Long considered the “standard” eyepiece that is often supplied with telescopes that are sold by various suppliers, the 25mm focal length category is perhaps the most used eyepiece in the world. Like the 20mm, these eyepieces are great for binoviewer devices, and the 25mm is probably the longest focal length of the mid-power range eyepieces before you get into longer focal lengths. Explore Scientific Waterproof Eyepieces - Comfort, Quality, Value. The Explore Scientific 52° Series eyepieces are excellent flat-field multi-element oculars that can transform your visual astronomy experience. With an apparent field of view of 52 degrees, these eyepieces offer incredible value in a very nice eyepiece. Expect exceptional quality, eye relief and value versus a comparably priced Plossl. Another great benefit of these eyepieces is the excellent contrast and ease with which you can gaze, not directly at an object, but slightly to either side. This skill, called using averted vision, is especially helpful when observing faint objects, because using averted vision puts the image on a part of your eye that is more sensitive to light, allowing you to see fainter images.