Nikon D750 User Manual page 166

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Minimum Shutter Speed
Auto shutter-speed selection can be fine-tuned by highlighting Auto
and pressing 2: for example, values faster than those usually selected
automatically can be used with telephoto lenses to reduce blur. Note,
however, that Auto functions only with CPU lenses; if a non-CPU lens is
used without lens data, minimum shutter speed is fixed at
Shutter speeds may drop below the selected minimum if optimum
exposure can not be achieved at the ISO sensitivity chosen for
Maximum sensitivity.
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Auto ISO Sensitivity Control
Noise (randomly-spaced bright pixels, fog, or lines) is more likely at
higher sensitivities. Use the High ISO NR option in the shooting
menus to reduce noise (see page 317). When a flash is used, minimum
shutter speed will be set to the value selected for Minimum shutter
speed unless this value is faster than Custom Setting e1 (Flash sync
speed, 0 345) or slower than Custom Setting e2 (Flash shutter
speed, 0 346), in which case the value selected for Custom Setting e2
will be used instead. Note that ISO sensitivity may be raised
automatically when auto ISO sensitivity control is used in combination
with slow sync flash modes (available with the built-in flash and the
optional flash units listed on page 433), possibly preventing the
camera from selecting slow shutter speeds.
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Enabling and Disabling Auto ISO Senstivity Control
You can turn auto ISO sensitivity control on or off by pressing the
W (S) button and rotating the sub-command dial. ISO AUTO is
displayed when auto ISO sensitivity control is on.
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