Visual acuity is the ability to focus so that two objects appear as distinct entities. This is the value that is measured in people using an eye chart. In animals (and in infants) this can be measured using retinoscopy.
Visual acuity in normal dogs has been estimated to be 20-40% of that of humans. This means that, at 20 feet, a dog can distinguish an object that a human could see at 90 feet.
Acuity is a function of the clarity of the structures of the eye (cornea, aqueous humor, lens, and vitreous) as well as the combined refractive powers to focus the image clearly on the retina.