A Nikon DK-20C -4.0 Diopter is a reliable and efficient accessory for your Digital SLR camera, designed to enhance your viewfinder's sharpness in harmony with your unique eyesight. This high-quality diopter correction lens, compatible with a wide range of Nikon AF and MF lenses, is specifically engineered to correct near-sightedness with a -4.0 diopter rating. Unlike standard eyepieces that come with Nikon SLR cameras, which offer no corrective value and serve only as dust protection, this Nikon eyepiece diopter provides a significant improvement in your camera's focus. It's user-friendly and comes with a rubber coating to minimize the risk of scratching your eyeglasses, making it a durable choice for photographers.Nikon SLR cameras without built-in adjustable eyepiece correction have a base diopter value of minus one, considered the ideal starting point for most SLR users with normal vision. The Nikon DK-20C -4.0 Diopter, when combined with the viewfinder's minus one value, delivers the final desired value without requiring any additional calculations. This product is not a replacement for the standard eyepiece but an enhancement. Using this diopter will change the camera's diopter value from minus one to zero. To determine the best Nikon diopter value for you, consult with your ophthalmologist, optometrist, or dispensing optician. They can help you determine your diopter from your corrective prescription when viewing an object at a distance of approximately one meter. For Nikon SLR cameras with built-in adjustable eyepiece correction, additional Nikon eyepiece diopters like the DK-20C -4.0 Diopter can be used if your vision requires further optimization beyond the built-in diopter adjustment control. Always refer to your camera's instruction manual to verify compatibility. In summary, the Nikon DK-20C -4.0 Diopter is a valuable accessory for enhancing your photography experience, offering a personalized focus correction solution for near-sighted photographers.