Measure of how strongly an optical system converges or diverges light
"Rear focal distance" redirects here. For lens to film distance in a camera, see Flange focal distance.
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The focal point F and focal length f of a positive (convex) lens, a negative (concave) lens, a concave mirror, and a convex mirror.
The focal length of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power. A positive focal length indicates that a system converges light, while a negative focal length indicates that the system diverges light. A system with a shorter focal length bends the rays more sharply, bringing them to a focus in a shorter distance or diverging them more quickly. For the special case of a thin lens in air, a positive focal length is the distance over which initially collimated (parallel) rays are brought to a focus, or alternatively a negative focal length indicates how far in front of the lens a point source must be located to form a collimated beam. For more general optical systems, the focal length has no intuitive meaning; it is simply the inverse of the system's optical power.
In most photography and all telescopy, where the subject is essentially infinitely far away, longer focal length (lower optical power) leads to higher magnification and a narrower angle of view; conversely, shorter focal length or higher optical power is associated with lower magnification and a wider angle of view. On the other hand, in applications such as microscopy in which magnification is achieved by bringing the object close to the lens, a shorter focal length (higher optical power) leads to higher magnification because the subject can be brought closer to the center of projection.
The focallength of an optical system is a measure of how strongly the system converges or diverges light; it is the inverse of the system's optical power...
In photography, the 35 mm equivalent focallength is a measure of the angle of view for a particular combination of a camera lens and film or image sensor...
the flange focal distance (FFD) (also known as the flange-to-film distance, flange focal depth, flange back distance (FBD), flange focallength (FFL), back...
In film and photography, a prime lens is a fixed focallength photographic lens (as opposed to a zoom lens), typically with a maximum aperture from f2...
In digital photography, the crop factor, format factor, or focallength multiplier of an image sensor format is the ratio of the dimensions of a camera's...
called the "circle of confusion". The depth of field can be determined by focallength, distance to subject (object to be imaged), the acceptable circle of...
photography and cinematography, in which the physical length of the lens is shorter than the focallength.: 93 This is achieved by incorporating a special...
{o} }} and image distance d i {\displaystyle d_{\mathrm {i} }} to the focallength f {\displaystyle f} : 1 d o + 1 d i = 1 f {\displaystyle {\frac {1}{d_{\mathrm...
the vertex and the focus, measured along the axis of symmetry, is the "focallength". The "latus rectum" is the chord of the parabola that is parallel to...
to a camera, or it might be interchangeable with lenses of different focallengths, apertures, and other properties. While in principle a simple convex...
In contrast, depth compression and expansion with shorter or longer focallengths introduces noticeable, and sometimes disturbing, distortion. Photographic...
camera lens elements for which the focallength (and thus angle of view) can be varied, as opposed to a fixed-focal-length (FFL) lens (prime lens). A true...
telescope's focallength f {\displaystyle f} divided by the eyepiece focallength f e {\displaystyle f_{e}} (or diameter). The maximum is limited by the focal length...
the origin O in the negative direction of the X3 axis, where f is the focallength of the pinhole camera. A practical implementation of a pinhole camera...
on the retina of the eye.) The amount of magnification depends on the focallength of the eyepiece. An eyepiece consists of several "lens elements" in a...
equal to the reciprocal of the focallength of the device: P = 1/f. High optical power corresponds to short focallength. The SI unit for optical power...
approximation in techniques such as ray transfer matrix analysis. The focallength, f, of a lens in air is given by the lensmaker's equation: 1 f = ( n...
distance from this point to the lens is the focallength, though it is negative with respect to the focallength of a converging lens. Convex-concave (meniscus)...
differs significantly from what the object would look like with a normal focallength, due to the relative scale of nearby and distant features. Perspective...
A varifocal lens is a camera lens with variable focallength in which focus changes as focallength (and magnification) changes, as compared to a parfocal...
typical at long focallengths. Transverse aberration occurs when different wavelengths are focused at different positions in the focal plane, because the...
This is a list of smartphones with a telephoto lens that offers a focallength (35mm equivalent) of at least 100mm or "4× optical zoom" with an imaging...
camera shake is a particular problem at slow shutter speeds or with long focallength lenses (telephoto or zoom). With video cameras, camera shake causes visible...
large telescopes which have mirrors consisting of several segments. The focallength of the paraboloid is related to the angular speed at which the liquid...
optical power rather than focallength is that the thin lens formula has the object distance, image distance, and focallength all as reciprocals. Additionally...
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