How do constant aperture lenses work?

Question

Cheaper zoom lenses usually are faster at the wide end and slower at the long end (for example, the $150 Canon EF-S 18-55mm f/3.5-5.6). More expensive constant-aperture zoom lenses have the same aperture regardless (for example, $800 Canon EF 17-40mm f/4.0 L).

My question is: are these good lenses sandbagging at the wider settings, or do they have a different optic system that allows them to maintain the same aperture throughout the zoom range?

Answer

They have different optics and are usually substantially bigger lenses for the same focal range (compare a 70-200mm f/2.8 to a 70-300mm f/4.5-5.6 and see that the latter is small in comparison). To get the constant aperture at the long end, you need to have a bigger barrel because the aperture is a ratio versus the focal length. However, if you do the math for your examples:

18mm f/3.5 means a 5.14mm opening 55mm f/5.6 means a 9.82mm opening

17mm f/4.0 means a 4.25mm opening 40mm f/4.0 means a 10mm opening

It's clear that the aperture's physical diameter can be bigger in both cases. So, in either case, you would theorize that at the widest end you should be able to be f/2.0 or thereabouts and your sandbagging scenario would then apply to both. On the other hand, for the latter, the optics may be simplified and thus approaching prime quality in result. So... Tradeoffs.

In any case, zooms have pretty complex construction involved, much more so than a prime lens ever would, and so there are a lot of considerations around optical correction at various focal lengths, the effect of the aperture on that correction, and so on. It may be, given the lens design and costs associated, that attempting get wider on the short end would result in a hugely unacceptable softness in the image or some other forms of abberation.

Finally, for certain there are different optical constructions between the two. Heck, there's different optical constructions between lenses of the same configuration but different manufacturers. It all comes down to cost versus benefit and, in the end, what price the market will bear for a lens of a given construction.

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