Understanding lens specs: how to read a lens name

Every lens name contains a focal length in millimetres and a maximum aperture as an f-number. Everything else in the name is a qualifier on those two.

Focal length sets the angle of view. Short means wide and takes in a lot; long means narrow and magnifies. One number — 50mm — means the lens is a prime and does not zoom. Two numbers separated by a dash — 18–50mm — mean a zoom, wide end first. On a full-frame sensor, roughly 50 mm gives an angle of view close to what feels natural to a person, which is why it is the reference point for everything else.

The f-number is the focal length divided by the width of the aperture as seen through the front of the lens — the entrance pupil, which the glass in front of the iris magnifies, not the iris itself. Smaller f-number, wider opening, more light. It is written as the maximum the lens can open to, because that is the limit you are buying. When a zoom shows two f-numbers — f/3.5–5.6 — the aperture closes as you zoom in: f/3.5 is available at the wide end only, and by the long end you have lost a stop and a half. A single f-number on a zoom means it stays constant across the range, which costs money, glass and weight.

Here is a lens from this catalogue, taken apart. It is an APS-C standard zoom at €450, which makes it one of the cheaper ways into a constant aperture.

Other makers use their own codes for the same four ideas — full-frame or crop coverage, mirrorless or SLR, stabilised or not, and a premium line badge — but the structure never changes. Find the focal length, find the aperture, then work out what the letters claim.

This catalogue has 28 primes with a median of €700 and 37 zooms with a median of €1,300. That gap is the whole trade in one line: a zoom asks you to pay for versatility in money, weight and aperture.

• A prime is simpler, so for the same money it opens wider, weighs less and usually renders better at its widest aperture. The catalogue floor of f/1.2 is prime territory; so are the €220 to €280 fifties.
• A zoom covers framing you cannot walk to. On a street, at a wedding, from a riverbank, the ability to go from 24 mm to 70 mm without changing lenses is worth real money.
• A prime makes you move, which is not a mystical benefit but a practical one: you learn one field of view properly and stop fiddling with framing at the moment something happens.
• A constant-aperture zoom is the expensive middle ground. It keeps its widest aperture across the range, which is why the heaviest lenses here — up to 2,115 g — are mostly telephoto zooms.

For a first purchase alongside a kit zoom, the prime wins on cost per unit of change, which is the argument made at length in lenses before bodies .

The f-number does two jobs at once, and most confusion about lenses comes from conflating them.

The first job is light. Each full stop halves or doubles it, and the standard series runs f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16 — each step is a factor of the square root of two in diameter and a factor of two in light. The range across these 65 lenses, f/1.2 to f/6.3, is about four and three-quarter stops. At the same shutter speed that is the difference between ISO 400 and ISO 10,000.

The second job is depth of field. Wider aperture, less of the scene in focus. But aperture is only one of three levers: depth of field also shrinks as the lens gets longer and as you get closer to the subject. A 50 mm at f/2.8 a metre from a face gives a thin slice of sharpness; the same f/2.8 on a wide lens across a room gives almost everything. If a background is not blurring as you expected, the fix is as likely to be moving closer or zooming in as opening up.

Optical stabilisation in the lens, or in-body stabilisation that moves the sensor, corrects for your hands shaking. Makers rate it in stops: four stops of stabilisation means a shot you would have taken at 1/500 s is plausible at 1/30 s. Treat those ratings as optimistic and useful, not exact.

It matters most on long lenses, because angular shake is magnified along with everything else. The old rule of thumb is to keep the shutter speed at or above one over the focal length — 1/200 s on a 200 mm lens — and stabilisation is what lets you break it.

• It does nothing about a moving subject. A running child at 1/30 s is blurred regardless, because the blur is theirs, not yours. Only shutter speed fixes that, and holding focus on them is a question about the body rather than the lens.
• It does nothing on a solid tripod, and on some older lenses it makes things fractionally worse by hunting for movement that is not there. Switch it off when the camera is locked down.
• It does nothing once the shutter speed is already short. At 1/2000 s there is nothing left to correct.
• It does not improve optical quality. A stabilised lens is not a sharper lens; it is a lens you can hold still for longer.

Two numbers describe how near you can work. Minimum focus distance is measured from the sensor plane, not from the front of the lens, which is why a stated 25 cm can leave only a few centimetres of actual working room on a long barrel. Maximum magnification is the honest one: it says how large the subject lands on the sensor compared with life size.

A true macro lens reaches 1:1, meaning a ten-millimetre insect covers ten millimetres of sensor. The word itself is not regulated, so a zoom badged "macro" frequently manages 1:3 or 1:4 — useful for flowers, nowhere near an insect's eye. Check the ratio, not the badge. This catalogue lists 6 lenses in the macro role, against 18 standard, 13 telephoto, 10 portrait, 9 all-round and 9 wide.

A focal length is a property of the lens and never changes. What changes is how much of the projected image the sensor keeps. A smaller sensor keeps a smaller piece, so the same lens gives a narrower field of view. Multiply the focal length by the crop factor — about 1.5 for most APS-C, 1.6 for Canon's, 2.0 for Micro Four Thirds — and you get the full-frame lens that would frame the same shot. That is why this catalogue quotes reach from 16 mm to 914 mm as full-frame equivalents: it is the only way to compare across six mounts.

The aperture needs the same treatment, and this is where most published comparisons go wrong. For exposure, f/1.8 is f/1.8 on any sensor: the same shutter speed and ISO give the same brightness. But for depth of field and for the total light the sensor collects, multiply the f-number by the same crop factor. The OM System 45mm f/1.8 at €280 frames like a 90 mm on full frame, and at the same framing its depth of field matches a full-frame 90 mm at f/3.6.

Neither half of that is a criticism of small sensors. More depth of field is an advantage for landscapes, for close-up work and for anything where focus accuracy is the problem. It only matters that you compare like with like — otherwise a smaller, cheaper, lighter lens looks as though it is giving you something it is not. How much to spend works through what that equivalence is worth in euro, and how to choose a camera covers which side of it your photographs sit on.