It is two apertures, not one: f/3.5 is the widest the lens opens at the short end of its zoom range, and f/6.3 the widest it opens at the long end. The second figure is the one that decides what the lens can still do as the light goes.
The pair is printed because the lens physically cannot hold the wider opening as it zooms, and makers print both ends because both are true.
What each end actually governs
The f-number is the ratio of the focal length to the diameter of the entrance pupil. As a zoom lens extends, the focal length grows faster than the opening does, so the ratio rises and the lens gets dimmer. f/3.5 belongs to the short end of the range; f/6.3 to the long end, and nothing in between is published.
That matters most in poor light, where the difference between f/3.5 and f/6.3 is a little over two and a half stops — the shutter speed has to fall, or the sensitivity has to rise, to compensate.
Where the figure is quoted alone
A listing that gives only f/3.5 is quoting the wide end. Where this catalogue’s titles carry a pair, both ends appear: 18-150mm F3.5-6.3, 75-300mm F4-5.6, 100-400mm F5.6-8. Where one figure appears beside a single focal length — 50mm F1.8 — the lens does not zoom, and the single number is the whole of it.
What the aperture pair says nothing about
Nothing about sharpness, which is a separate matter and rarely published in a form two lenses can be compared on. Nothing about stabilisation, which is its own claim. And nothing about depth of field on its own: that follows from the aperture together with the focal length, the subject distance and the size of the frame behind the lens.
Where this page and the maker’s own manual disagree, the manual decides. Every focal length, aperture, guide number, stop of stabilisation, load rating and sealing claim on this page is reported from the manufacturer’s own documentation, with its unit written as published, and named as such — none of it is measured here. An engraved figure is reported as the maker engraves it, and is never restated in another convention, because the two are not the same number. And the second statement repeated rather than assumed: a zoom quoted f/3.5-6.3 has two maximum apertures, not one. The first belongs to the short end of the range and the second to the long end, and the value moves between them as the barrel extends. Both ends are written here every time, because the second is the one that decides what the lens can still do as the light goes.
What people ask before they commit
Which of the two figures should I compare between lenses?
The long end, because that is where a variable-aperture zoom is at its dimmest and where two lenses differ most. Comparing the wide ends of two zooms compares them at their most flattering, and a lens that opens to f/3.5 at 18 mm tells you nothing about what it does at 150 mm.
What is a constant aperture lens?
One that holds the same maximum aperture across its whole zoom range, published as a single figure — f/2.8 rather than f/3.5-6.3. It is a different design with a different price, and the single figure is the honest shorthand for it.
Last reviewed 12 September 2026