Focus has three meanings, namely: 1. In JavaScript, focus is the area of concern, that is, the position where the cursor is currently activated; 2. In geometric mathematics, focus refers to the special point that constructs the curve. ; 3. In physics, focus means that an optical system has two focuses, namely the object-side focus and the image-side focus.
The operating environment of this article: Windows 7 system, Dell G3 computer.
What does focus mean?
In fact, focus has three meanings:
1. In JavaScript, focus is the area of concern, that is, the position where the cursor is currently activated, and the small flashing icon on the page screen. A vertical line indicates that a control on the web page is selected and can be operated. Click the mouse to get the cursor, and the Tab key can switch focus according to the set Tabindex.
Methods for JavaScript to obtain focus and remove focus:
JavaScript can obtain focus through the focus() method and remove focus through the blur() method.
2. In geometric mathematics, focus or foci (UK: /foʊkaɪ/, US: /foʊsaɪ/), focus refers to the special point where a curve is constructed. For example, one or two foci can be used to define a conic section, the four types of which are circle, ellipse, parabola and hyperbola. Furthermore, two foci are used to define Cassini ellipses and Cartesian ellipses, and more than two foci are used to define n-ellipses.
3. In physics, focus means that an optical system has two focuses: object-side focus and image-side focus. The object-side focus is the position of the object at infinity, and the image-side focus is the position of the image at infinity. There are two methods to determine the position of the two focus points. One is the front focal length and the back focal length relative to the first and last faces of the system. The front focal length is the distance between the object focus and the first face of the system, and the back focus is the distance between the image focus and the last face of the system. Another method is relative to the main plane of the system, that is, the so-called main focal length, object-side main focal length or first main focal length, which is the distance of the object-side focus relative to the first main plane of the system, the image-side main focal length or the second main focal length. Focal length is the distance between the image focus and the second principal plane of the system. The focal length value of a system often refers to the main focal length value. The plane perpendicular to the common axis of the optical system and passing through the focus is the focal plane. Because there are two focal points, there are also two focal planes: the first focal plane (object-side focal plane) and the second focal plane (image-side focal plane). ).
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