Quote:
Originally Posted by Larne
The bit that I don't understand is why a lens doesn't have the same maximum aperture diameter (D) throughout its range but that probably has to do with the mechanics of how the lens is built and operates - maybe someone else can enlighten us on that bit.
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Some zoom lenses do just that. For instance, the 70-200 f2.8 is f2.8 throughout the range, as is the 18-55 f2.8. Snag is that for this constant aperture, the physical size of the lens is quite big and they are heavy. The optics that are found in the lenses with variable apertures throughout the range are much smaller and lighter, so these lenses tend to be more compact and lighter.