Originally posted by Leumas Additive: What is this even good for???
Think long exposure time similar effect to average but meant to be used when it it dark and say you want to shoot at ISO 100 but need longer than a 30 second exposure. While you could use bulb mode you will eventually get artifacts from an excessive shutter speed as the sensor warms but additive avoids that. Also for things like star trails it would produce the trails but you would have a brightened foreground and the sky background would be brighter.
I actually don't use the built in composite modes and instead use a release cable I made, put the camera into high speed continuous mode and set the shutter speed, f-stop, and ISO to what I want and just flip the switch letting it rip until I get enough shots. Then I process and combine on the computer allowing me to do what ever I want. With the built in modes you end up stuck with 14 bits of data from the composites where as combining on the computer results in the full 16bits and with a large enough number of shots you get a real 16 bits of data. A while back someone posted some starnge effects that they got using a large number of shots in one of the composite modes. I did
find the thread and eventually we wandered into the weeds of technical details on the subject. In reading that you will learn more than you ever wanted to on the subject.