以后means in the fruture what will happen is right. time could be longer~
之后means when you talk something, it include two or three cases, after one thing it could happen or later what may come up. for example: you want to eat dinner and watch TV, at first you eat dinner, then watch TV, you can use 吃了晚饭之后看电视.
my english is not very good, but i wish it can help you sovle the promble