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Why do zoom lenses and compact cameras have varied maximum aperture across the zoom range?

It is rather common with zooms that the maximum aperture changes (in fact, decreases) with increasing focal lenght. At the same time, it is possible to make zooms with constant aperture (typically and not surprisingly, much more expensive).

What is the technical reason for the decreased aperture? After all it seems naively that the blades could be opened independently of what is happening to the focal length.

Is it to mantain the same level of image quality (distortion, aberrations, vignetting) at each focal length?

Or is there some "more fundamental" reason for this engineering decision, which can be overcome only in more complex/expensive designs?

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Francesco
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  • @barry my fault, I had searched but had missed it! – Francesco Jul 31 '11 at 07:51
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    The search feature on this site needs some work. Having a few duplicates is actually okay — we close them as duplicates but don't delete them, and there's more for the search engine to find. :) – mattdm Jul 31 '11 at 13:05

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The notation f/4.0 is usually taken to mean the size of the opening left by the aperture blades is equal to the focal length divided by 4, so for a 400mm f/4.0 you'd expect to find a 100mm opening. So given the maximum size of the opening is clearly fixed, if your lens has a variable focal length you would expect f ratio to change as you zoom.

This is not quite correct however, f/4.0 only means that the image of the opening as seen through the front of the lens appears to be 100mm - if you look at the barrel of such a lens you'll see there's no space for a 100mm iris. In fact the aperture is smaller but is "magnified" by the lens elements.

So via clever (and expensive) optics its possible to make the aperture appear to get larger as you zoom, giving you a constant f number.

Matt Grum
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  • Excellent point, I had understood that it was the size of the aperture and not the size of the IMAGE of the aperture. I wonder why this distinction is not usually spelled out: I always missed it. I learned a new thing today :) – Francesco Jul 31 '11 at 07:55
  • So @Matt what you're saying here is that mechanically, the aperture does vary even in the zoom lenses that have a fixed max f stop and it only "appears" to be constant on zooming because of the clever optical techniques? – Rish Jul 31 '11 at 10:35
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    @Rish mechanically the aperture (the actual hole in the lens where the blades are) doesn't vary, even in a constant aperture lens (that would be quite a feat to build a lens with stretchable aperture stop) but it does indeed act as if it varies due to the optics. – Matt Grum Jul 31 '11 at 10:51
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    This question has been marked as a duplicate, but I found this answer much easier to understand than the answer on the duplicate question - especially the bit about perceived aperture size. This strikes me as the sort of thing that could be very nicely explained with a video. I just spent some time staring down the lens of my Canon G12 24-140mm equiv and Nikon D80 kit 18-135mm, and I understand much more having read this answer, but I don't have more expensive lenses to hand: thus why I think a demo video could be such an asset. Does anyone know of an existing video, or feel like making one? – M_M Sep 15 '15 at 13:22