What do the markings on an eyepiece mean?
If you look at a typical eyepiece you will almost certainly see some numbers and possibly some letters printed on the eyepiece. For example, you might see the marking “25mm” or “7.5mm” (or any other number of values ranging from around 4mm to possibly 60mm). This numeric value is known as the focal length of the eyepiece (measured in millimeters). This is probably the most important characteristic of an eyepiece because it allows you to calculate how much magnification the eyepiece will provide. The actual magnification an eyepiece provides depends on the focal length of the telescope. You may see other letters or markings on the eyepiece (such as “H”, “SR”, “Pl”, etc), these indicate the type of eyepiece (we will discuss these in an upcoming section). For now, the thing to take away from this section that the number printed on the eyepiece is the focal length and understand that it is used to calculate the magnification the eyepiece will provide.
