Here I shot 7 exposures with my 200mm lens and combined them all into a single frame. All of the shots were 1/3 second, so if the smoke were a constant density/thickness, each image would look the same, but as you can see the brightness of the images vary. Each exposure is 5 minutes apart, the last one ended near midnight (local time).
(06 June 2015 - 400-406-412-418-424-x-436-442.jpg)