Telescope Magnification Calculator

Calculate the magnification of telescopes with different eyepieces. Determine optimal magnification for various astronomical targets, exit pupil size, field of view, and maximum usable magnification based on aperture.

Telescope Focal Length (mm)

or select common:

Eyepiece Focal Length (mm)

or select common:

Telescope Aperture (optional)

Needed to calculate exit pupil and maximum magnification

Eyepiece Apparent Field of View (degrees)

Optional: For calculating true field of view

About Telescope Magnification Calculator

Understanding Telescope Magnification

Telescope magnification is one of the most fundamental yet often misunderstood aspects of telescope performance. While many beginners focus heavily on magnification power, experienced astronomers understand that it's just one of several important factors that determine what you can see through a telescope.

The magnification (or power) of a telescope is calculated using a simple formula:

Magnification Formula:

Magnification = Telescope Focal Length ÷ Eyepiece Focal Length

For example, a telescope with a 1000mm focal length used with a 25mm eyepiece will produce 40× magnification (1000 ÷ 25 = 40).

Important Concepts:

Changing eyepieces changes magnification
Shorter eyepiece focal length = higher magnification
Different telescopes with the same eyepiece produce different magnifications
Higher magnification is not always better - it reduces brightness and field of view

A Common Misconception:

Many beginners assume more magnification is always better. In reality, excessive magnification often results in dim, blurry images because the same amount of light is spread over a larger area, and any imperfections in the optics or atmosphere are also magnified.

Maximum Useful Magnification

Every telescope has a maximum useful magnification beyond which the image begins to deteriorate. This limit is primarily determined by the telescope's aperture (the diameter of its main lens or mirror).

Calculating Maximum Magnification:

Theoretical Maximum:

Max Magnification = Aperture (mm) × 2

Example: A 150mm telescope has a theoretical maximum magnification of 300×

Practical Maximum:

Practical Limit = Aperture (mm) × 1.4

For the same 150mm telescope, a more practical limit would be 210×

Why this limit exists: This limitation is based on the physics of light diffraction through a circular aperture. The resolving power of a telescope is fundamentally constrained by its aperture—larger apertures can resolve finer details and support higher magnifications.

Atmospheric Seeing:

Even with a large telescope that could theoretically support very high magnification, Earth's atmosphere often becomes the limiting factor. Turbulence in the air causes the image to shimmer and blur, effectively limiting practical magnification to around 200-250× on most nights, regardless of aperture. Only during moments of exceptional atmospheric stability ("good seeing") can higher magnifications be effectively used.

Exit Pupil and Its Importance

The exit pupil is the diameter of the light beam exiting the eyepiece. It's a critical factor in determining image brightness and the optimal magnification range for your eye.

Calculating Exit Pupil

Exit Pupil = Aperture ÷ Magnification

Exit Pupil = Eyepiece Focal Length ÷ Telescope f-ratio

For example, a 150mm telescope at 50× magnification produces a 3mm exit pupil (150 ÷ 50 = 3)

Optimal Exit Pupil Sizes

Exit Pupil	Best Application
0.5-1mm	High magnification (planets, double stars)
2-3mm	General purpose, ideal for most observing
4-5mm	Low power, wide field viewing
6-7mm	Maximum size for dark-adapted eye

Note: The human eye's pupil can dilate to about 7mm in youth but decreases with age (often to 5-6mm for middle-aged observers)

Understanding Exit Pupil Effects:

If the exit pupil is larger than your eye's pupil, light is wasted
A smaller exit pupil makes the image dimmer but can increase contrast
Different astronomical targets benefit from different exit pupil sizes
In bright conditions (like lunar observing), smaller exit pupils work well
For faint deep-sky objects, an exit pupil of 3-5mm often provides the best balance of brightness and detail

Field of View

The field of view (FOV) refers to the angular width of sky visible through your telescope. As magnification increases, the field of view decreases, showing you a smaller portion of the sky in more detail.

Apparent vs. True Field of View

Apparent Field of View (AFOV):

This is a property of the eyepiece alone—the angular width of the eyepiece's field as perceived by your eye, typically ranging from 40° to 100°.

True Field of View (TFOV):

This is what you actually see of the sky, calculated as:

TFOV = AFOV ÷ Magnification

For example, a 50° AFOV eyepiece at 100× magnification gives a 0.5° true field of view

Field of View Reference

Object	Angular Size
Full Moon	0.5° (30 arcminutes)
Jupiter	~0.7 arcminutes
Andromeda Galaxy (M31)	3° × 1°
Pleiades Cluster (M45)	~2°
Orion Nebula (M42)	~1°

Low magnification with a wider field is often preferred for large deep-sky objects, while high magnification with a narrow field works better for planets and double stars.

Field of View Considerations:

Different eyepiece designs offer different apparent fields of view:
Plössl: Typically 50-52° AFOV
Wide-angle: 60-70° AFOV
Ultra-wide: 80-100° AFOV
Wider AFOV eyepieces generally cost more but provide a more immersive viewing experience
For photography, field of view calculations differ and depend on camera sensor size

Choosing the Right Magnification for Different Targets

Different astronomical objects are best observed at different magnifications. Here's a guide to help you select the optimal power for various targets:

Target	Recommended Magnification	Notes
Moon	50-150×	Lower power for whole moon, higher for crater detail
Planets (Jupiter, Saturn, Mars)	120-250×	Use highest practical power that atmospheric conditions allow
Double Stars	100-200×	Higher power helps split close doubles
Star Clusters	50-150×	Lower for open clusters, higher for globular clusters
Nebulae	30-100×	Usually benefit from lower magnification to maintain brightness
Galaxies	30-100×	Lower power for extended galaxies, higher for small galaxies

Building an Eyepiece Collection:

A well-rounded eyepiece collection typically includes:

Low Power

For wide field views of large objects and star fields. Creates an exit pupil of 3-7mm.

Example: 25-40mm eyepiece

Medium Power

For general viewing of most objects. Creates an exit pupil of 1-2mm.

Example: 10-15mm eyepiece

High Power

For planets, lunar details and double stars. Creates an exit pupil of 0.5-1mm.

Example: 4-6mm eyepiece

Note: Actual focal lengths depend on your telescope's focal length. For a 1000mm focal length telescope, these ranges would provide approximately 25-100× (low), 65-100× (medium), and 165-250× (high) magnification.

Frequently Asked Questions

What is telescope magnification?

Telescope magnification refers to how much larger an object appears through the telescope compared to viewing it with the naked eye. It's calculated by dividing the telescope's focal length by the eyepiece's focal length. For example, a telescope with a 1000mm focal length used with a 25mm eyepiece produces 40× magnification (1000 ÷ 25 = 40), meaning objects appear 40 times larger than they would to the unaided eye.

Is higher magnification always better?

No, higher magnification is not always better. While it makes objects appear larger, it also spreads the same amount of light over a larger area, making the image dimmer. Higher magnification also amplifies any imperfections in the optics or atmospheric turbulence, often resulting in blurry views. Additionally, it reduces the field of view, making objects harder to find and track. Each astronomical target has an optimal magnification range, and conditions like seeing (atmospheric stability) also limit useful magnification.

What is the maximum useful magnification for my telescope?

The theoretical maximum useful magnification for a telescope is about 50× per inch of aperture, or 2× per millimeter. For example, a 6-inch (150mm) telescope has a theoretical maximum of about 300×. However, a more practical limit is usually around 25-35× per inch (1-1.4× per mm) due to optical quality and atmospheric conditions. Even with large telescopes, Earth's atmospheric turbulence often limits useful magnification to 200-250× on most nights, regardless of aperture size.

What does exit pupil mean and why is it important?

The exit pupil is the diameter of the light beam that emerges from the eyepiece and enters your eye. It's calculated by dividing the telescope aperture by the magnification, or by dividing the eyepiece focal length by the telescope's f-ratio. Exit pupil is important because it affects image brightness and detail. A larger exit pupil (4-7mm) provides brighter views ideal for deep-sky objects, while a smaller exit pupil (0.5-2mm) can improve contrast and detail for planets and the Moon. The human eye's pupil can dilate to about 7mm in complete darkness (less as we age), so exit pupils larger than this waste light.

Why does field of view matter for telescope viewing?

Field of view (FOV) is the angular width of sky visible through your telescope and is important because different astronomical targets require different viewing areas. A wider field of view makes it easier to locate objects and observe larger targets like star clusters and nebulae. As magnification increases, field of view decreases. The true field of view is calculated by dividing the eyepiece's apparent field of view by the magnification. For context, the full Moon spans about 0.5 degrees (30 arcminutes) in the sky, so a true field of view of 1 degree would fit the Moon with space to spare.

How do I choose the right eyepieces for my telescope?

When choosing eyepieces, aim for a range that provides low, medium, and high magnifications. For low power (wide field views), select an eyepiece that gives you about 3-5× per inch of aperture. For medium power (general viewing), aim for 10-15× per inch. For high power (planets and details), choose one that delivers 20-30× per inch, not exceeding your telescope's practical limits. Consider eyepiece quality, eye relief (especially if you wear glasses), apparent field of view, and compatibility with your telescope's focuser. A good starter set might include a low-power eyepiece (~25mm), a medium-power (~10mm), and a high-power (~6mm).

What is the best magnification for viewing planets?

The best magnification for viewing planets typically falls between 120× and 250×, depending on atmospheric conditions and your telescope's aperture. Planets like Jupiter, Saturn, and Mars benefit from higher magnifications to resolve details like Jupiter's cloud bands, Saturn's rings, or Mars' polar caps. Start with medium power (around 100×) and increase magnification if the image remains sharp. On nights with excellent seeing conditions, you might push to 250× or even higher with large telescopes. Remember that a steady, clear image at 150× will reveal more detail than a blurry image at 300×, so prioritize image quality over raw magnification.

How does atmospheric seeing affect telescope magnification?

Atmospheric seeing refers to the stability and clarity of Earth's atmosphere, which significantly impacts telescope performance at high magnifications. Even with a large, high-quality telescope, atmospheric turbulence causes images to shimmer, blur, or appear "boiling," especially at magnifications above 200×. Seeing conditions vary from night to night and location to location. On nights with poor seeing, even a large telescope might be limited to 100-150× for clear views. On exceptional nights with steady air, the same telescope might perform well at 300× or more. This is why experienced astronomers adjust their magnification based on the night's seeing conditions rather than always using maximum power.

Additional Resources

Sky & Telescope: Choosing Your Telescope's Magnification Cloudy Nights: Optimum Magnification & Exit Pupil

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