This widely praised Nikon 105mm macro lens has a maximum f-stop value of 2.8. I've held it in my hands, it's a big lens. Meanwhile, this 50mm Nikon lens can go up to f/1.2, despite being 25% cheaper and a LOT smaller. So at least with these two lens, there is not a very direct correlation between price and f-stop ability.
What is it that determines a given lens' f-stop range? Why can't that 105mm go sub-2?
Answer
The pupil (aperture opening) area is proportional to the square of the focal length (at the same f-stop). So 105mm being about twice the focal length of the 50mm, it would need 4x the pupil (area) to be f/1.2.
In other words f/1.2, or any f-stop, doesn't correspond to a fixed diameter - it increases for larger focal lengths.
That also assumes both lenses gather and transmit the same amount of light to the aperture. Given the 50mm has a wider field of view, it will tend to gather more light, so it has a further advantage there.
The maximum aperture area is clearly restricted by the format of the camera - it can't be bigger than the lens mount. A lens can compensate by gathering more light, which is why big 300mm f/2.8 and 600mm f/4 lenses have enormous front lens elements.
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