Nikon Z 9 Reference Manual page 155

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The Photo Shooting Menu [ISO Sensitivity Settings] Option
ISO senstivity can also be adjusted using the [ISO
sensitivity settings] item in the photo shooting
menu.
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Hi 0.3–Hi 2.0
A setting of [Hi 0.3] corresponds to an ISO sensitivity approximately 0.3 EV
higher than ISO 25600 (ISO 32000 equivalent) and [Hi 2.0] to an ISO
sensitivity approximately 2 EV higher (ISO 102400 equivalent). 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 64 (ISO 50
equivalent). [Lo 1.0] is roughly 1 EV below ISO 100 (ISO 32 equivalent). Use
for larger apertures or slower shutter speeds when lighting is bright.
Highlights may be overexposed. In most cases, ISO sensitivities of ISO [64] or
above are recommended.
ISO Sensitivity
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