f-stop 的鑑定

We spotted a great blue heron walking along the shore, and we stopped to watch it for a minute. We then noticed an otter foraging along the bank just in front of it. The otter swam downstream through the river, stopping along the shore frequently to poke around. The heron followed close behind it for quite a while (perhaps a few hundred meters?) and seemed to be opportunistically hunting for fish that were spooked by the otter. We saw the heron strike a few times in the water where the otter had just left, and it caught a small fish at least once. When the otter would leave the water temporarily, the heron would wait around. It once seemed to lose track of the otter, but when it spotted it ~30 m downstream, it flew immediately to where the otter was and waited just behind it once again. This whole process went on for perhaps 15 minutes. Eventually, the otter crawled up onto the shore and groomed itself in the sun. The heron remained waiting where the otter had last been in the water. We left them alone at this time, but when we returned ~20 minutes later, they were both in the same positions.