Definition of focal length in English:

focal length


  • 1The distance between the center of a lens or curved mirror and its focus.

    • ‘The focal length of a lens, for example, varies depending on the colour of light that is being focused.’
    • ‘The shorter the focal length of the lens, the wider the field of vision.’
    • ‘When you increase the focal length of the lens (or decrease the film size) an apparent ‘compression’ of the image occurs.’
    • ‘The focal length of this lens can be varied somewhat without compromising the quality of the data.’
    • ‘This produces a rounded lens with a small focal length, so the drop can focus light only to a point near itself.’
    • ‘In 1647 Cavalieri published an important contribution to optics when he gave the relationship between the curvature of a thin lens and its focal length.’
    • ‘Yet Stork determined the probable focal length of a concave mirror made by inverting and silvering the convex mirror shown in the painting.’
    • ‘A double convex lens with a focal length of 50 cm is fixed in a PVC pipe about 60 cm long.’
    • ‘Changing the lens's curvature changes the focal length, so that images at different distances appear in focus.’
    • ‘Long focal length lenses require less diffusion than short ones to achieve the same visual effect.’
    • ‘That behavior is not affected by the medium in which the light travels, so the mirror's focal length would be no different in air or water.’
    • ‘Varying the voltage across the lens, the researchers could change the focal length of the lens, because this alters its refractive index.’
    • ‘I suggest that you look at any one that has a range in the focal length of the lens.’
    • ‘To overcome this, we must use a large number of mirror shells and as long a focal length as possible.’
    • ‘Telephoto lenses have focal lengths that range from about 85 mm (good for portraits) to supertelephotos' 300 mm to 600 mm or longer.’
    • ‘This will give a negative focal length for a convex lens of air inside water.’
    • ‘A dioptre is 1 divided by the focal length of the lens in centimetres.’
    • ‘Three things control how great the depth of field will be in a photograph: lens focal length, aperture, and distance from the subject.’
    • ‘The focal length of the lens that you use, depends on where you are in relation to the fireworks and the composition you're looking for.’
    • ‘Our team chose two tube lenses with focal lengths that provided the necessary magnifications and fields of view.’
    1. 1.1The equivalent distance in a compound lens or telescope.
      • ‘One evening he was observing with a rather crude refractor (a lens telescope), with a focal length of 24 feet.’
      • ‘The tube of the Gregorian telescope is thus shorter than the sum of the focal lengths of the two mirrors.’
      • ‘He developed a general rule for the focal length of lenses and described a reflecting telescope.’
      • ‘By changing the distance between particular lenses, you can adjust the magnification power - the focal length - of the lens as a whole.’
      • ‘He devised numerous ways of reducing the physical size of telescopes of long focal length, and machinery with which to move them.’
      • ‘Huygens later went back to the study of optics and developed long focal length lenses used in ‘aerial telescopes.’’
      • ‘Small phase change leads to a long focal length, which is more suitable for telescope and satellite imaging applications.’
      • ‘The magnification is simply the focal length of objective lens divided by the focal length of eyepiece.’
      • ‘The higher graze angle allows a shorter focal length and also preserves the collecting area of the telescope.’


focal length

/ˈfōkəl ˌleNGTH/