- Defocus aberration
Optical aberration Distortion
In optics, defocus is the aberration in which an image is simply out of focus. This aberration is familiar to anyone who has used a camera, videocamera, microscope, telescope, or binoculars. Optically, defocus refers to a translation along the optical axis away from the plane or surface of best focus. In general, defocus reduces the sharpness and contrast of the image. What should be sharp, high-contrast edges in a scene become gradual transitions. Fine detail in the scene is blurred or even becomes invisible. Nearly all image-forming optical devices incorporate some form of focus adjustment to minimize defocus and maximize image quality.
The degree of image blurring for a given amount of focus shift depends inversely on the lens f-number. Low f-numbers, such as f/1.4 to f/2.8, are very sensitive to defocus and have very shallow depths of focus. High f-numbers, in the f/16 to f/32 range, are highly tolerant of defocus, and consequently have large depths of focus. The limiting case in f-number is the pinhole camera, operating at perhaps f/100 to f/1000, in which case all objects are in focus almost regardless of their distance from the pinhole aperture. The penalty for achieving this extreme depth of focus is very dim illumination at the imaging film or sensor, limited resolution due to diffraction, and very long exposure time, which introduces the potential for image degradation due to motion blur.
The amount of allowable defocus is related to the resolution of the imaging medium. A lower-resolution imaging chip or film is more tolerant of defocus and other aberrations. To take full advantage of a higher resolution medium, defocus and other aberrations must be minimized.
Defocus is modeled in Zernike polynomial format as a(2ρ2 − 1), where a is the defocus coefficient in wavelengths of light. This corresponds to the parabola-shaped optical path difference between two spherical wavefronts that are tangent at their vertices and have different radii of curvature.
For some applications, such as phase contrast electron microscopy, defocused images can contain useful information. Multiple images recorded with various values of defocus can be used to examine how the intensity of a the electron wave varies in three dimensional space, and from this information the phase of the wave can be inferred. This is the basis of non-interferometric phase retrieval. Examples of phase retrieval algorithms that utilise defocused images include the Gerchberg-Saxton algorithm and various methods based on the transport of intensity equation.
- Smith, Warren J., Modern Optical Engineering, McGraw-Hill, 2000, Chapter 11, ISBN 0-07-136360-2
Wikimedia Foundation. 2010.
Look at other dictionaries:
Aberration — has several meanings:*An abnormality of chromosomes *A disorder in one s mental state (see Mental illness) In optics: *Optical aberration, an imperfection in image formation by an optical system **Spherical aberration, which occurs when light… … Wikipedia
Optical aberration — v · d · e Optical aberration … Wikipedia
Chromatic aberration — On top is corner detail in a photograph taken with a higher quality lens; bottom is a similar photograph taken with a wide angle lens showing severe visible lateral chromatic aberration. The effect is pronounced on the right side of the building… … Wikipedia
Eyeglass prescription — Using a phoropter to determine a prescription for eyeglasses An eyeglass prescription is an order written by an eyewear prescriber, such as an optometrist or ophthalmologist, that specifies the value of all parameters the prescriber has deemed… … Wikipedia
Focus (optics) — An image that is partially in focus, but mostly out of focus in varying degrees. In geometrical optics, a focus, also called an image point, is the point where light rays originating from a point on the object converge. Although the focus is… … Wikipedia
Blur — can refer to:Unfocused imagery Blur generally refers to the appearance of an unfocused image. See Defocus aberration, eyeglass prescription, lens and eye. Specific terms using this meaning of Blur include:* Bokeh, the appearance of out of focus… … Wikipedia
High-resolution transmission electron microscopy — (HRTEM) is an imaging mode of the transmission electron microscope (TEM) that allows the imaging of the crystallographic structure of a sample at an atomic scale. [cite book |title=Experimental high resolution electron microscopy |last=Spence… … Wikipedia
Transmission electron microscopy — A TEM image of the polio virus. The polio virus is 30 nm in size. Transmission electron microscopy (TEM) is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it… … Wikipedia
Circle of confusion — For the closely related topic in microscopy, see Point spread function. In optics, a circle of confusion is an optical spot caused by a cone of light rays from a lens not coming to a perfect focus when imaging a point source. It is also known as… … Wikipedia
Crystallographic image processing — (CIP) is a set of methods for determining the atomic structure of crystalline matter from high resolution electron microscopy (HREM) images obtained in a transmission electron microscope (TEM). The term was created in the research group of Sven… … Wikipedia