Focal length is the distance, measured in mm, from the telescope’s primary lens or mirror to the point where light converges to form an image. Your specific choice depends on whether you prioritize seeing large areas of the sky or magnifying small, distant objects like planets.
Which Focal Length Suits Your Astronomy Goals?
| If this is you | The Focal Length that suits you | Why |
|---|---|---|
| You want to view large nebulae or the Milky Way and prefer a wide field of view. | Short focal length (300mm to 600mm) | Shorter lengths capture more of the sky in a single view, making it easier to find and frame large deep-sky objects. |
| You primarily want to view the Moon and planets like Jupiter or Saturn. | Medium focal length (700mm to 1,200mm) | These values provide enough magnification to see surface details on planets without making the image too difficult to keep steady. |
| You are an advanced observer focusing on high-resolution planetary imaging. | Long focal length (1,500mm to 2,500mm+) | Longer lengths provide the high magnification required to resolve tiny details on planets, though they require very stable mounts. |
| You want a portable setup for travel or easy setup in a small backyard. | Short focal length (under 500mm) | Shorter focal lengths are typically associated with smaller, lighter tubes that are easier to transport and handle. |
Where these choices show up in our reviews
Prices across these 3 run from $272.59 to $1199; each one has a full review here.
130EQ Newtonian Reflector Telescope
$359.99 price checked August 2026
Read our full 130EQ Newtonian Telescope review
If you need steady tracking for deep-sky objects, explore our comparison of manual equatorial telescopes.
Orion SkyScanner 100mm TableTop Reflector
For a more mature hobbyist, see the best astronomy telescope for adults.
$272.59 price checked August 2026
How Do I Find the Focal Length of a Telescope?
Finding the focal length of a telescope involves identifying the specific mm value printed on the manufacturer’s specifications or the telescope tube itself.
Where the Number is Usually Written
Most manufacturers print the focal length directly on the main optical tube, often near the eyepiece holder or on a small metal plate. If it is not on the tube, the value is listed on the original manual or the manufacturer’s website under the “Optical Specifications” section.
Understanding the physics of light is easier when you learn about different optical designs for telescopes.
Understanding the mm Notation
Focal length is always expressed in millimeters (mm). You will often see it paired with an aperture measurement, such as “900mm f/10.” In this example, 900mm is the focal length, while f/10 is the focal ratio. The focal length is the primary number that determines the magnification potential of the telescope.
What Happens if the Focal Length is Wrong for My Needs?
Choosing a focal length that is too short or too long for your specific goals results in different types of visual limitations during your time spent observing.
The Problem with a Focal Length That is Too Short
If the focal length is too short for your goals, the image will appear too small. This occurs when you try to view a planet or the Moon and find that the object takes up very little of your field of view, making it difficult to see surface details. You may find yourself wishing for more magnification to bring the object closer.
The Problem with a Focal Length That is Too Long
If the focal length is too long for your goals, the field of view becomes too narrow. This creates a “magnifying glass” effect where you can only see a tiny portion of the sky at once. This makes it difficult to find large objects like nebulae or galaxies because they may not fit entirely within the frame, or they may be impossible to center.
Mounting and Stability Constraints
Longer focal lengths also increase the impact of telescope shake. Because the focal length determines the magnification, a long focal length makes even the slightest vibration from your hands or the wind much more noticeable. If a telescope has a long focal length but the mount is not sufficiently stable, the image will blur and shake, preventing clear viewing.
Portability depends on size, so consider how tube weight affects telescope portability.
What Should I Do if I Fall Between Two Focal Lengths?
If your requirements sit between two common focal length values, you should decide based on which “failure mode” you would prefer to avoid.
Choosing the Longer Value
If you are primarily interested in the details of planets and the Moon, choose the higher focal length. It is generally easier to find and center a large object in a narrow field of view than it is to try and “zoom in” on a tiny planet using a telescope with a short focal length.
Choosing the Shorter Value
If you intend to spend most of your time looking at large deep-sky objects like the Andromeda Galaxy or large nebulae, choose the shorter focal length. It is much easier to navigate the sky and keep large objects in view with a wider field of view than it is to try and find a large object in a very narrow, high-magnification view.
Checking Sizing Policies
Because focal length is a fundamental physical property of the telescope, it cannot be changed once purchased. If you are unsure, check the return policy of the seller to see if they allow for returns on large optical equipment if the focal length does not meet your requirements.