When perusing the store shelves or digital catalogs for new telescopes and telescope accessories, you will undoubtedly come across lovely looking eyepieces that catch your attention. There are many people out there who simply buy eyepieces that they think look good or buy eyepieces in a set knowing they are going to get a low magnification piece and a high magnification piece. Each eyepiece always has a corresponding number in mm that many choose to ignore.

What are these numbers on each eyepiece? Are they important? What can they tell us about the eyepieces themselves? These are simple questions that I can quickly answer and more.

Simply put, the mm on eyepieces stands for what you think it does: millimeters. That’s right, those little m’s are simply the denotation for millimeters. So what does that mean for your eyepieces?

**The number in millimeters corresponding to each eyepiece is a measurement of its focal length, which is the length of the optical tube within the eyepiece itself. The focal length determines how much magnification the eyepiece will give to the user so reading the length in millimeters of each eyepiece can tell you a lot about what you are actually buying.**

At first, millimeters can be a little offputting for people who follow the imperial measurement system. You will get used to reading things in millimeters over time. Telescopes are scientific instruments, even if we use them for fun, which means they subscribe to the metric system for most of their measurements. Metrics are the measurements of science afterall.

Most eyepieces run a focal length of 3mm to 20mm but there are a great number of eyepieces available on the market today.

## Inverting Expectations

A lot of the time, bigger numbers correlate with bigger results. In the case of eyepiece focal length, that is not the case. The focal length of your telescope eyepieces, in millimeters (mm) has an inverse relationship with magnification.

That means that the smaller the focal length in millimeters, the larger the magnification. The larger the length in millimeters, the smaller the magnification.

For example, an eyepiece that is 5mm in length is going to have a much larger magnification level than an eyepiece that is 20mm in length.

This inverse relationship occurs due to the way light moves through telescopes and interacts with the eyepiece. In short, the less distance the light has to travel, the better it will manifest and the stronger the magnification.

Think about the eyepiece as a diluting agent for the light coming through the telescope. Although, the eyepiece is necessary to render images into a form that our eyes can properly see, the longer the eyepiece, the more it dilutes the image making the magnification smaller.

When looking at eyepieces, or telescope that includes eyepieces, pay close attention to the size of each piece so that you can figure out what kind of magnification range you are getting. Many telescopes only come with a single eyepiece and sometimes two eyepieces which generally cover a low magnification and high magnification level.

If you are looking to fill a magnification hole in your telescope accessory array, it is important to know how these numbers work in order for you to make the best decision regarding your telescope’s equipment.

Just remember, the smaller the focal length of each eyepiece, the larger the magnification. It is an inverse relationship.

## Other Important Eyepiece Numbers

There is another important eyepiece number which is usually given in inches rather than millimeters. That is the eyepiece compatibility number. Eyepieces are either fitted as a 1.25” fitting or a 2” fitting. Knowing which one your telescope takes is just as important as the focal length.

The vast majority of telescopes operate on a 1.25” eyepiece attachment size but some telescopes have the larger 2” hook up. If you do not know which size eyepieces your telescope can take, it can lead to a frustrating unboxing situation in which you get an eyepiece which will not fit your rig. That will lead to a bad day.

While the number in millimeters tells us how long the eyepiece is and consequently how much magnification it offers, knowing what size eyepiece attachment your telescope can naturally take is just as important to ensure everything works properly.