Sensor Noise in Series Shots
When shooting under low-light conditions without tripod, ISO must be set higher and exposure times must frequently be extended beyond the time safely held by hand.
The unwanted movement of the hand is considered to be stochastic and it has been suggested to make a series of exposures if working close to your personal hand held-time limit for a given lens so that a shot of the series least affected by camera shake induced blur can be selected.
Having done this for quite some time I noted that I frequently chose the FIRST shot in a series.
Searching for the reason, I found that this holds even for shots of my body cap taken without lens at t=1/25s and ISO = 160. Shown below are two pictures shot in series mode of the camera. Crops of upper left corner, Raw conversion done via PICASA.
It can be seen that the first shot in the series has MORE dark spots and is thus LESS effected by noise. This is consistent over different ISOs, exposure times; same results are obtained in Lightroom 3.2. I consider the increased noise in the second and further shots a result of sensor warming.
I have found that when shooting a series it is useful to wait about at least 10secs for the next shot in order to not have the adverse effect seen here. Given the result, it seems recommendable to shoot the more noise prone LOWER EXPOSURE pic of a bracket series FIRST ( -0+ sequence).