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This article is about the lack of observable focus. For other uses of the term Blur, please see Blur (disambiguation).

Blur is to render obscure by making the form or outline of confused and uncertain, as by soiling, or to smear or to make indistinct and confused. The word is first attested in literature around 1548 as a noun, and 1581 as a verb. It is related to the much older word 'blear' (c. 1300), describing an object that is watery or rheumy. This may relate to the print of manuscripts becoming blurred while damp or the impression of a woodcut becoming blurred due to an excess of ink.

In photography, a blur generally refers to the appearance of an unfocused image. Technically in optical physics, a blur is referred to as a defocus aberattion. There are two types of aberrations: A spherical aberration occurs because curvature in a lens or mirror causes rays falling on the outer edges to be brought to a focus at a different point than those falling on the middle. This makes the images formed appear blurred. Chromatic aberration, which occurs in lenses but not mirrors, is the failure of a lens to focus all colours (wavelengths) of light in the same plane; the image appears blurred and shows rainbow-coloured fringes around the edges. For example, violet light is bent more than red and thus is brought to a focus nearer the lens than red. No single lens can ever be free of chromatic aberration, but by combining lenses of different types, the effects of the component lenses can be made to cancel one another. Such an arrangement is called an achromatic lens.

The degree of image blurring for a given amount of focus shift depends inversely on the square of the lens focal ratio. Low focal ratios, such as f/1.8 to f/3, are very sensitive to defocus and have very shallow depths of field. High focal ratios, in the f/16 to f/32 range, are highly tolerant of defocus, and consequently have large depths of field. Extreme depth of field in very dim illumination at the imaging film or sensor, and very long exposure times, introduces the potential for image degradation due to motion blurring.

Some photographers use blurring to achieve an artistic effect, both during the photographic process, and post-image. One technique for creating a blurred background image while keeping the foreground or primary subject in focus is called Bokeh, a photographic term which also describes the subjective aesthetic qualities of out-of-focus areas in an image produced by a camera lens. Blurring post-image can be achieved using various software including Blur, a paint program filter, which throws the image slightly out of focus. It can be calibrated until the desired effect is achieved. Motion blur is a filter that blurs the image along a specified axis to give the effect of motion. Gaussian blur is another type of filter commonly used to blur an object. It may be used to blur the entire image or to produce a drop shadow effect.