Five marks, read in this order, tell you what a lens is.
The focal length, which is a property of the lens
Engraved in millimetres, and unchanged by the body it is mounted on. A smaller sensor takes a narrower rectangle out of the same projection, so framing tightens — but the number on the barrel is the number of the lens. An equivalent figure is that number multiplied by a ratio of frame widths: arithmetic, performed for comparison, and named as such on this site.
The aperture, which is one figure or two
One figure means a fixed focal length. Two means a zoom whose maximum aperture falls as it extends, and the second is the one that governs low light. A constant-aperture zoom publishes a single figure across its range and costs and weighs more for it.
The mount, which ties you to a system
Proprietary, published by the camera maker, and shared across that maker’s own formats. A suffix marking a lens as built for the smaller format — EF-S in six titles here, RF-S in three — is the statement that its image circle is the smaller one. Without a suffix, the lens covers the larger frame.
The filter thread, which belongs to this lens alone
Printed beside a ⌀ symbol. Four titles here name one, carrying three different diameters. Two lenses on the same body routinely differ, and the figure to match is on the lens.
The minimum focus distance, measured from the sensor
Not from the front of the lens. Subtract the camera and the lens and what remains is your working space, which is why two lenses quoting the same figure can leave very different room in front of the glass. A magnification ratio — 1:1 meaning life size — is the separate figure that distinguishes a macro lens from a lens that merely focuses close.
The three things it never tells you
Sharpness at each aperture, how it renders what is out of focus, and how it behaves against the light. None is published in a form two lenses can be compared on, which is why this site compares what is engraved and says plainly when a claim has no figure behind it.
What people ask before they commit
Does a longer zoom range mean a better lens?
It means a wider compromise. The extreme case in this catalogue is a bridge camera printing 20-1200 mm with f/2.8-5.9 on a single fixed lens: sixty times the reach, and nearly three stops darker at the long end. Range and aperture are traded against each other in every design.
What is the difference between a prime and a zoom at the same focal length?
Usually the maximum aperture and the weight. A prime holds one focal length and buys light with the space saved; a zoom covers a range and pays for it in aperture. Sharpness is the property people assume distinguishes them, and it is the one no listing publishes comparably.
Last reviewed 13 September 2026