5d4 and 90ex exposure
I know that 90ex is discontinued, but it has several interesting features, such as small size and ability to act as an optical master.
I have recently connected it to 5d4 and all the shots are really dark when the flash fires. I can make them reasonable by setting EV+5.
Any ideas about what would the reasons be, and if there are any ways to make the camera measure exposure correctly? I understand that the flash is very weak, but I don't see any reason why the camera can't increase ISO to make the exposure correct.