Nikon D300 Complete Manual page 244

9. Use the Direction pad to navigate to the SHOOTING
menu (camera icon tab) and press the
Direction pad to select it.
10. Use the Direction pad to navigate to the JPEG
compression option. Press the > key on the Direction
pad to see the sub-options.
11. Use the Direction pad to navigate to the option you
want to use. Press the > key on the Direction pad or
the OK button to complete the settings.
Some confusion exists about the difference between Size
priority and Optimal quality.
Normally, JPEG compression is variable in the file size it
creates. If you shoot something with lots of detail in it (a
complex landscape with lots of trees and bushes, for example)
at JPEG fine you'll get a bigger file size than if you shoot a
subject with very little detail (e.g. a plain sky). That's because
of the way JPEG compression varies with the amount of detail
in the underlying data. What the JPEG compression option
provides you is with a way to force the JPEG compression to
react to the detail differently.
Thom Hogan's Complete Guide to the Nikon D300
V1.02
key on the
>
Page 244