Blur

generally refers to the appearance of an unfocused image. Specific terms include:
Bokeh, the appearance of out-of-focus areas from an image produced by a lens
Box blur, a graphic-art effect
Gaussian blur, a graphic-art effect
Motion blur, a photographic and optical effect
Shallow focus, a photographic technique where the subject is in focus and the background and foreground are blurred.

Gaussian blur

From Wikipedia, the free encyclopedia

Gaussian blur describes blurring an image by a Gaussian function. It is a widely used effect in graphics software, typically to reduce image noise and reduce detail. The visual effect of this blurring technique is a smooth blur resembling that of viewing the image through a translucent screen, distinctly different from the bokeh effect produced by an out-of-focus lens or the shadow of an object under usual illumination. Gaussian smoothing is also used as a pre-processing stage in computer vision algorithms in order to enhance image structures at different scales—see scale-space representation and scale-space implementation.

Mathematically, applying a Gaussian blur to an image is the same as convolving the image with a Gaussian function; this is also known as a two-dimensional Weierstrass transform. By contrast, convolving by a circle (i.e., a circular box blur) would more accurately reproduce the bokeh effect. Since the Fourier transform of a Gaussian is another Gaussian, applying a Gaussian blur has the effect of reducing the image’s high-frequency components; a Gaussian blur is thus a low pass filter.

Bokeh = (derived from Japanese, a noun boke 暈け, meaning “blur” or “haze”) is a photographic term referring to the appearance of point of light sources in an out-of-focus area of an image produced by a camera lens using a shallow depth of field.[1] Different lens bokeh produces different aesthetic qualities in out-of-focus backgrounds, which are often used to reduce distractions and emphasize the primary subject.

more about bokeh on  Wikipedia

Box blur

A box blur is an image filter in which each pixel in the resulting image has a value equal to the average value of its neighboring pixels in the input image. It is a form of low-pass (“blurring”) filter and is a convolution.

Due to its property of using equal weights it can be implemented using a much simpler accumulation algorithm which is significantly faster than using a sliding window algorithm[1].

Box blurs are frequently used to approximate a Gaussian blur[2]. By the central limit theorem, if applied 3 times on the same image, a box blur approximates the Gaussian kernel to within about 3%, yielding the same result as a quadratic convolution kernel.

In the frequency domain, a box blur has zeros and negative components. That is, a sine wave with a period equal to the size of the box will be blurred away entirely and wavelengths shorter than the size of the box may be phase reversed, as seen when two bokeh circles touch to form a bright spot where there would be a dark spot between two bright spots in the original image.

sursa: Wikipedia

Motion blur is the apparent streaking of rapidly moving objects in a still image or a sequence of images such as a movie or animation. More on Wikipedia

Shallow focus
From Wikipedia, the free encyclopedia

Shallow focus is a photographic and cinematographic technique incorporating a small depth of field. In shallow focus one plane of the image is in focus while the rest is out of focus. Shallow focus typically is used to emphasize one part of the image over another. Photographers sometimes refer to the area that is out of focus as bokeh.

The opposite of shallow focus is deep focus, in which the entire image is in focus. Deep focus photographic technique more closely approximates what is seen by the human eye.
1 How to
2 Examples

How to

A shallow focus is achieved by using a big aperture in conjunction with a long lens or a close subject distance. Common lenses used in still photography to achieve a shallow focus are 50/1.4, 85/1.8, and 135/2.

Further, using a camera with a large sensor helps; in still photography, large format inherently has narrower focus than 35 mm, which is still narrower than compact digital cameras. If you can choose what format to use, go with the largest sensor available.

Shallow focus is often used in portraiture, to isolate the subject from the background and to deemphasize skin imperfections or vellus hair.

Examples

In the film, The Rules of the Game (1939), a couple flirts in the foreground while the woman’s husband enters in the background. Director Jean Renoir chooses to keep the husband out-of-focus so that his presence is hinted, but not emphasized.