The specific meaning of STF is: smooth focus, smooth image. It is also the abbreviation of staff;

Generally, the light diffusion of a lens in Jiao Wai is equal, so the light distribution of its virtual image is uniform. In view of this, an optical element called apodization filter is installed on the STF lens near the aperture, which makes the luminous flux in the central part of the lens more, but the luminous flux decreases as it moves to the periphery. Therefore, the contour fades in the out-of-focus imaging part, forming an ideal soft virtual image.

Extended data:

This STF lens is different from ordinary lens. Figure 1 explains the difference between STF lens and ordinary lens. The actual image point becomes a real image at point A through the lens, and is displayed as a virtual image at points B and C before and after it. The virtual images of ordinary lens at point B and point C are all circles with uniform brightness;

The defocus imaging of STF lens is due to the apodized optical elements, and the brightness of the circular image decreases as it goes to the periphery, which can realize soft and beautiful foreground and background imaging. On the other hand, it avoids the so-called "bilinear defocus" that deteriorates the foreground and background imaging, making this lens especially suitable for portraits and photography that require high texture performance like precious metals.

Usually it is a virtual image before and after the focused object. In some cases, there is a tendency called bilinear, which destroys the foreground and background imaging. Therefore, the spherical aberration can be adjusted by moving a part of the lens group to improve the foreground and background imaging, but usually only one of the background imaging and the foreground imaging can be improved, and the blur degree before and after can not be improved at the same time. The degree of blur depends on the brightness distribution of some Jiao Wai images.