Eyepieces make the image formed by a telescope usable to the human eye.  They determine

• how large the image appears,
• how wide the view is,
• how bright it looks,
• and how comfortable it is to observe.

The telescope collects light, but the eyepiece defines how that light is experienced.

Understanding eyepieces and eyepiece accessories  makes it easier to choose the right accessories and get the best performance from any telescope.

Inside every telescope, light is gathered and brought to focus as a small image. The eyepiece magnifies that image and delivers it to the eye. 

Swapping the eyepiece changes magnification, the width of the view, brightness, and overall clarity.

The same telescope can feel completely different depending on the eyepiece used.

Focal length controls how an eyepiece behaves in a telescope and is the basis of magnification.

Telescope focal length

• is the distance light travels inside the telescope to reach focus. It is usually printed on the tube (e.g. 400 mm, 900 mm, 1200 mm). In most refractors and Newtonian reflectors, this number is often close to the physical length of the telescope tube.
• With the same eyepiece, a longer focal length telescope gives higher magnification and a narrower view, while a shorter focal length gives a wider view with less magnification. 

Eyepiece focal length

• Is printed on the eyepiece itself (e.g. 32 mm, 25 mm, 10 mm, 6 mm).
• It tells you how strongly that eyepiece will magnify the image produced by the telescope.

Longer eyepiece focal length (e.g. 25–32 mm): lower magnification, wider and brighter view.
Shorter eyepiece focal length (e.g. 4–10 mm): higher magnification, narrower and dimmer view.

Physically, longer focal length eyepieces are often longer in size and easier to look through. Shorter focal length eyepieces tend to be smaller and can be less comfortable unless designed with long eye relief.

• A large number on the eye piece = Low magnification

• A small number on the eye piece = High Magnification

The actual magnification you get depends on both the telescope and eyepiece.

It is a calculation where Magnification = Telescope focal length ÷ Eyepiece focal length

• For example (with a 900 mm telescope and three eyepieces a 25, 10 & 5mm ) the magnification is as follows 
  
  36x  ( 900 ÷ 25mm )
  90x  ( 900 ÷ 10mm )
  180x ( 900 ÷ 5mm )
  

Magnification is not printed on the eyepiece because it changes on different telescopes.

However, while magnification can be increased by using shorter eyepiece focal lengths (or adding a Barlow lens), the maximum useful or clear magnification is set by the telescope’s aperture — not by the eyepiece. This is because the aperture controls how much light and detail the telescope can actually resolve.

Aperture is the diameter of the telescope’s main lens or mirror. It determines how much light is collected and how much detail the telescope can resolve - meaning how clearly it can separate or show small features without them blurring together. 

Even with a short / high power eyepiece, magnification is only useful up to the point where the telescope’s aperture can still produce a sharp image.

Useful magnification also a calculation is approx. 2x the telescope aperture in mm so for example 

• a 70mm aperture will have a maximum useful magnification of 140x
• a 120mm aperture will have a maximum useful magnification of 240x
• a 200mm aperture will have a maximum useful magnification of 400x

Beyond this, the image usually becomes dim, soft, and difficult to focus.

A small telescope can technically use a high-power eyepiece, but the image may not improve.

As magnification increases, the amount of the scene you can see at once becomes smaller. Field of view describes how much of the sky or landscape is actually visible through the telescope.

Every eyepiece has an apparent field of view (AFOV) – for example 40°, 50°, 68°. This is built into the eyepiece design and is the angular width the view appears to your eye.

The true field of view is the real area you see through the telescope.

It depends on both the eyepiece’s apparent field and the magnification being used. Two eyepieces may both have a 52° apparent field, but the one giving lower magnification (e.g. a 32 mm) will show a much larger area than a 10 mm eyepiece with the same AFOV.

Eye relief is the distance your eye needs to be from the eyepiece lens to see the full image.

Short eye relief means your eye must be very close to the eyepiece, which can be uncomfortable and impractical for people who wear glasses.

Long eye relief allows a more relaxed position and is available in many modern eyepiece designs, particularly in longer focal lengths and purpose-built “long eye relief” models.

Not all eyepieces are built the same. They use different lens designs, and this affects how sharp the image looks, how wide the view appears, how bright it is, and how comfortable it is to look through.

Some designs are very basic and are best suited to low magnification or casual viewing, while others give sharper, brighter, or wider views and are preferred for more detailed observing.

The differences are not just optical — some eyepieces are heavier, some are easier to use for longer periods, and some are better matched to certain types of telescopes. 

 

• Huygenian eyepieces
  are one of the earliest designs, using just two lenses. They have a  narrow field of view and can show some colour fringing and softness around the edges. 
  
  
• Kellner eyepieces
  are a simple three-lens design and are a step up from Huygenian in clarity and brightness. 
  
  
• Super eyepieces (sometimes called “Super Modified Achromats” or just “Super”)
  are a variation of the Kellner design. They provide slightly better contrast and edge performance than standard Kellners, with a modestly wider field of view. They are still considered basic but are a practical improvement over older designs.
  
  
• Plössl eyepieces
  are one of the most commonly recommended upgrades. They use four lenses in two groups, producing noticeably sharper images, better contrast and more consistent sharpness across the field. With an apparent field of view of around 50–52 degrees, they offer a natural viewing experience and work well on most modern telescopes. For many users, a Plössl represents the best balance between performance and cost.
  
  
• Wide-angle eyepieces
  increase the apparent field of view to around 60–70 degrees. They feel more open and make it easier to follow or locate objects. The Saxon Cielo HD eyepieces fall in this category. They use higher-quality glass (ED elements) to improve sharpness and colour correction while offering a wider view than a Plössl.
  
  
• Ultra-wide or premium eyepieces
  extend the apparent field even further, often to 80–100 degrees. These provide an immersive view with less visible edge to the field of vision. The Celestron Luminos eyepieces are an example of this type, with an 82-degree apparent field. They offer a wide, space-like view but are heavier and more expensive than standard eyepieces.
  
  
  

Eyepieces are made in different barrel diameters, and they need to match the size of the telescope’s focuser or diagonal.

Most modern telescopes use 1.25-inch eyepieces, which is the standard size for general use.

Some larger telescopes also accept 2-inch eyepieces, which allow wider fields of view and are useful for panoramic or deep-sky viewing.

A few very small or older telescopes use 0.965-inch eyepieces, which restrict field of view and limit upgrade options. Telescopes that accept 2-inch eyepieces usually include an adapter so 1.25-inch eyepieces can also be used.

Eyepieces in the 25–32 mm range - Low magnification

• provide low magnification with a wide, bright field of view. They are ideal for scanning large areas of sky or landscape, locating targets, and for general daytime use where brightness and stability are more important than image size.

Eyepieces around 15–20 mm - Medium magnification

• give medium magnification and are often used the most. They offer a good balance between image size and brightness and work well for the Moon, planets, star clusters and distant terrestrial objects.

Eyepieces between 5–10 mm - High Magnification

• produce high magnification and are used when fine detail is needed — such as looking at craters on the Moon, planets like Jupiter and Saturn, or fine detail in distant land objects. These eyepieces work best when the telescope has enough aperture and the viewing conditions are steady; otherwise, the image may appear soft or unsteady.

 

Diagonals are optical attachments that sit between the telescope and the eyepiece. They redirect the light path to provide a more comfortable viewing angle and, in many cases, correct the image so it appears the right way up and the right way around.

This is especially useful for daytime and terrestrial viewing, where a correctly oriented image is important. In astronomy, diagonals are also used to make it easier to look through the telescope when it is pointed high in the sky. T

They are most commonly used with refractor and cassegrain (Maksutov and Schmidt-Cassegrain) telescopes.

A 90-degree star diagonal

• is the most common type used for astronomy. It bends the light path by 90 degrees so you can look down into the eyepiece instead of crouching behind the telescope. The image appears upright but reversed left-to-right. This mirror-reversed view doesn’t affect astronomy but can be confusing for land-based observing or navigation.
  

A 45-degree erecting diagonal

• is designed mainly for terrestrial use. It provides an image that is both upright and the correct way around left-to-right, which is useful for viewing landscapes, wildlife, or ships at sea. The viewing angle is more comfortable for horizontal targets but becomes more difficult when the telescope is aimed high into the sky, which is why it is not usually preferred for night-time astronomy.
  

A 90-degree erecting prism diagonal (Amici prism)

• also produces a fully correct, upright and left-right accurate image. Unlike a 45-degree diagonal, it is usually set at 90 degrees, so it can be used for astronomy or terrestrial viewing. Because of the way the light is split inside the prism, some models may show a faint line through bright objects like the Moon or planets, but higher-quality Amici prisms minimise this effect.
  

Reflector telescopes (Newtonians) generally do not use diagonals because the viewing angle is already built into the telescope design via the secondary mirror.

A Barlow lens increases magnification by extending the telescope’s focal length.

• 2× Barlow doubles magnification,
  
• 3× Barlow triples it.
  

For example: a 10 mm eyepiece becomes the equivalent of a 5 mm when used with a 2× Barlow.

Quality differences:

• Basic Barlows work but can reduce sharpness and contrast slightly.
  
• Mid-range models (e.g. Celestron Omni) use better glass and coatings for a clearer image.
  
• Higher-end versions (Celestron X-Cel LX, ED glass Barlows) maintain brightness and sharpness more effectively and add less colour distortion.
  

Should I use a 2× Barlow with a 10 mm eyepiece, or buy a 5 mm eyepiece?
Both approaches work, but they are not identical:

• Using a 10 mm with a good Barlow is often more comfortable because it keeps longer eye relief.
  
• A dedicated 4–6 mm eyepiece may offer slightly sharper detail, but usually has very short eye relief and can be harder to use, especially for long viewing sessions or glasses wearers.
  

For most users, a good 2× Barlow paired with a medium eyepiece (e.g. 10–12 mm) is a flexible and cost-effective option.

Filters screw into the bottom of an eyepiece or Barlow and can improve contrast, reduce glare, or make certain features easier to see.

Neutral Density / Moon Filter
Reduces brightness, especially on the Moon, making surface detail easier to see without glare.

Colour Filters – are used mainly for the Moon and planets.

• Light Blue: Improves contrast in Jupiter and Saturn’s cloud bands.
  
• Dark Blue: Brings out Jupiter’s Great Red Spot and Martian polar caps.
  
• Red: Enhances dark surface markings on Mars and increases contrast in Jupiter’s belts.
  
• Orange: Softer alternative to red; useful on Jupiter, Saturn and Mars.
  
• Yellow: Slightly boosts contrast on the Moon and planetary cloud bands while reducing haze.
  
• Green: Sharpens lunar features and helps show polar ice on Mars or atmospheric storms on Jupiter.
  

Solar filters allow you to safely observe the Sun through a telescope. They reduce sunlight to a safe brightness level and block harmful ultraviolet (UV) and infrared (IR) radiation.

Without a proper solar filter, even a brief look at the Sun through a telescope can cause permanent eye damage or destroy the telescope’s optics.

Where the filter must go
A safe solar filter always fits over the front (aperture) of the telescope. This blocks almost all the light before it enters the optical system. The image that reaches the eyepiece is already reduced to a safe and usable level.

Filters that fit on the eyepiece (often included with very cheap telescopes) are unsafe because they sit at the point where sunlight is concentrated — they can crack or overheat without warning and must not be used.

Types of safe solar filters

• Full-aperture white-light filters (film or glass): Fit over the front of the telescope and allow safe viewing of sunspots, solar rotation and the mottled solar surface (photosphere). These give a white, blue-white, or yellow-orange image depending on the material.
  
• Herschel Wedge (for refractors only): A specialised prism diagonal that safely reflects most sunlight away and sends a small, safe portion to the eyepiece. It must always be used with an additional neutral density or polarising filter.
  
• Hydrogen-alpha (Hα) solar telescopes or filter systems: These are purpose-built systems that show solar prominences, flares, and surface detail in the Sun’s hydrogen emission line. They are specialised equipment and not general-purpose filters.
  

Important safety notes

• Never use an eyepiece-end “Sun filter” — if it screws into the eyepiece, it should not be used.
  
• Never observe the Sun through a finder scope unless it also has a safe solar filter or is capped.
  
• Always check that the solar filter is secured firmly with no pinholes, damage, or loose fit before use.
  

A zoom eyepiece changes focal length by twisting a rotating barrel — for example, 8–24 mm. It allows different magnifications without changing eyepieces.

Zoom eyepieces offer convenience, especially for travel or learning, but they often have a narrower field of view at low power and may not be as sharp as fixed eyepieces at the same focal length.

Eyepieces define what you see through a telescope as much as the telescope itself does. Magnification comes from the combination of telescope and eyepiece focal lengths. Aperture limits how much magnification is useful. Field of view, eye relief and optical design affect how natural, bright and comfortable the view appears.

Accessories such as diagonals, Barlow lenses and filters expand how eyepieces can be used for both astronomy and terrestrial viewing. With an understanding of these components, choosing suitable eyepieces becomes straightforward, and even simple telescopes can perform well.