To what end are you trying to separate them?
If you're trying to track the expenses and income from the different types of photography as if they are different companies, you can use Classes to help you run reports. But there's more involved in keeping this much detail on every expense or sale.
But if you're just looking to see how much you did in Portrait vs. Commercial, etc. (what't the 3rd? Ad?) That could just be a Service Item you set up and put in your invoice. Even on one invoice you could put 2hr Commercial and 6hrs Portrait and later run a report to see how much you invoiced for Portrait.
