Lenses with larger apertures ( the lower the f number the larger the aperture ) let in more light. That requires larger diameter elements. Notice the front element is larger on a 1.2 than say a 1.7. More glass/more weight.
Lenses with larger apertures ( the lower the f number the larger the aperture ) let in more light. That requires larger diameter elements. Notice the front element is larger on a 1.2 than say a 1.7. More glass/more weight.