O P Q R S T U V W X Y Z |
| • HOME • ABOUT • CATEGORIES • USE OF TEXT AND IMAGES • NEWSLETTER   
|
focal ratio
The ratio of the focal length (F) of a mirror or lens to its diameter (D) expressed as a number (also called the f-number or f-stop), f/# = F/D; it is a measurement that describes the relative opening or aperture. For example, a lens with a 50mm focal length with the aperture set at f/2 indicates an aperture diameter of 25mm,whereas f/4 indicates an aperture of 12.5mm. Borrowing the language of photography, small focal ratios, below about f/6, are said to be fast and result in a brighter image for a given aperture. Large focal ratios, equal to or above about f/8, are said to be slow.
Related category
• OPTICS AND OPTICAL PHENOMENAAlso on this site:
Encyclopedia of Alternative Energy & Sustainable Living
Encyclopedia of History