Which instrument is commonly used for performing visual field tests?

The commonly used instrument for performing visual field tests is the perimeter. This device is specifically designed to assess the peripheral vision of a patient and can detect various types of visual field defects, such as those caused by glaucoma, retinal diseases, or neurological conditions. The perimeter provides a controlled environment where test stimuli are presented at various locations to map the patient's field of vision accurately.

In contrast, other instruments serve different purposes in eye care. The retinoscope is used to measure refractive error by evaluating the light reflex in the eye. The Tonopen is utilized for measuring intraocular pressure to assess for conditions like glaucoma. The ophthalmoscope is employed for examining the interior structures of the eye, such as the retina and optic nerve. Each of these instruments plays an important role in comprehensive eye examinations, but the perimeter is uniquely suited for visual field testing, making it the correct choice.