maximum aperture size in zoom lenses

[photosbybrian]

Bob, Based on my understanding that the f-stop number of a lense is basically the focal length divided by the maximum physical aperture opening size, how come a zoom, such as the 70-200 2.8L can have a constant f 2.8? It seems to me that at 200mm /2.8, the aperture size would be 71.4mm or thereabouts. Well if this lense has a maximum aperture of 72mm, why would it not be an f/1.0 at 70mm focal length?
I'm sure there are technical details to which I am not priviledged as yet to know, but as Buffet might say (Jimmy, not Warren) "He went to Atkins, lookin' for answers, to questions that bothered him so..."

Thanks,

Brian

[Bob Atkins]

In a zoom like a 70-200/2.8 the whole of the front element isn't used at 70mm (or other intermediate focal lengths). The effective aperture is controlled by the internal zooming optics.

The reason is probably to maintain image quality and/or to maintain a constant aperture, which is something photographers used to want since before the days of auto exposure it meant you didn't have to change exposure settings when you zoomed.

Of course there are many lenses which change aperture, such as the 70-300/4-5.6. In theory I suppose you could make it a 70-300/1.4-5.6 since it has something like a 55mm front element. However when you consider the cost and complexity of a lens like the 85/1.2L prime, you can get some idea of the difficulty of making such a fast lens. The image quality of the 70-300/1.4-5.6 would likely be awful at 70mm f1.4 (or even 2., especially if the cost was low.

For 35mm lenses, you simply don't see zooms faster then f2.8 because of the complexity (and cost) of the optics that would be needed to make an f2 zoom with high image quality. If you drop the frame size to even smaller than APS-C, such as the Olympus "4/3" format (18 x 13.5 mm), then you can make an f2 zoom with decent optics like the Olympus 35-100/2, though it will cost you $2500! With even smaller formats such as are used on camcorders, even faster zooms are possible.