Constant aperture zooms could be faster?

[J Veitch]

It's just something that nags at me... My understanding is that for a given aperture size as you shorten the focal length the f stop would be faster (as it's a ratio). I believe that constant aperture lenses have a second aperture diaphragm effectively slowing them down as the zoom goes wider to maintain the constant f stop. Therefore they all could actually be even faster at the wide end (or an i missing something)? Is it just marketing that drives then or a hangover from the past?

 

[Snapsh0t]

But then they'd be variable aperture and people prefer constant aperture......

 

[Brachytron]

While you are right in principle, that as you zoom out, the change in focal length should reduce the aperture (which it does on variable aperture lenses), your belief that a second aperture is present is incorrect. f-stop value is dependent on the ratio of the focal length to entrance pupil diameter, not the physical aperture diameter, which is the size of the aperture viewed from the front of the lens. In fixed aperture zooms, the elements in front of the diaphragm magnify the diaphragm to enlarge the entrance pupil and this ratio is size is kept constant throughout the zoom range. This is more complex to achieve which is why fixed aperture zooms are bigger and more expensive. In variable aperture zooms, the design does not need to keep this entrance pupil diameter to focal length ratio fixed so the lens can be smaller and lighter and less complex.

 

[J Veitch]

Thank you for your information, it would make more sense that most lenses are constructed as such. I wish I could find it, but I was sure I had read a while ago that someone had taken apart a lens (now I can't remember what one or where I read it) and it did have two aperture diaphragms.

 

[HoppyUK]

Most zooms are indeed variable aperture, and faster at the wide end. So that's the theory working, though the extra aperture is nothing like you might expect from a simple calculation of f/number vs focal length. Many zooms adjust the aperture slightly to maintain a constant f/number as the lens is zoomed, but I've never seen a dual aperture and you certainly get every drop of light that the lens is capable of delivering. And BTW, it's pretty much impossible to measure the f/number just by looking down the barrel.

Theory is one thing, but practise is another, and lens design is always overlaid with constraints of size, weight and cost vs optical performance. There is no marketing smoke and mirrors, just physics, though if there is some sleight of hand, it's that f/numbers can sometimes be more optimistic than the actual light transmission (t/stop) and zoom focal lengths often fall a bit short of stated at the long end, and then get significantly shorter at closer focusing distances (focus breathing).