The Honest Sentence Needs Two Lenses, Not One

Panasonic put two lenses on a test bench this week and most readers will clock the wrong difference between them. The Lumix S 20mm f/2.5 is a prime: one focal length, one aperture, cut and finished to do exactly that job and apparently doing it well enough to earn ePHOTOzine's Highly Recommended. The Lumix S 18-40mm f/4.5-6.3 standing next to it looks, by the reviewer's own account, virtually identical — same compact barrel, same quiet metal ring — until you read the number after the slash.

A fixed f/2.5 means the lens lets in the same amount of light at every focal length, because there is only one. A variable f/4.5-6.3 means the opposite: wide open at 18mm the lens gathers a stop and a bit more light than it does zoomed to 40mm, because the glass doing the zooming has to get smaller relative to the aperture as the barrel extends. That is not a flaw bolted on to save weight. It is the oldest trade in lens design — one ring of glass that does many jobs will always be slower than one ring that does only one — and it is the trade every compact travel zoom has made since the format existed.

What a number like f/4.5-6.3 buys you is not a worse picture. It is a different decision at the moment you pick up the camera: one lens for a fast indoor corner, a wide window of range for whatever the next hour throws at you outdoors. The 20mm prime will outrun the zoom in a dim room by more than a stop at its own focal length and nowhere else. The 18-40mm will follow you from a doorway to the far end of a street without a lens change. Neither number is the honest one on its own; the honest sentence has both lenses in it.

I have not had either lens in my hands, and until I do I am not going to tell you which one earns a place in a bag over the other — that call belongs to whoever is actually carrying it up a hill. But a company putting a slow, variable zoom on the shelf next to a lens it already let reviewers call Highly Recommended is not hiding anything. It is betting that enough of us can read a fraction and decide for ourselves what we are willing to trade for it.

On the bench the two lenses sit less than three centimetres apart, almost the same black cylinder twice, and the only thing that actually separates them is four characters stamped near the mount that most people will never crouch down to read.