Photographic lens - Introduction
Photographic lens - Introduction | ||
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A simple glass lens gives you a much better image than a pinhole. However, its quality is still a long way short of the standard needed for photography. Simple single lenses often distort shapes, create odd colour fringes or give a general 'misty' appearance. Occasionally such results work well as interpretative romantic images, but it is better to have a lens capable of producing utmost image clarity and detail. Then you can add a diffuser to the camera or later manipulate results digitally when you want pictures of the other kind. The main object of photographic lens design and manufacture is to produce lenses which minimize optical defects (known as 'aberrations') while increasing resolution of detail and image brightness. To achieve this a range of special optical glasses is used, each type having different refraction and dispersion properties. So a photographic lens has a 'compound' construction, containing a series of elements of different shapes and made from different glass types to help neutralize aberrations. In fact, a camera lens of normal focal length typically has 5–8 elements. Their centering and spacing within the metal lens barrel is critical, and can be upset if the lens is dropped or roughly knocked. But even the number of elements causes problems, as the tiny percentage of light reflected off every glass surface at the point of refraction multiplies as scattered light. If uncorrected, the result would be images that lack contrast and sparkle – like looking through a window with multiple double-glazing. Modern lenses therefore have their elements surface-coated with one or more extremely thin layers of a transparent material which practically eliminates internal reflections under most conditions. However, light may still flare if you shoot towards a bright light source just outside the picture area and fail to use a lenshood. Your camera or enlarging lens is therefore a relatively thick barrel of lens elements, all of them refracting light but together having an overall converging effect. Every photographic lens has its focal length (usually in millimetres) clearly engraved around the lens barrel or front-element retaining ring. | ||