Nikon D300 User Manual page 229

quarter of a normal exposure for each shot so that I'll have a
normally exposed background when the four shots are taken.
The reason I mentioned this in such a repetitive fashion is that
it took me a little while to wrap my brain around the
confusing presentation of this fact in the user's manual.
Whoever heard of "gain" meaning dividing something into
parts? What I think the manual writer was trying to say is that
each shot "gains" a portion of the normal exposure so that in
the end, the exposure is complete and correct. I hope this
makes sense to you.
Auto gain is like an automatic normal-exposure
"divider-upper" for multiple exposures. It divides the
exposure into appropriate sections—so you won't have to fool
with it.
When should one use Auto gain? Only when you have no
need for controlling exposure differently for each frame but
instead can use an exact division of similar exposures.
Auto gain works fine if you're not using masks. When you
use a mask, you want a full normal exposure for each of the
uncovered (non-masked) sections of the image, so Auto gain
will not work for this. In this case, you should use manual
exposure, with Auto gain turned Off.
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