Nikon Z 5 Reference Manual page 148

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High ISO Sensitivities
The higher the ISO sensitivity, the less light needed to make an exposure,
allowing pictures to be taken when lighting is poor and helping prevent
blur when the subject is in motion. Note, however, that the higher the
sensitivity, the more likely the image is to be affected by "noise" in the
form of randomly-spaced bright pixels, fog, or lines.
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Hi 0.3–Hi 1.0
A setting of [Hi 0.3] corresponds to an ISO sensitivity approximately
0.3 EV higher than ISO 51200 (ISO 64000) and [Hi 1.0] to an ISO sensitivity
approximately 1 EV higher (ISO 102400). Note that pictures taken at these
settings are particularly prone to "noise" in the form of randomly-spaced
bright pixels, fog, or lines.
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Lo 0.3–Lo 1.0
[Lo 0.3] corresponds to an ISO sensitivity about 0.3 EV below ISO 100
(equivalent to ISO 80). [Lo 1.0] is roughly 1 EV below ISO 100 (ISO 50). Use
for larger apertures or slower shutter speeds when lighting is bright.
Highlights may be overexposed. In most cases, ISO sensitivities of ISO
[100] or above are recommended.
The S Button
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