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Light gathering power

  • Maja 

The light gathering power (also light grasp) is an ability of a telescope to collect more light than the human eye.

It is expressed as the ratio of the objective area to the human eye pupil area. 

  • For the eye pupil, the value ‘7 mm’ is most frequently used. 

The larger the size of the primary mirror/objective lens (aperture), the more light the telescope can collect (brighter image)!

Light gathering power

Star cluster M13. On the left image, viewed with small aperture telescope - on the right image, viewed with large aperture telescope (Source:

Light gathering power compared to the human eye

Area is proportional to the square of the diameter.  

Light gathering power


D = objective diameter (mm)

d =  eye pupil diameter (mm)





Light gathering power

Telescope with the objective diameter 125mm gathers 319x more light than the human eye.

This calculation doesn't account for loses!

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