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Effective focal length (EFL) The effective focal length is the inverse of the optical power of an optical system, and is the value used to calculate the magnification of the system. [1] The imaging properties of the optical system can be modeled by replacing the system with an ideal thin lens with the same EFL. [2]
It measures visual acuity loss: positive values indicate vision loss, while negative values denote normal or better visual acuity. This scale is commonly used clinically and in research because the lines are of equal length and so it forms a continuous scale with equally spaced intervals between points, unlike Snellen charts, which have ...
Light from a single point of a distant object and light from a single point of a near object being brought to a focus. The accommodation reflex (or accommodation-convergence reflex) is a reflex action of the eye, in response to focusing on a near object, then looking at a distant object (and vice versa), comprising coordinated changes in vergence, lens shape (accommodation) and pupil size.
For example, P <2% means that less than 2% of the population have vision loss worse than measured [19] Pattern standard deviation (PSD): Derived from the pattern deviation and thus highlights focal loss only. A high PSD, indicating irregular vision, is therefore a more useful indicator of glaucomatous progression, than the MD [3]
If the medium surrounding an optical system has a refractive index of 1 (e.g., air or vacuum), then the distance from each principal plane to the corresponding focal point is just the focal length of the system. In the more general case, the distance to the foci is the focal length multiplied by the index of refraction of the medium.
Two horse lenses suspended on water by cling wrap with four approximately parallel lasers directed through them. The 1 cm spaced grid indicates a near focus focal length of around 6 cm. Accommodation is the process by which the vertebrate eye changes optical power to maintain a clear image or focus on an object as its distance varies.
How focal length affects perspective: Varying focal lengths at identical field size achieved by different camera-subject distances. Notice that the shorter the focal length and the larger the angle of view, perspective distortion and size differences increase. Lenses are often referred to by terms that express their angle of view:
The original application called for placing the chart at a distance 26 times the focal length of the imaging lens used. The bars above and to the left are in sequence, separated by approximately the square root of two (12, 17, 24, etc.), while the bars below and to the left have the same separation but a different starting point (14, 20, 28, etc.)