Telescopic sights are classified in terms of the optical magnification and the objective lens diameter, e.g. 10x50.

Second World War era night glasses usually had a lens diameter of 56 mm or more with magnification of seven or eight.

Of particular relevance for low-light and astronomical viewing is the ratio between magnifying power and objective lens diameter.

Objective lens diameter - The diameter of the objective lens determines how much light can be gathered to form an image.

Divide the magnification into the lens diameter and you get the exit pupil diameter, which tells you how bright the scene will appear to the eye.

With a magnification factor of eight and an objective lens diameter of thirty millimetres the binoculars are compact, lightweight and rugged, classic all-purpose glasses.

In general terms, larger objective lens diameters, due to their ability to gather larger amounts of light, provide a larger exit pupil and hence provide a brighter image at the eyepiece.

However, past efforts to make large aperture GRIN lenses have had limited success due to restricted refractive index change, poor control over index profiles, and/or severe limitations in lens diameter.

Luxation can also be a secondary condition, caused by trauma, cataract formation (decrease in lens diameter may stretch and break the zonules), or glaucoma (enlargement of the globe stretches the zonules).

The fact that they're increasing the focal length makes it seem more likely to me that they're increasing the sensor size again, and need a longer focal length to keep the same FOV, while at the same time increasing the lens diameter.

In these cases, a 4x81 (4x magnification) sight would be presumed to have a brighter sight picture than a 2.5x70 (2.5x magnification), but the objective lens diameter would not bear any direct relation to picture brightness, as brightness is affected also by the magnification factor.

The rings controlling the effective lens diameter (from f/2.8 to f/22) and the shutter speed are mechanically linked so that once one combination has been matched to the ambient lighting conditions, a faster shutter speed can be selected and a larger effective lens diameter will automatically be set when possible.