Placed near the lens aperture is, in effect, a concentric graduated ND filter that has the highest light transmission at its center (the opposite of a central ND-type filter).
Combined with the lens’s high spherical aberration correction, the APD element is designed to achieve high resolution at the image plane, with smooth foreground and background blur. The darkened edges of the APD element also eliminate the bright-edge effects of out-of-focus highlights.
Although not the first lens with an APD element to use AF, the Sony’s low transmission of T5.6 wide-open means it’s less suited to dimly-lit scenes than the F2.8 maximum aperture
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