Why no inversed aperture zoomlenses?

The perfect walk around lens for me (and I suspect a lot of other people) would be something like a 24-105/f4-f1.8 with the aperture opening up the more you zoom in. For wider shots, a large aperture isn't necessary most of the time. The more you zoom in, the more a large aperture makes sense, especially for portraits. Every zoom lens has either a fixed aperture or an aperture that closes down the more you zoom in.

So I'm wondering why there has never been a lens like this. Is this something limited to physics?

Physics! The f ratio is basically the focal length divided by the diameter of the len aperture. So for a lens that is say 25 mm in diameter if the focal length is 25 mm the f number is f/1.0. If you could zoom that to 100mm then the f number becomes f/4.0.

So for a fixed diameter lens the f number has to get smaller as the focal length gets longer.

Alan

Actually what amazes me is that we have some zoom lenses with constant aperture (e.g.70-200 f2.8)- very convenient, but how come zooming out doesn't make a brighter image?

Martin Dixon wrote in post #17067680
Actually what amazes me is that we have some zoom lenses with constant aperture (e.g.70-200 f2.8)- very convenient, but how come zooming out doesn't make a brighter image?

Because of a completely different operating principle; it's essentially a fixed-focal length lens (say 70mm 2.8) with a magnifying glass in front of it, so unlike a variable zoom lens, it doesn't physically change the focal length, it increases the magnification.. This is a gross simplification though, there are usually a number of things that go on when you change the zoom level on a 24-70 or 70-200.

I see (I think!) - this helped a lot - with diagrams!

http://www.reddit.com …_aperture_zoom_​lens_work/

This diagram in particular:

http://www.pierretosca​ni.com …ocal_length/Fig​ure-44.jpg

Martin Dixon wrote in post #17068331
I see (I think!) - this helped a lot - with diagrams!

This diagram in particular:

Yep, that's pretty much what I was talking about... and if you're wondering "but wait, doesn't mean that this magnification should negatively impact image quality?", then you're exactly right, most 70-200 lenses have the poorest performance at 200mm, as well the most vignetting too. Enough vignetting in fact, that at 200mm most f/2.8 zooms let in about t/3.4 worth of light at best!

Usually, in a wide constant aperture zoom, the magnification happens in the rear elements, on a standard zoom it's a combination of the two, and in a tele zoom, it all happens in front.