What focal length is
The focal length of a telescope is the distance from its main lens or mirror to the point where light comes to a focus. It is normally given in millimetres, such as 650 mm or 1000 mm. Unlike aperture, which controls how much light is collected, focal length mostly controls image scale: how large objects appear at the focus.
In compound scopes (such as Schmidt-Cassegrain designs), mirrors fold the light path, so the tube is much shorter than the stated focal length.
Where to find it
Most manufacturers print the focal length and aperture on the tube, the focuser, or the box, often as something like “70/700” (70 mm aperture, 700 mm focal length). If it is not marked, check the manual or product listing. Focal length divided by aperture gives the focal ratio.
Focal length and magnification
Magnification equals the telescope’s focal length divided by the eyepiece’s focal length.
- A 1000 mm scope with a 25 mm eyepiece gives 1000 / 25 = 40x.
- The same scope with a 10 mm eyepiece gives 100x.
- A 2000 mm scope with the same 25 mm eyepiece gives 80x.
So a longer focal length gives more magnification with the same eyepiece. How much is actually useful is covered in telescope magnification power.
Focal length and field of view
Longer focal length gives higher magnification and a narrower field. Shorter focal length gives lower magnification with a wider field, which suits large objects such as big star clusters and wide nebulae. Field of view also depends on the eyepiece’s apparent field of view: roughly, true field is the eyepiece’s apparent field divided by the magnification.
What the eyepiece focal length means
Eyepieces have their own focal lengths, also in millimetres. A smaller number means more magnification. Common eyepieces run from about 4 mm to 40 mm.
Can you see Jupiter with a 25 mm eyepiece? Often yes. On a 1000 mm scope that is 40x, which usually shows Jupiter as a small disc with its brightest moons, and perhaps hints of cloud bands. For more detail, a shorter eyepiece (giving higher magnification) is generally used, as long as the air is steady and the scope can support it.
Focal length vs focal ratio
Focal ratio (f/number) is focal length divided by aperture. A 100 mm scope with 500 mm focal length is f/5; with 1000 mm it is f/10. This ratio affects how forgiving the scope is with eyepieces and how it behaves for photography. See fast vs slow telescopes.
Choosing a focal length
- Shorter (roughly 400 to 700 mm). Wider views, good for sweeping star fields and large deep-sky targets.
- Longer (roughly 1000 mm and up). Higher magnification with ordinary eyepieces, which suits the Moon and planets, at the cost of a narrower field.
Neither is better overall; it depends on your targets. Many observers end up using a mix of eyepieces to cover both.