Ocular lenses or eyepieces are very important accessories attached to the focal point of the objective, making your view-through more comfortable. They are implemented on the housing of a hunting optics such as monocular and binocular, and also on the telescopes and microscopes. They offer a nice support when looking through optic devices, so it's very important to choose an ideal design based on your needs. Some optical devices can adjust the focus on both eyepieces at the same time, by moving them nearer or further from the objective, and some have special shaft that doesn't require eyepieces movements, such as central focus wheel on the bridge of binoculars.
When the user wears eyeglasses or sunglasses, the eyepieces must be at least 16 mm away from the eyes to get a proper eye relief. The further away from the ocular lenses are the eyes, the more comfortable are they to use. Eyepieces are usually mounted to binoculars permanently (using them to have a pre-determined magnification and field of view), but some of them can be easily removed, convenient especially when increase or decrease the magnification.
Eyepieces on Roof prism binoculars are usually multi-positioned with twist-up and twist-down mechanism. Tethered with soft rubber to feel comfortable to the face, provide a proper eye relief with or without eyeglasses. Multi-position eyepieces provide “click-stop” mechanism so eye relief doesn’t unexpectedly change when viewing through.
Binoculars with Porro prisms have usually fold-up, fold-down eyepieces or provide only one position. This is less convenient for glass wearers and it’s highly recommended trying them on and making sure you can adjust the proper eye-relief for your needs.