1. The telescope has a fast focal ratio of f/4.
这台望远镜的焦比很快，为 f/4。
2. A lower focal ratio generally means a wider aperture.
较低的焦比通常意味着更大的光圈。
3. Choosing the right focal ratio is essential for astronomical photography.
选择合适的焦比对天文摄影至关重要。

A: What is the focal ratio of this new camera lens?
A：这款新相机的镜头的焦比是多少？

B: It has a fast focal ratio of f/1.4.
B：它的焦比很快，是 f/1.4。

1) The ratio of the focal length to the aperture diameter.

2) A measurement used to describe lens speed in photography.

3) A key parameter in optical design for telescopes.

1. 焦比

e.g. The focal ratio affects the image brightness.
焦比会影响图像的亮度。

noun phrase

1. The ratio of the focal length of an optical system to its aperture diameter.