What light gathering power is
Light gathering power (LGP) describes how much light a telescope collects compared with a reference, most often the dark-adapted human eye. It depends on the area of the main lens or mirror, so it comes straight from telescope aperture.
The formula
Light collecting area scales with the square of the diameter. To compare two apertures:
LGP ratio = (D1 / D2)^2
Both diameters must be in the same units. To compare a telescope with the eye, use the eye’s pupil diameter as D2. A commonly used figure is about 7 mm for a fully dark-adapted young eye, though many people have a smaller pupil, especially with age. Some sources use 6 mm or 5 mm instead, so quoted results differ.
Worked examples
An 8-inch telescope. 8 inches is about 203 mm. Using a 7 mm pupil: (203 / 7)^2 = 29^2, which is roughly 841. So the scope collects around 800 times more light than the unaided eye. With a 6 mm pupil the figure is about 1,150.
A 10-inch vs a 5-inch. (10 / 5)^2 = 4. The 10-inch collects four times as much light as the 5-inch.
A 70 mm scope. (70 / 7)^2 = 100, so about 100 times the eye.
Rules of thumb
- Double the aperture: about 4 times the light.
- Increase aperture by about 40 percent (a factor of 1.4): about 2 times the light.
- Use consistent units. Do not mix inches and millimetres.
Real-world losses
The calculated figure is an upper bound. In practice:
- Lenses and mirror coatings do not transmit or reflect all light.
- In reflectors and compound scopes, the secondary mirror blocks part of the aperture.
- Eyepieces add small losses of their own.
So the real gain is somewhat lower than the formula suggests, and it differs between designs.
What it means for what you see
More light makes faint stars and extended objects easier to see. A larger aperture reaches fainter stars, and galaxies and nebulae show more structure. It does not make objects look larger; that is magnification, set by focal length and eyepiece. Bright targets like the Moon and planets benefit less from extra light and more from steady air.