I have a question about zoom lenses and aperture and I'm hoping there's an optical physicist in the room that can help me. There are 2 distinct families of lenses, one with constant aperture thru the various focal lengths and the other with an increasing (or decreasing, not sure which adjective is the right one) aperture as you increase the focal length.
So my question is this - why?
Take for instance a 70-300mm F/ 4-5.6 Why is it that when you increase the focal length you must decrease the maximum aperture? Clearly the larger aperture is mechanically available within the lens itself. Is it because the beam of light has narrowed as the focal length has increases such that the light itself is no larger than 5.6 so by virtue of the light rays themselves you cannot have a larger aperture? Then compare this with a constant aperture lens such as the 70-200mm F 2.8, how is it that at full zoom you can still have access to a 2.8 f-stop? From a mechanical standpoint, it seems like it would be easier to make a lens who's aperture is not a function of the focal length, doesn't it? But yet the constant aperture lenses are much more expensive which implies they're more demanding to manufacture.
Hopefully someone out there in internet-land will understand my rambling question and have a good answer for me.
TIA
So my question is this - why?
Take for instance a 70-300mm F/ 4-5.6 Why is it that when you increase the focal length you must decrease the maximum aperture? Clearly the larger aperture is mechanically available within the lens itself. Is it because the beam of light has narrowed as the focal length has increases such that the light itself is no larger than 5.6 so by virtue of the light rays themselves you cannot have a larger aperture? Then compare this with a constant aperture lens such as the 70-200mm F 2.8, how is it that at full zoom you can still have access to a 2.8 f-stop? From a mechanical standpoint, it seems like it would be easier to make a lens who's aperture is not a function of the focal length, doesn't it? But yet the constant aperture lenses are much more expensive which implies they're more demanding to manufacture.
Hopefully someone out there in internet-land will understand my rambling question and have a good answer for me.
TIA