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Definition: a measure of how strongly an optical system focuses or defocuses light
Various types of optical systems (e.g. microscope objectives and curved laser mirrors) can focus or defocus light, and the focal length is used for quantifying such effects. The simplest case is that of a thin focusing lens (Figure 1a). If a sufficiently large collimated beam of light is incident on the lens, the beam will be focused, and the focal length is the distance from the lens to that focus (assuming that the lens is surrounded by vacuum or air, not by some dense substance with a significant refractive index). For a defocusing lens (Figure 1b), the focal length is the distance from the lens to the virtual focus (indicated by the dashed lines), taken as a negative value.
Focal Length of a Lens
An ideal thin lens with a given focal length f creates a radially varying phase delay for a laser beam according to the following equation:
This formula ignores the constant part of the phase change as well as aberrations.
The curvature radii are taken as positive values for convex surfaces and negative for concave surfaces. Positive results are obtained for focusing lenses, negative results for defocusing lenses. The last term is relevant only for thick lenses with substantial curvature on both sides.
The equation holds for paraxial rays, not too far from the symmetry axis.
Focal Length of a Curved Mirror
Curved mirrors are often used for focusing or defocusing light. For example, within laser resonators curved laser mirrors with dielectric coatings are more commonly used than lenses, mainly because they introduce lower losses.
A mirror with a curvature radius R of the surface has a focal length f = R / 2, if the beam axis is normal to the mirror surface. (We take positive signs for concave curvatures and focusing mirrors.) If there is some non-zero angle θ between the beam axis and the normal direction, the focal length is ftan = (R / 2) · cos θ in the tangential direction (i.e., within the plane of incidence) and fsag = (R / 2) / cos θ in the sagittal direction.
Curved laser mirrors usually have a curvature radius somewhere between 10 mm and 5 m. The fabrication of dielectric mirror coatings can be more difficult for very strongly curved mirror substrates, but with refined techniques it is possible to reach focal lengths of only a few millimeters, as required for some miniature lasers.
Focal Length of an Extended Optical System
For an optical system, which may consist of multiple lenses and other optical elements, the above definition of the focal length cannot be used, as it is not clear a priori for an extended system where to measure the distance to the focus: from the entrance into the optical system, from the exit, the middle, or some other position? In principle, an arbitrary definition of a reference point (e.g. the entrance) can be used, but this will in general mean that some common rules can not be applied, which e.g. hold for the radius of the beam waist at a focus behind some lens with a given focal length (see below), or the possible magnification of a telescope containing that optical system. Some authors use terms such as the front focal distance (rather than front focal length) for indicating the distance of the focus from the front surface of the optics, rather than an effective focal length which allows that value to be used in equations for the size of the focus (see below), for example.
Effective Focal Length of a Photographic Objective
Considerable confusion arises from the fact that in the context of photography the term effective focal length is also used with a completely different meaning, as explained in the following.
The angle of view of the camera is determined by the ratio of the image size on the film and the focal length. Film-based cameras have for a long time mostly used 35-mm film (also called 135 film according to ISO Standard 1007), where the image size on the film is typically 36 mm × 24 mm. (The width of the film spool is 35 mm, somewhat larger than 24 mm, as the picture does not extend to the edges of the spool.) A standard objective then has a focal length of 50 mm. However, modern digital cameras (particularly the more compact ones) often contain image sensors which are smaller than 36 mm × 24 mm, so that an objective lens with a correspondingly smaller focal length (e.g. 32 mm instead of 50 mm) is required for obtaining the same field of view. As many photographers are still used to the previously valid relation between focal length and angle of view, it has become common to specify the effective focal length of an objective of a digital camera as that focal length which would give the same angle of view in combination with ordinary 35-mm film. For example, an objective with a true focal length of 32 mm may then be said to have an effective focal length of 50 mm and thus function as a standard objective, rather than e.g. a macro or tele objective.
It can be expected that this type of conversion will be abandoned as 35-mm film becomes less and less common.
Optical Systems with an Adjustable Focal Length
For some applications, in particular for focusing of imaging systems, it is essential than the focal length of an optical system can be fine adjusted. The following physical principles can be used:
- If a lens is made of deformable material, applying some mechanical pressure can alter its shape, and this can change the focal length. This principle is used in the eye's lens. The focal length is somewhat reduced for focusing on nearby objects.
- When an optical system contains multiple optical elements (e.g. lenses), the focal length may be tuned by adjusting the relative distances between the optical elements. This principle is used e.g. in photographic zoom objectives.
Wavelength Dependence of the Focal Length; Using Curved Mirrors
Ordinary lenses, working on the basis of refraction, have a focal length which is slightly wavelength-dependent due to the wavelength dependence of the refractive index (–> chromatic dispersion). This effect leads to chromatic aberrations of imaging systems and similar problems in other applications where an optical system is used for a wide range of optical wavelengths. Lens combinations (e.g., objectives for photographic cameras) can be designed such that chromatic aberrations are minimized. Most common is the use of achromatic doublets, i.e., lenses consisting of two different glass materials chosen such that the overall chromatic aberrations are largely canceled.
One may eliminate chromatic aberrations altogether by using optical systems with mirrors only. A curved mirror with radius of curvature R has a focal length f = R / 2 (for normal incidence), determined only by the geometry and thus independent of the wavelength. On the other hand, for non-normal incidence the focal length in the tangential direction is decreased by the cosine of the angle of incidence, and increased by the inverse cosine of that angle in the sagittal direction. Therefore, such mirrors can introduce astigmatism.
The dioptric power (also called focusing power) of a lens is defined as the inverse of the focal length. This means that a strongly focusing lens has a small focal length, but a large dioptric power. The dioptric power is measured in units of m−1, also called diopters (dpt). For prescription glasses, it is common the specify the dioptric power, whereas the focal length is specified for standard lenses, microscope objectives, and photographic objectives.
In many cases, the dioptric power is a more natural quantity than the focal length. For example, the dioptric power of the thermal lens in a laser crystal is proportional to the dissipated power. The width of the stability zones of a laser resonator with respect to dioptric power of the thermal lens depends only on the minimum mode radius in the laser crystal and on the optical wavelength, whereas the stability range in terms of focal length has a more complicated dependence.
Focusing of Divergent Beams
If a divergent (rather than collimated) beam hits a focusing lens, the distance b from the lens to the focus becomes larger than f (Figure 2). The lens equation states that
where a is the distance from the original focus to the lens. This shows that b ≈ f if a >> f, but b > f otherwise. That relation can be intuitively understood: a focusing power 1 / a would be required to collimate the incident beam (i.e. to remove its beam divergence), so that only a focusing power 1 / f − 1 / a is left for focusing.
If a ≤ f, the equation cannot be fulfilled: the lens can then not focus the beam.
Note that the lens equation applies for rays, assuming that the paraxial approximation is valid, i.e., all angles relative to the beam axis remain small.
Achievable Beam Waist Radius
where it is assumed that the beam radius at the focus is much smaller than the initial beam radius w0. (This condition is violated for beams with a too small incident radius; the focus is then larger than according to the given equation.) Also, it is assumed that the beam radius is significantly larger than the wavelength λ, so that the paraxial approximation is valid.
The equation shows that what determines the minimum possible beam radius is not the focal length f alone, but rather the ratio of f to the radius of the open aperture of the lens, which sets a maximum to the input beam radius w0. That ratio is essentially the numerical aperture of the lens.
Whether this rule can also be applied to an extended optical system with focal length f depends on the applied definition of f. It is useful to specify an effective focal length which is valid for such relations.