Focal length is the distance in mm from the telescope’s objective lens to the point where light rays converge to form a sharp image. The specific number you need depends on whether you want to see large areas of the sky or high-magnification details of small objects.
Which Refractor Telescope Focal Length Suits You?
| If this is you | The Focal Length that suits you | Why |
|---|---|---|
| You want to view large nebulae, star clusters, and wide-field galaxies. | Short focal length (300mm to 500mm) | A shorter focal length provides a wider field of view, making it easier to frame large deep-sky objects. |
| You primarily want to observe the Moon and planets like Jupiter or Saturn. | Medium focal length (600mm to 900mm) | This range balances the ability to see planetary surface details with a manageable field of view for the solar system. |
| You are an advanced astrophotographer focusing on high-resolution lunar shots. | Long focal length (1000mm to 1500mm) | Longer focal lengths provide the high magnification required to resolve fine textures on the lunar surface or planetary disks. |
| You have a very small balcony or limited space for a tripod. | Short focal length (under 400mm) | Short focal length telescopes are physically more compact and easier to balance on smaller, less stable mounts. |
| You want a versatile “all-rounder” for general backyard observing. | Medium focal length (700mm to 1000mm) | This range is a common middle ground that allows for decent planetary views while still being able to find larger targets. |
Where these choices show up in our reviews
Prices across these 3 run from $188.59 to $218.49; each one has a full review here.
SpectrumOI TourStar Pro 90 AZ
If you are just starting out, you should see our comparison of the best refractor telescope for beginners.
$188.59 price checked August 2026
SpectrumOI 80mm EQ Refractor Telescope
$199.99 price checked August 2026
Read our full SpectrumOI Refractor Telescope review
For those who need mobility, view our ranking of the best refractor telescope for travel.
How do I determine my required focal length before ordering?
To determine your required focal length, you must first decide on your primary target type and then work backward from your desired magnification.
Identify your primary observation targets
Determine if your priority is “wide-field” or “high-magnification.” Wide-field observing focuses on large structures like the Andromeda Galaxy or the Pleiades. High-magnification observing focuses on small details, such as the Great Red Spot on Jupiter or craters on the Moon.
Calculate the necessary focal length
Magnification is calculated by dividing the telescope’s focal length by the eyepiece’s focal length. For example, a 1000mm focal length telescope paired with a 25mm eyepiece provides 40x magnification. If you know you want 100x magnification on a planet and plan to use a 10mm eyepiece, you would need a telescope with a 1000mm focal length.
Locate the focal length on technical specifications
The focal length is almost always listed in millimeters (mm) on the manufacturer’s spec sheet or on the telescope tube itself. It is often paired with the aperture (the diameter of the lens). Look for a value such as “900mm” or “f/10” (where the focal length is the aperture multiplied by the f-number).
What happens if I choose the wrong focal length?
Choosing a focal length that is too long or too short for your goals results in specific visual limitations that can make the telescope frustrating to use.
You should also consider how to choose the right weight for refractor telescopes before making a purchase.
The consequences of a focal length that is too long
A focal length that is too long for wide-field targets makes it difficult to find and frame large objects. If you are trying to view a large nebula through a 1500mm telescope, the object may only occupy a tiny portion of your view, making it hard to see the full structure. Additionally, long focal length telescopes are heavier and harder to keep steady on a mount.
The consequences of a focal length that is too short
A focal length that is too short for planetary observation results in a lack of detail. If you want to see the rings of Saturn clearly but use a 300mm telescope, the planet will appear as a small, blurry dot. You will lack the “zoom” necessary to resolve fine surface features, even with high-magnification eyepieces.
What do I do if I fall between two focal length values?
If your requirements sit between two common focal length ranges, you should choose the direction based on your primary target priority.
Prioritize your most frequent observation type
If you spend 80% of your time on planets and 20% on galaxies, choose the longer focal length. If you spend 80% of your time on deep-sky objects and 20% on the Moon, choose the shorter focal length. It is easier to use a slightly longer telescope to find a large galaxy than it is to use a short telescope to see a small planet.
Consider the limitations of your mount
If you are choosing between a 900mm and a 1200mm focal length, check the weight capacity of your mount. A longer focal length telescope is generally heavier and requires a more stable mount to prevent “mount flexure,” where the image drifts because the tripod cannot hold the weight perfectly still. If your mount is small, the shorter focal length is the safer choice.