Appertures changing on zoom lens


Hello everyone,  I'm puzzled by zoom lenses ithat seem to decrease aperture size as the focal kengrh increases. If I buy, say, a 20-35 f.2.8, surely I want the aperture to stay constant as i zoom in and out? Can soneone explain what, if any advantages this confers please.

And how do I know what rhe appeture is as i zoom in and out?

My aim isn't to be 'obnoxiously pedantic'.

As you want to teach it, the aperture gets bigger as it gets smaller. So an aperture of 2.8 is bigger than one of 16.

A constant aperture lens has an aperture that changes.

An aperture size of 6mm is the same as an aperture size of 18mm and bigger than one of 9mm.

That's a whole load of contradictions that someone has to 'understand'. Just use the terminology correctly, they go away.

It's OK for you and others to confuse your terminology and your concepts for yourself, but when helping beginners it behoves you to get it right. If someone is a beginner, they aren't saddled with the sloppy terminology that you use, and there is no reason that they have to be.

Otherwise, they get entrenched in a web of confusion of which the OP's query is quite typical.

Manufacturers, I'm afraid, are more interested in marketing slogans than informing people. As I said, when people say 'aperture' they tend to mean 'f-number', and it should be written f/2.8, not 'F2.8'. So, the f-number is the aperture expressed relative to the lens focal length, and the main reason that we do this is that an f-number of, say, 2.8 will indeed let the same light in at every focal length. In your 11-16mm lens, at 11mm, f/2.8 the aperture is 3.9mm. The lens sees a much bigger scene at 11mm than it does at 16mm, so if we kept the aperture the same size, collecting light from a narrower angle, the lens would collect less light. To keep it collecting the same light we adjust it to 5.7mm, which is f/2.8 when 'f' = 16mm.

Hope that helps. Your confusion is quite natural, given the way manufacturers and some here mix and match terms to their own advantage.

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