When an elderly person undergoes cataract surgery, one eye is replaced with a monofocal lens while the other eye is replaced with a multifocal lens, there will be a difference in the way they see, the exact difference is related to the elderly person’s own visual condition and the type of monofocal lens chosen. Cataract is a common ophthalmic disease that usually requires surgical treatment. A clear IOL can be used to replace the original cloudy lens, thus restoring the patient’s vision. When implanting an IOL, there are many different types of crystals to choose from, such as monofocal crystals and multifocal crystals. Monofocal IOLs have only one focal point, which can only address the need for near or far vision, while multifocal IOLs have multiple focal points, which can address the need for distance, center and near vision at the same time. For example, if an elderly person with presbyopia uses both a monofocal lens and a multifocal lens for distance vision, the eye with the multifocal lens will see clearly in the center and close up, while the eye with the monofocal lens may experience blurred vision. When one eye is switched to a monofocal lens and the other eye is switched to a multifocal lens, this may result in an imbalance in vision between the two eyes, causing visual fatigue. Patients should choose the IOL to be implanted in their eyes according to their own eye needs and financial situation under the guidance of their doctors.