Nikon D300 User Manual page 653

Figure 9-9 shows the D300 in Shutter priority auto mode (S).
To use the S mode, select a shutter speed that you feel will
work for your current shooting situation and set it with the
Main command dial. The D300(S) will now assist you by
controlling the aperture to maintain correct exposure.
If the light changes drastically and the D300(S) cannot
maintain a correct exposure, it will inform you by replacing
the normal aperture reading with either HI or Lo. They mean
what they imply: HI means there is too much light for a good
exposure, and Lo means there is not enough light for a good
exposure.
While you have your camera in S mode, you can set your
shutter speed anywhere between 30 seconds and 1/8000
second. You can also set it at x250, which is one click below
the slowest shutter speed on the dial (30s). For more
information on x250, see Custom setting e1 on page 281 of
the D300(S) User's Manual or page 288 of the D300 User's
Manual.
A – Aperture Priority Auto Mode
Nature and macro shooters, and anyone concerned with
carefully controlling depth of field, will often leave their
D300(S) set to Aperture priority auto mode (A). Figure 9-10
shows the A for Aperture priority auto mode, followed by the
shutter speed and aperture to the right of the A.
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