Visual acuity 4.2 is equivalent to 0.15, which means that when you sit 5 meters away from the visual acuity table to check your vision, you can only see the 3rd line of the visual acuity table, which is not normal. Normally, the best corrected visual acuity should not be less than 0.5 for children between the ages of 3 and 5, and 0.7 for children between the ages of 6 and 7. Poor visual acuity should be examined immediately by a dilated eye exam at a hospital. under 10 years of age, the pupil should be dilated with a chronic dilator, atropine cream or atropine gel for 3 days (or as prescribed by the doctor), and over 10 years of age, the pupil should be dilated with a rapid dilator, tropine amide, for 3 to 6 times. If the corrected visual acuity still does not reach normal after pupil dilatation, further diagnosis is needed to clarify the cause of the vision loss.