Almost 150 years after Herman Snellen (Snellen 1862, 1864), the renowned ophthalmologist from the University of Utrecht, published his table of optotypes for assessing the visual status of the eye, we continue to utilise optotype test charts as the mainstay of our clinical evaluation of vision. Although the term ‘visual acuity’ is almost synonymous with the assessment of central visual status, using optotypes, it is however more specific and refers to the ability of the visual system to resolve spatial detail. It is a measure of the angular size of detail that is just resolvable by the observer and its limitations are imposed by a combination of optical and neural factors.