Canon EOS REBEL T7i Instruction Manual page 256

3 Focusing with AF
If focusing is difficult in the magnified view, return to the normal view and
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perform AF.
If you perform AF in the normal view and then use the magnified view,
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accurate focus may not be achieved.
AF speed differs between normal view and magnified view.
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When in magnified view, Movie Servo AF (p.303) will not function.
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With the magnified view, achieving focus becomes more difficult due to
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camera shake. Using a tripod is recommended.
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