Sony DSLR-A300K/N Lenses and Accessories Selection Guide - Page 14

Lens Technology Circular aperture In general, if an aperture uses 7 or 9 aperture blades, then the shape of the aperture becomes a 7-sided or 9-sided polygon as the aperture is made smaller. However, this has a certain undesirable effect in that the defocusing of point light sources appears polygonal and not circular. lenses overcome this problem through a unique design that keeps the aperture almost perfectly circular from its wide-open setting to when it is closed by 2 stops. Smoother, more natural defocusing can be obtained as a result. Aspherical lens Spherical lens Focal plane specially developed aspherical lens elements. This advanced optical technology minimizes spherical aberration in all types of lenses, including large-aperture lenses. Aspherical lenses maintain high contrast in images taken with the aperture wide open, and effectively suppress image distortion. Also, because less lens elements are required, the entire lens stays compact. ED (Extra-low Dispersion) glass Optical glass ED glass Focal plane Multi-layered coating Although most of the light that falls on an optical glass element passes through directly, some of it reflects at the surface to cause flare or ghost images. In order to avoid this problem, a thin layer of anti-reflective coating must be applied to the lens surface. Lenses use multi-layered coating to effectively suppress such problems over a wide spectrum of wavelengths. Double telephoto zoom Through the use of a telephototype lens structure, the length of a fixed focal length lens can be made shorter than its actual focal length. A double telephoto zoom lens takes this principle of design one step further, by combining two such telephoto-type structures to produce a telephoto zoom lens that is extremely compact. Aspherical lens Focal plane Spherical aberration occurs in conventional lenses due to a misalignment on the focal plane between light coming from the periphery of the lens and light coming from the center of the lens. Despite the development of various methods to correct this aberration, the flaw remains apparent particularly in wideangle lenses with large apertures. Unless, that is, the lens includes Focal plane As focal lengths get longer, lenses built with conventional optical glass have difficulties with chromatic aberration, and as a result, images suffer from lower contrast, lower color quality, and lower resolution. To counter such problems, ED glass was developed and is included in select lenses. It dramatically improves chromatic aberration at telephoto ranges, and provides superior contrast across the entire image, even at large aperture settings. Floating mechanism Ideally, lens aberrations can be adequately compensated at any shooting distance. In reality, compensation is quite difficult at close distances. The floating mechanism addresses this problem by allowing groups of lens elements to move closer or farther apart, which makes it possible to correct various aberrations at practically all focusing distances. A double floating mechanism is included in the SAL50M28 and SAL100M28. 26 Internal focusing Only the middle groups of the optical system are moved to achieve focusing, which leaves the total length of the lens intact. Benefits include fast autofocusing and a short minimum focusing distance. Also, the filter thread at the front of the lens does not rotate, which is convenient if you're using a polarizing filter. Rear focusing By moving only the rear group of lenses to focus, the lens allows for speedy AF operation and a shorter minimum focusing distance. Also, because the front of the lens does not rotate, operability is improved when you're shooting with a polarizing filter attached. Aluminum alloy lens barrel Aluminum alloy is used in the construction of the G series and other high-end lenses to ensure high optical performance. This material is lightweight, durable, and highly resistant to the effects of temperature change. Focus hold button Once focus has been adjusted on the desired part of the frame, pressing this button on the lens barrel will hold focus if you wish to recompose the shot. Alternately, depth of field preview can be assigned to this button through the camera's custom setting menu. Focus range limiter This function saves you a bit of time during AF operation by setting a limit on the focusing range. In macro lenses, this limit can be on either the close or distant range (as pictured). In the SAL70200G, the limit is set on distant ranges only. In the SAL300F28G, focusing can be limited either to a distant range or to a range that you specify yourself. Distance encoder The distance encoder is a lens component that directly detects the position of the focusing mechanism, and sends a signal to the CPU in order to measure distance to the subject. During flash photography, this data is very useful in calculating how much flash output is appropriate to the scene. The distance encoder plays an integral part in ADI flash metering, which delivers high-precision flash metering that is unaffected by the reflectance of subjects or backgrounds. ADI flash metering Advanced Distance Integration flash metering is available when the built-in flash, HVL-F56AM external flash, or HVL-F36AM external flash is used together with a lens that has a built-in distance encoder.* It provides automatic metering that is virtually unaffected by the reflectance of subjects or backgrounds. Precise distance information is obtained through the encoder, and this data is used to compensate the flash output accordingly. This yields accurate exposures more reliably than conventional TTL (through-the-lens) flash metering, which can be thrown off by overly reflective or overly dark subjects and backgrounds. * For details, please see the specifications chart on p.24-25 SSM (Super Sonic wave Motor) SSM is a piezoelectric motor that contributes to smooth and silent AF operation. The motor produces high torque at slow rotation, and provides immediate start and stop responses. It is also extremely quiet, which helps keep autofocusing silent. Lenses that feature SSM also include a position-sensitive detector to directly detect the amount of lens rotation, a factor that improves AF precision overall. 27