For a 4N25/26 I tried to plot the CTR vs LED Current graph by simulating the following circuit. To obtain results similar to the one in the datasheet I keep Vce constant at 10V and sweep the forward LED current between 0.1mA upto around 50mA.
As you see the peak CTR occurs when the LED current is around 1.8mA.
But in the datasheet here, at Fig. 2 or Fig. 3 the graph looks very different and the peak CTR occurs at 10mA LED current:
CTR plot is basically Ic/If when Vce is kept constant. But I obtain very different plot. Am I interpreting something wrong?