No. The focal length engraved on the barrel is a property of the lens and does not change when the lens is mounted. What changes is how much of the image the sensor sees, and the equivalent figure often quoted is a multiplication rather than a measurement.
The millimetres on a barrel describe the optic. They are the same number in the box, on the body, and on a body of any other format.
What the sensor changes
The lens projects a circle of light; the sensor takes a rectangle out of the middle of it. A smaller sensor takes a smaller rectangle, so the field of view narrows — the picture looks as though it were taken with a longer lens, while the lens itself is unchanged.
Where the equivalence comes from
It is a division. Take the width of a 36 mm frame, divide by the width of the sensor in question, and multiply the engraved focal length by the result. Twelve titles in this catalogue print APS-C and seven print the full-frame equivalent; the exact widths behind those names are published per maker rather than by any organisation, which is why the ratio differs in its second decimal place between systems.
How this site writes it
The engraved focal length first, because that is what the maker cut and printed. Where an equivalent figure is useful, it is named as a calculation and the sensor format it assumes is stated. A page that prints only the equivalent has removed the one figure that belongs to 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 statement repeated rather than assumed: an equivalent focal length is arithmetic, not a measurement. The millimetres engraved on a barrel describe the optic itself and never change; what changes is how much of the projected circle a given sensor takes out of it. An equivalence is that ratio applied to the engraved figure, and it appears here as a calculation with its format named, never in place of the number on the lens.
What people ask before they commit
So why does everyone quote an equivalent focal length?
Because it lets two systems be compared in one number, using 35 mm film as the shared reference. It is useful shorthand and it is arithmetic every time: the engraved figure multiplied by the ratio of frame widths. Written as though it were a measurement, it becomes a claim the lens never made.
Does the aperture change too?
The f-number does not: it is a ratio of focal length to entrance pupil, and neither changes on mounting. Depth of field at the same framing does change with frame size, because framing from a different distance changes the geometry — a consequence, not a property the format publishes.
Last reviewed 12 September 2026