默认前提当然是放大率相同的情况了。刚才打错了,应该是f^2/光圈/crop系数,所以你的20饼干弥散圆直径会更小。 http://photo.net/learn/optics/lensTutorial If we frame an object the same way with two different lenses, i.e. M is the same both situations, the shorter focal length lens will have less front depth of field and more rear depth of field at the same effective f-stop. (To a first approximation, the depth of field is the same in both cases.) Another important consideration when choosing a lens focal length is how a distant background point will be rendered. Points at infinity are rendered as circles of size C = f M / N So at constant object magnification a distant background point will be blurred in direct proportion to the focal length. This is illustrated by the following example, in which lenses of 50mm (red) and 100 mm (green) focal lengths are both set up to get a magnification of 1/10. Both lenses are set to f/8. The graph shows the circles of confusions as a function of the distance behind the object. N f-stop (focal length divided by diameter of entrance pupil) M magnification (M=f/(So-f))