When changing your aperture to defocus the background, the light sources appear blurred. This bokeh' effect of the blurred background can be enhanced with circular aperture blades used in this lens. Conventional aperture blades have flat sides creating unappealing polygonal shaped defocussed points of light. α 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.
ED (Extra-low Dispersion) glass
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. ED glass dramatically reduces chromatic aberration at telephoto ranges, and provides superior contrast across the entire image, even at large aperture settings. Super ED glass provides enhanced compensation for chromatic aberration.
SAM (Smooth Autofocus Motor)
Rather than using the focus drive motor in the camera body, SAM lenses feature an autofocus motor built into to the lens itself that directly drives the focusing element group. Since the built-in motor directly rotates the focus mechanism, operation is significantly smoother and quieter than conventional coupled autofocus drive systems.
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. 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.
Because aperture blades form a near circle at the wide openings used for low-light shots, spot-light sources have a pleasing circular defocused effect.