Nikon D90 - Manual (Page 60)

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A The Built-in Flash
If additional lighting is required for correct exposure in i mode, the
built-in flash will pop up automatically when the shutter-release
button is pressed halfway. Flash range varies with aperture and ISO
sensitivity (pg. 266); remove lens hoods when using the flash. If the
flash is raised, photographs can only be taken when the flash-ready
indicator (
M) is displayed. If the flash-ready indicator is not displayed,
the flash is charging; remove your finger briefly from the shutter-
release button and try again.
To save battery power when the flash is not in use, return it to its closed
position by pressing it gently downward till the latch clicks into place.
For more information on using the flash, see page 70.