These have lenses with a single focal power tuned so that near objects come into focus.

这些镜头具有经过调整的单一焦度，因此可以聚焦近处的物体。

Today we also have progressive lenses which get rid of the line by smoothly varying the focal power from top to bottom.

今天我们还有渐进镜片，它通过从上到下平滑地改变焦度来摆脱线条。