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Topic: Focal length

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Focus (optics)

Zoom lens

Lens (optics)

Wide angle lens

Telephoto lens

Focal point

Normal lens

Photographic lens

Magnifying glass

Macro photography

Chromatic aberration

Collimated light

Newtonian telescope

Resolving power

Reflecting telescope

	Focal length - Wikipedia, the free encyclopedia
		The focal length of a lens is the distance along the optical axis from the lens to the focus (or focal point).
		For a negative (diverging) lens, the focal length is negative, and is the distance in front of the lens to the point at which a collimated beam appears to be emerging from after passing through the lens.
		For a spherically curved mirror, the focal length is equal to half the radius of curvature of the mirror.
	en.wikipedia.org /wiki/Focal_length (337 words)

	Focal Length of a Lens (Site not responding. Last check: 2007-11-07)
		The distance from the lens to that point is the principal focal length f of the lens.
		The lens strength in diopters is defined as the inverse of the focal length in meters.
		The principal focal length of a lens is determined by the index of refraction of the glass, the radii of curvature of the surfaces, and the medium in which the lens resides.
	hyperphysics.phy-astr.gsu.edu /hbase/geoopt/foclen.html (278 words)

	Actual focal length determination WRT distance focused
		Since a lens'; actual focal length increases with lens extension upon focusing (law of optics), it's actual focal length is naturally longer than marked and this is then the focal length Leica uses for their frame lines.
		The real focal length of a lens is always measured at infinity and is by definition the distance from the optical center of the lens at which something like the sun forms a single point.
		There may be a slight variation in focal length with lenses that focus internally, retrofocus and such that move the groups/elements individually when focusing rather tahn the whole lens back and forth, but a 50mm lens is essentially a 50mm lens, give or take a mm or 2.
	www.photo.net /bboard/q-and-a-fetch-msg?msg_id=00AQbO (905 words)

	Celestron -- Telescope Basics
		Most manufacturers specify the focal length of their various instruments; but, if it is unknown and you know the focal ratio you can use the following formula to calculate it: focal length is the aperture (in mm) times the focal ratio.
		This is the ratio of the focal length of the telescope to its aperture.
		To calculate, divide the focal length (in mm) by the aperture (in mm).
	www.celestron.com /tb-trms.htm (2665 words)

	Focal Length: Optical: Glossary: Learn: Digital Photography Review
		The focal length of a lens is defined as the distance in mm from the optical center of the lens to the focal point, which is located on the sensor or film if the image is "in focus".
		A change in focal length allows you to come closer to the subject or to move away from it and has therefore an indirect effect on perspective.
		Focal lengths of digital cameras with a sensor smaller than the surface of a 35mm film can be converted to their 35mm equivalent using the focal length multiplier.
	www.dpreview.com /learn?/Glossary/Optical/Focal_Length_01.htm (379 words)

	Focal Length of a Camera Lens
		The focal length of a camera lens is the distance between the center of the lens and the film when an in-focus image is formed of an object very far away.
		The focal length of a camera lens is displayed on the barrel of the lens along with the measurement of the largest aperture and the maker.
		The focal length of a normal lens for a 35 mm SLR camera is approximately 50 mm.
	hypertextbook.com /facts/2002/MiriamJanove.shtml (549 words)

	Focal length - Open Encyclopedia (Site not responding. Last check: 2007-11-07)
		For a thick lens (one which has a non-negligible thickness), or an imaging system consisting of several lenses (e.g., a photographic lens), two focal lengths can be defined: the front focal length (FFL) is the distance from the first (front) focal point of the system to the first optical surface.
		The back focal length (BFL) is the distance from the second (back) focal point to the last optical surface of the system.
		For a thin lens (one which has a negligible thickness), the two focal lengths are measured from the same point, the middle of the lens.
	www.open-encyclopedia.com /Focal_length (271 words)

	Measuring Focal Length
		If a lens actually had the focal length that was marked on it, the rear nodal point would be one focal length in front of the film (or sensor) plane when the lens was focused at infinity.
		So to measure the focal length you either have to determine where the rear nodal point is, or you have to use a method of measurement which doesn't require you to know where it is.
		The focal length of the lens is thus 345/((6-(1/6)) = 345/5.833 = 59.14mm.
	www.bobatkins.com /photography/technical/measuring_focal_length.html (2288 words)

	focal length
		The focal length of a lens is inversely proportional to the power of that lens (the greater the power the shorter the focal length).
		For a convex mirror or concave lens, it is the distance from the centre to the point from which rays of light originally parallel to the principal axis of the mirror or lens diverge after being reflected or refracted.
		The human eye has a lens of adjustable focal length to allow the light from objects of varying distance to be focused on the retina.
	www.tiscali.co.uk /reference/encyclopaedia/hutchinson/m0017397.html (282 words)

	Telescope Numbers (Site not responding. Last check: 2007-11-07)
		The focal length is the distance from the primary mirror (or objective lens, for refractors) to the focal point (which is also called the prime focus).
		Magnification is the focal length of the telescope divided by the focal length of the eyepiece (use the same units, if the eyepiece is specified in mm, then make sure the telescope focal length is also expressed in mm).
		Or, the exit pupil is also equal to the focal length of the eyepiece (in mm) divided by the focal ratio of the telescope (the f number).
	www.bicomnet.com /ritchieobs/pages/numbers.htm (1086 words)

	Focal length (Site not responding. Last check: 2007-11-07)
		The focal length of a lens is the distance along the optical from the lens to the focus (or focal point).
		For a negative (diverging) the focal length is negative and is distance in front of the lens to point at which a collimated beam appears be emerging from after passing through the
		For a spherically curved mirror the focal length is equal to the radius of curvature of the mirror.
	www.freeglossary.com /Focal_length (523 words)

	focal length
		The focal length of a compound optical system, given as if it were a single optical element, is called the equivalent focal length.
		The focal length at which an optical system seems to be working in a given situation is known as the effective focal length.
		The back focal length is the distance between the last surface of a compound optical system and the focal plane of the system and may be quite different from the actual focal length.
	www.daviddarling.info /encyclopedia/F/focal_length.html (198 words)

	* Focal length - (GIS): Definition (Site not responding. Last check: 2007-11-07)
		Focal length: the distance from the middle of the camera lens to the focal plane (i.e.
		Longer focal lengths create a higher zoom level, capture a smaller area of the target scene, and introduce smaller displacement errors...
		f-number-Representation of the speed of a lens determined by the focal length divided by diameter of the lens.
	www.bestknows.com /gis/focal_length.html (112 words)

	Focal Length of Eyeglasses
		Converging lenses are convex with a positive focal length and diverging lenses are concave with a negative focal length.
		Focal length is not stated directly in a prescription for eyeglasses.
		For example a if the refractive power of a lens is 2.0 D then the focal length would be the inverse: 0.5 m.
	hypertextbook.com /facts/2002/RomaKaur.shtml (469 words)

	Focal Length Photography digital camera (Site not responding. Last check: 2007-11-07)
		The focal length is the length of the path through a telescope which incoming light follows.
		The reason for this is that the secondary mirror in a Schmidt-Cassegrain has the effect of multiplying the focal length of the primary mirror by a factor of about 5.
		The magnification of a telescope is determined in part by the focal length of the scope.
	qbox.x3fusion.com /focal-length.html (133 words)

	Telescopes dimensions focal length ratio magnifications
		The distance between the center of the mirror and the point of it being focused by the eyepiece is the focal length.
		A refracting telescope's lens is similar except the light travels through the lens where it is bent by its curvature to the focal length and then magnified by the eyepiece.
		Eyepieces also have focal lengths and these are written in millimetres on the side.
	www.telescopes-astronomy.com.au /telescopes017.htm (187 words)

	Focal length comparisons
		Below are shown five different focal lengths and an example of their coverage.
		This is noticeable in the slight haze visible in the longer focal length shots.
		The last frame was with the 300mm and a 2X extender, for an effective focal length of 600mm.
	www.luminous-landscape.com /tutorials/focal_lengths.shtml (148 words)

	Stoopid lens question
		Yes, if the lens focal length is accurately advertised, a 28mm lens should provide the same angle of view and DOF in a fixed focal length or as part of a zoom's range.
		If I’m using the term correctly the effective focal length is the focal length of the lens as used whereas the focal length proper is the actual focal length of the lens when focused to infinity.
		The focal length of a lens is specified as the distance from the rear nodal point of the lens to the focal plane (film) when the lens is focussed at infinity.
	www.photo.net /bboard/q-and-a-fetch-msg?msg_id=008uKQ&unified_p=1 (1630 words)

	Focal Length - Digital Photography Tutorial - Photoxels
		The focal length of the lens is then defined as the distance from the middle of the lens to its focal point.
		The focal length of a lens is usually displayed on the lens barrel.
		Any lens with a focal length less than 50mm (or 55mm) can be considered as wide-angle; any lens with a focal length greater than 50mm (or 55mm) can be considered as telephoto.
	www.photoxels.com /tutorial_focal_length.html (1083 words)

	Focal Length
		A lens' focal length determines the level of magnification of the image captured in a photograph.
		A lower focal length leads to images appearing smaller in a photograph which will include more area from the scene (a larger field of vision).
		It is generally easier to use the Optical Zoom feature than focal length to specify your requirements.
	www.logipick.com /~toPage-PageDefDvcFlen (235 words)

	Focal Length and Perspective
		The longer lens focal lengths narrowed the angle of view to crop and enlarge the area of interest in the scene.
		Again a longer focal length was used to crop and enlarge the area of interest.
		At first glance it appears that the focal lengths of the various lenses have caused the differeces in perspective, but if you look again at the photos at the top of the page you will see that this is not the case.
	jamesmskipper.tripod.com /jamesmskipper/perspective.html (1580 words)

	Photography - Focal Length - Angle Subtended (Site not responding. Last check: 2007-11-07)
		A measure of focal length is in units of "length"; usually units are millimeters (mm).
		The focal length is that distance from the center of the lens to the point in space where the sunlight converges into an intense "hot" spot.
		Lenses with focal lengths greater than 50-mm are referred to as telephoto and zoom lenses (a variable telephoto).
	www.ncc.sdccd.cc.ca.us /resource/qtvr/qtvrlm2/lm2p1.html (326 words)

	Focal length (Site not responding. Last check: 2007-11-07)
		The focal length of a lens is the distance along theoptical axis from the lens to the focus (or focal point).
		For a negative (diverging)lens, the focal length is negative, and is the distance in front of the lens to the point at which a collimated beam appears tobe emerging from after passing through the lens.
		For a spherically curved mirror, the focallength is equal to half the radius of curvature of themirror.
	www.therfcc.org /focal-length-109705.html (138 words)

	The Panorama Factory -- Focal length fine tuning does not work for 24-bit images
		There is a major problem with the Fine tune option on the Focal length dialog for 24-bit images.
		Transfer the focal length value that results from fine tuning to the Focal length dialog in the original project.
		Adjust the focal length and restitch the panorama.
	www.panoramafactory.com /trouble/trouble_38.html (310 words)

	Focal Length Multiplier: Optical: Glossary: Learn: Digital Photography Review
		The smaller sensor with FLM of 1.5 captures the same field of view as the 200mm lens on a 35mm camera by using a lens with a shorter focal length of 133mm (200mm/1.5).
		Also because the 35mm equivalent field of views are achieved with shorter focal lengths, the depth of field is larger (2).
		Because of the very small focal lengths used, the depth of field is much larger (2) than digital SLRs or 35mm film cameras with the same field of view.
	dpreview.com /learn?/Glossary/Optical/Focal_Length_Multiplier_01.htm (807 words)

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