Canon EOS 90D Advanced User's Manual page 280

Movie Recording
During movie recording, avoid changing the shutter speed, aperture, or ISO
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speed. Doing so may record the changes in the exposure or create more
noise at high ISO speeds.
When recording a movie of a moving subject, a shutter speed of approx.
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1/25 sec. to 1/125 sec. is recommended. The faster the shutter speed, the
less smooth the subject's movement will look.
If you change the shutter speed while recording under fl uorescent or LED
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lighting, image fl icker may be recorded.
With ISO Auto, exposure compensation can be set in a range of ±3 stops.
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When ISO Auto is set, you can press the < A > button to lock the ISO speed.
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After locking the ISO speed during movie recording, you can cancel it by
pressing the < S > button. (ISO speed lock is maintained until you press the
< S > button.)
If you press the < A > button and recompose the shot, you can see the
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exposure level diff erence on the exposure level indicator compared to when
the < A > button was pressed.
With the camera ready to shoot in the < 4 > mode, you can display the
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histogram by pressing the < B > button.
ISO Speed in < a > Mode
You can set the ISO speed manually or select [AUTO]. For details on the
ISO speed, see = 620.
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