✅ How to Choose Right Eyepieces
1. Focal Length and Magnification
2. Eye Relief and Corrective Lenses
3. Barrel Size
4. Optical Correction
5. How Exit Pupil Relates to Power
6. What Does Parfocal Mean?
7. Illuminated-Reticle Eyepieces
8. So, How Many Eyepieces Do I Really Need?
9. To Zoom or not to Zoom?
10. Using a Barlow Lens with Eyepieces
A telescope simply will not work without an eyepiece. The optical ele-ments of an eyepiece allow you to focus light collected by a telescope, so you can observe a sharp view of the object or area where the telescope is pointing. With an eyepiece installed in the telescope focuser, you can bring extremely distant objects into focus for magnified study.
While most new telescopes include one or two eyepieces to get started with, purchasing additional eyepieces can significantly increase the functionality of any telescope, new or old.
Let's take a few moments to learn a bit more about the most common terms and specifications used to describe features of telescope eyepieces. With this knowledge, you'll be well-prepared to select ideal eyepieces for your own telescopes.
1. Focal Length and Magnification
Perhaps the most important specification of an eyepiece is its focal length. The focal length of an eyepiece, along with the focal length of the telescope the eyepiece is used with, determines the magnification the combination provides.
So the first step in choosing eyepieces is to decide what magnifications, or powers, you want to use and what eyepiece focal lengths will give them. Since both eyepiece and telescope focal lengths are expressed in millimeters, the formula used to determine magnification is:
Magnification = Telescope focal length (mm) ÷ Eyepiece focal length (mm)
Or, put another way,
Eyepiece focal length (mm) = Telescope focal length (mm) ÷ Magnification
For example, a telescope with a 2000mm focal length used with a 20mm eyepiece will give 100 power (2000 ÷ 20 = 100).
The above formula dictates that a telescope eyepiece with a shorter focal length yields a higher magnification than an eyepiece with a longer focal length. For example, a 10mm eyepiece will always provide a higher magnification than a 25mm eyepiece. This relationship is important to remember while choosing eyepieces: the lower the eyepiece focal length, the higher the relative magnification will be. The actual magnification will depend on the focal length of the telescope the eyepiece is used with.
If you've ever used a single telescope at different powers, you know that you have a choice of a small, sharp, bright image at lower magnification; or a big, blurred, dim image at higher power. The reason is twofold. First, the telescope gathers a fixed amount of light, and at higher magnifications, or powers, you're spreading the same amount of light over a larger area, so the image will always be dimmer. Second, because light consists of waves, even an optically perfect telescope picks up only a limited amount of fine detail in the image. Magnifying the image beyond a certain point does not reveal more; it just makes the image look blurry. This is called "empty magnification" and can change depending on the object or area viewed.
2. Eye Relief and Corrective Lenses
The optical design of an eyepiece determines the eye relief, which is the distance from your eye to the eyepiece lens when the image is in focus. If you wear corrective lens eyeglasses while using a telescope, we recommend looking for telescope eyepieces with at least 15mm, and more preferably 20mm, of eye relief to see the entire field of view comfortably. With insufficient eye relief the outer portion of the viewing field will be cut off, resulting in a "keyhole effect" which can be frustrating. In more traditional telescope eyepiece designs, eye relief is proportional to focal length: the shorter the focal length, the shorter the eye relief. However, some of the more modern eyepiece designs provide luxuriously long eye relief regardless of focal length -- a real boon to eyeglass wearers. If you like to keep your eyeglasses on while using a telescope, the eye relief of an eyepiece is a very important specification to consider.
3. Barrel Size
Most quality telescope eyepieces come in two different barrel diameters, 1.25", and 2". A smaller, 0.965" barrel size is found mostly on low-end "department store" telescopes and should be avoided, if possible. Most amateur telescopes are designed to accommodate the 1.25" eyepiece size. Larger 2" eyepieces are typically used with larger aperture telescopes that feature a 2" focuser. Big 2" eyepieces traditionally feature long eye relief for comfortable views, and they often offer wider fields of view compared to 1.25" eyepiece models.
4. Optical Correction
The main goal of any telescope eyepiece design is to get all the light rays collected by the telescope to form a sharp image. Depending on the f-ratio of the telescope, this can be a difficult task. Telescopes with low f-ratios require more highly corrected eyepieces because the cone of light entering the eyepiece is converging more sharply. With a relatively low f-ratio telescope, such as an f/4 optical tube, only the best modern eyepieces will yield completely sharp images all the way out to the edge of the field of view. Some older designs may result in blurred views around the edge of the field of view, but the center will remain sharp. In telescopes with a relatively high f-ratio, such as an f/10 telescope, any well-made eyepiece will give a sharp image.
5. How Exit Pupil Relates to Power
The powers or magnifications at which a telescope will work well depend on the aperture of the instrument. In general, a larger telescope gathers more light and captures a broader wavefront, giving sharper images. One handy way to classify powers is in terms of "power per inch" of aperture. For example, 80x on an 8"-aperture telescope is 10 power per inch. Another way is to go by the size of the exit pupil. The term "exit pupil" describes the size of the bundle of light rays coming out of the eyepiece. Exit pupil size in inches is the reciprocal of power per inch. More commonly, exit pupil size is calculated in millimeters using these formulas:
Exit pupil size (mm) = Telescope aperture in mm ÷ Telescope magnification
Exit pupil size (mm) = Eyepiece focal length in mm ÷ Telescope f-ratio
The exit pupil must be smaller than the pupil of your eye, or else some of the light rays will not make it into the pupil, meaning the light will essentially be wasted. A young person's fully dark-adapted eyes may have 7mm-wide pupils. As you age, maximum pupil diameter decreases. For middle-aged adults, the practical maximum is closer to 5mm.
At the other end of the scale, at magnifications that yield an exit pupil in the range of 0.5mm to 1.0mm, empty magnification begins to set in, depending on the quality of your telescope and your eyes. In other words, this much magnification really starts to degrade the image you see. Here's a table of how various powers stack up:
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