LensCalc

Telescope Magnification Calculator

How much magnification can your scope actually use?

Telescope Focal Length (mm)

Eyepiece Focal Length (mm)

Aperture (mm)

Magnification

48x

Max Useful Magnification: 300x
Exit Pupil: 3.13 mm
Focal Ratio: f/8

What you entered

Telescope Focal Length (mm): 1200Eyepiece Focal Length (mm): 25Aperture (mm): 150

1
Magnification
1,200 mm / 25 mm= 48x
2
Focal ratio
1,200 / 150= f/8
3
Maximum useful magnification (2x aperture in mm)
150 x 2= 300x
4
Exit pupil
150 / 48= 3.125 mm

Result

Magnification: 48x

Telescope magnification by eyepiece

Magnification is the telescope's focal length divided by the eyepiece's. Exit pupil is aperture divided by magnification, and maximum useful magnification is about twice the aperture in millimetres.

Telescope	Aperture	Eyepiece	Magnification	Exit pupil	Max useful
650 mm f/5.0	130 mm	25 mm	26.0x	5.00 mm	260.0x
900 mm f/8.8	102 mm	25 mm	36.0x	2.83 mm	204.0x
1000 mm f/7.7	130 mm	25 mm	40.0x	3.25 mm	260.0x
1000 mm f/7.7	130 mm	10 mm	100.0x	1.30 mm	260.0x
1200 mm f/6.0	200 mm	10 mm	120.0x	1.67 mm	400.0x
1000 mm f/7.7	130 mm	6 mm	166.7x	0.78 mm	260.0x

Aperture, not magnification, is what determines how much you can actually see. Pushing past roughly twice the aperture in millimetres gives a bigger but dimmer, blurrier image. An exit pupil below about 0.7 mm starts to show the eye's own imperfections.

Aperture sets the useful limit

Magnification is free — just fit a shorter eyepiece — but resolution is not. Beyond roughly 2x the aperture in millimetres you are enlarging blur rather than revealing detail, so a 150mm scope tops out near 300x no matter what eyepiece you use.

Exit pupil and low-power views

Exit pupil is aperture divided by magnification, and it should stay under about 7mm or light spills past your iris and is wasted. That sets a practical low-power limit, which matters for wide-field views of large targets like nebulae.

Frequently asked questions

My telescope has a 1200mm focal length and I'm using a 25mm eyepiece — what's the magnification?

Magnification = 1200 ÷ 25 = 48x. That's a good starting magnification for lunar craters and planet-finding. Swap to a 10mm eyepiece for 120x to see Saturn's rings clearly.

How is telescope magnification different from the focal length equivalent calculator?

The focal length equivalent calculator converts camera lens focal lengths across sensor sizes for photography framing. Telescope magnification divides the scope's focal length by the eyepiece's — it's about visual observation power, not photographic framing.

My 6-inch (150mm) scope — what's the max useful magnification?

About 300x (2× the aperture in mm). Beyond that you're magnifying atmospheric turbulence and diffraction blur rather than real detail. Most nights, seeing conditions limit you to 150-200x regardless of the scope.