Spatial contrast sensitivity may decrease differently at different spatial frequencies when vision is impaired. Patients with central scotoma may have normal or decreased sensitivity at low spatial frequencies, their Snellen acuity often is lower than grating acuity estimated on the basis of contrast sensitivity values. In retinitis pigmentosa contrast sensitivity may decrease at the low spatial frequencies already in the early teens. Because the ability to discern low contrasts is important in daily living, spatial contrast sensitivity measurements should become a part of clinical evaluation of vision.