How do constant aperture zooms work?

I've recently bought a Sigma 28-70mm F2.8 fixed aperture zoom and noticed that when I zoom out at the maximum aperture, the diaphragm closes slightly as it approaches the wide end of the zoom range.

Is this how all constant aperture zooms work? If so, why do manufacturers do this instead of leaving the diaphragm fully open throughout the entire range and giving the lens a 'faster' rating at the wide end?

it's actually how almost all zooms, constant aperture and not, work.

'Constant' aperture is actually a misnomer, it should be 'constant f number'.

The original reason for constant aperture zooms is one of ease of use. When the aperture was controled by an aperture ring on the lens it could be ambiguous what aperture you had actually set unless you were at one end of the range or the other. A constant aperture zoom meant that if you set f/5.6 you would get f/5.6 at any zoom setting.

With auto aperture control from the body you always get the aperture you ask for anyway (at least with modern lenses, some early A type zooms always report as if at their widest setting), and it has become more about offering the widest opening possible throughout the range.

I see, thanks.

So, the perception that constant f-number/aperture zooms are better is really more to do with ease of use on manual cameras than optical quality.

yes and no,

the fact that they have constant f-number doesn't help the optical quality, BUT

the constant aperture lenses have always tended to be the higher end models and that remains true, so constant zooms tend to be higher quality for that reason.

It is hard to make a zoom lens much faster than f/2.8, but the best and most expensive lenses tend to be the fastest so constant f/2.8 has become a badge of quality in a zoom.

F-number are a nicer word focal ratio is defined by dividing the focal length through the aperture diameter. So a shorter focal length need a smaller aperture to keep the focal ratio constant.
Makes sense right?

I can't believe that the likes of Sigma and Tamron haven't marketed an 'ultrawide' mode (in a similar way to the macro focussing modes on many zooms), where the diaphragm is opened fully at the wide end.

After all, low light performance must be one of the main reasons people buy fast zooms (it was for me, anyway).

I'm tempted to do a bit of strategic disassembly to see how it works...

But the diaphragm is opened fully at the wide end.
And you've a turning point in aperture when the lens goes through the register distance. Because wide angle lenses are basicly reversed tele lenses.

But we have lenses like the 17-70mm f/2.8-4.5

I'm not sure that the diaphragm is open fully at the wide end on the lens I was looking at (it was late yesterday evening and I haven't got it to hand now), which is what surprised me a bit.

From what Mongoose has said, it sounds like the diaphragm is closed slightly towards the wide end to give constant f-number performance. So for the example of the 17-70mm f/2.8-4.5, presumably they could have designed it as a constant f/4.5 by closing up the diaphragm towards the wide end of the zoom?

I was a bit surprised that manufacturers do this, as I would have thought that an f/2-f/2.8 28-70 zoom might appear more useful to the uninitiated (like me) than a constant f/2.8 over the same range.

There is something called apparent aperture actually that's the aperture what matters.
However my constant DA*50-135 however the aperture stay fix in the back and its wide open, It all comes down to lens design.

Constant aperture lenses are most often of better design because there is value in having a constant aperture which the advance user can appreciate.
I use the DA*50-135 for portrait a lot at f/2.8 with the camera in manual mode if that would change while I zoom that would mean I have to adjust exposure as well therefore it's also easier to learn how to use manual mode with a constant aperture lens.

Mechanically compensated fixed aperture zooms have a slight stopping down applied at the wider end of the range. This opens up as the lens is zoomed.

This fits in with the usual slight reduction of resolution at the telephoto end of a zoom. At the long end the lens is literally wide open, but at the wide end it is slightly stopped down to maintain the same f number.

Some zooms are optically compensated, but I think this is expensive and quite rare.

Thanks for the insight everyone, that's given me plenty to think about and read up on.