One stop of light, which halves the shutter speed you need, and a shallower depth of field at the same framing. What it also buys is weight and price, and those two are published while the optical difference mostly is not.
A stop is a doubling, and this one is the difference between two lenses that otherwise look alike on a listing.
What the stop buys
Shutter speed. At f/2.8 the same scene needs half the exposure time it needs at f/4, which is the difference between a sharp frame and a blurred one at the edge of hand-holding.
Depth of field. At the same framing and distance, the wider aperture renders less of the scene in focus — useful for separating a subject, a nuisance when two people stand at slightly different distances.
What it costs, in published figures
Weight and price, both stated. A constant-aperture zoom quoted f/2.8 across its range is a larger object than one quoted f/3.5-6.3, and the filter thread usually grows with it — this catalogue’s four listings that print a diameter carry three different ones, and the wider lens is rarely the smaller number.
What is not published either way
Sharpness at each aperture, which is where most of the real difference between two lenses lives. It is not stated in a form two listings can be compared on, which is why this site compares what is printed — the two aperture figures, the focal length, the thread, the mount — and says when a claim has no figure behind it.
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
Is one stop worth the difference in price?
It is when the stop is the constraint you keep meeting — indoors, at dusk, or with a moving subject. It is not when your pictures are made in daylight, where both lenses stop down to the same apertures anyway. The honest test is which figure your current lens runs out of first.
Does the wider lens focus better in low light?
Usually yes, and not because of the autofocus system: the focus mechanism reads the scene with the lens wide open, so a wider maximum aperture delivers more light for it to work on. That is a side effect of the aperture rather than a separate published feature.
Last reviewed 12 September 2026