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Grand Vision acquired Vision Express in 1997. The French parent company Grand Vision has clinics under various brands across Europe, and the rest of the world. Under franchise agreement there is a strong Vision Express brand present also in Latvian and Lithuanian markets.
A lens may be considered a thin lens if its thickness is much less than the radii of curvature of its surfaces (d ≪ | R 1 | and d ≪ | R 2 |).. In optics, a thin lens is a lens with a thickness (distance along the optical axis between the two surfaces of the lens) that is negligible compared to the radii of curvature of the lens surfaces.
Sleeping in lenses not designed or approved for extended wear is a common cause of complications. Many people go too long before replacing their contacts, wearing lenses designed for 1, 14, or 30 days of wear for multiple months or years. While this does save on the cost of lenses, it risks permanent damage to the eye and even loss of sight.
This list covers optical lens designs grouped by tasks or overall type. The field of optical lens designing has many variables including the function the lens or group of lenses have to perform, the limits of optical glass because of the index of refraction and dispersion properties, and design constraints including realistic lens element center and edge thicknesses, minimum and maximum air ...
A "slow" lens (one that is not capable of passing a lot of light through) might have a maximum aperture from 5.6 to 11, while a "fast" lens (one that can pass more light through) might have a maximum aperture from 1 to 4. Fast lenses are, by definition, larger than slow lenses (for comparable focal length), and typically cost more. [2]
Today the complex surfaces of a progressive lens can be cut and polished on computer-controlled machines, allowing 'freeform surfacing', as opposed to the earlier casting process, thus explaining the difference in price. In short, the price is based on the technology used and the year the lens came to market.