Lenses before bodies: the advice everyone gives, argued properly

The advice is usually delivered as a slogan, which is why it gets ignored. Stated properly it is narrower and more useful: for a given total budget, moving money from the body to the lens buys a larger visible change in the photograph, up to the point where the body stops being able to focus on your subject.

That version can be argued with, which is the point. It has a mechanism — the two components control different things — and it has a stated limit. The rest of this guide is the mechanism, the arithmetic and the limit.

Five properties of a photograph are decided by the glass in front of the sensor. No firmware update and no new body touches any of them.

• Field of view. How much of the world lands on the sensor. The lenses here span 16 mm to 914 mm in full-frame-equivalent terms, which is the difference between a whole room and a bird at the far end of a lake.
• Depth of field. How much is sharp in front of and behind the subject. It is set by aperture, focal length and distance — two lens properties and one you choose with your feet.
• Maximum aperture. How much light gets to the sensor at all. The range here is f/1.2 to f/6.3, which is roughly four and three-quarter stops. That is the difference between a handheld shot indoors and no shot.
• Close focus. How near you can get before the lens refuses. This is the whole of macro, and it is also what decides whether you can photograph a coffee cup across a table.
• Character. Flare, the shape of out-of-focus highlights, distortion, how colour falls away at the edges. This is the part reviews argue about and the part you recognise in other people's photographs without knowing why.

Every one of those is a physical property of a piece of glass. Put a lens on a twelve-year-old body and it delivers all five unchanged. That is the asymmetry the rule rests on, and reading a lens name tells you which of the five you are buying.

The other side, stated just as plainly, because a rule you can only defend by ignoring the counter-evidence is not worth having.

• Autofocus. Whether the camera finds the eye, and whether it holds on to it when the subject turns away and comes back. This is the single biggest difference between a body from 2016 and one from today.
• Burst and buffer. The catalogue spans 3 to 40 frames per second. A lens has no opinion on this at all.
• Video. Resolution, frame rates, codecs, recording limits, how badly the frame skews when you pan. Almost entirely a sensor and processor question.
• Resolution. 9 to 102 MP here. It sets how hard you can crop and how large you can print, and nothing else worth arguing about.
• The low-light ceiling. Sensor size and generation decide where noise stops being texture and starts being mush.
• The things you touch. Viewfinder quality, weather sealing, battery life, whether the buttons fall where your hand is. Unglamorous, and the reason people keep a body they should have replaced.

Take a common case. A kit zoom is typically f/3.5 at the wide end and f/5.6 at the long end. A €250 fifty is f/1.8 all the way through. From f/5.6 to f/1.8 is about three and a third stops; from f/3.5 it is about two. Three and a third stops is 1/30 s becoming 1/250 s, or ISO 6400 becoming ISO 640 — the margin between a blurred indoor frame and a clean one.

The same change transforms depth of field. At the same distance and focal length, f/1.8 gives roughly a third of the depth of field of f/5.6, which is how a background stops competing with a face. Note that the two scales differ: that same change is about ten times the light but only about three times less depth of field, because light goes with the area of the opening and depth of field goes with its width. No sensor does that. A newer body, by comparison, buys perhaps a stop of high-ISO cleanliness and much better focus — real, but a stop against three and a third.

The prices make the comparison starker. The cheap fast primes here are €220 to €350, against a lens median of €950 and a body median of €1,800. Nothing else in either catalogue changes this much for this little, which is the practical core of the argument set out in what to spend .

1. 1

   Pull up your last two hundred photographs

   Not the best ones. All of them, including the ones you deleted in your head. This is evidence, not a portfolio.

2. 2

   Sort the failures into two piles

   Pile one: out of focus, missed the moment, subject moved. Pile two: too dark, too cluttered, could not get close enough, could not get wide enough, background too busy.

3. 3

   Count them

   Pile one is a body problem. Pile two is a lens problem. Most people find pile two is three or four times larger, which is where the rule comes from.

4. 4

   Check which focal length you actually shoot

   Your editing software can chart it from the metadata. If eighty per cent of your keepers sit at one end of the zoom, that is the prime to buy, and you already know what you will do with it.

5. 5

   Buy the one lens, not the system

   One purchase, then six months of shooting. The lens finder will narrow the 65 here by mount, role and budget in a couple of minutes.

Buying glass first means committing to a mount before you have committed to a camera, and the mount is the longer commitment of the two. In this catalogue, the bodies split as Sony E 12, Nikon Z 9, Canon RF 8, Fujifilm X 7, Micro Four Thirds 7, L-mount 4 and 11 fixed-lens cameras, with single entries for Pentax K, Fujifilm GFX and Hasselblad XCD. The lenses split similarly: Sony E 15, Canon RF 13, Nikon Z 11, Fujifilm X 10, Micro Four Thirds 9 and L-mount 7.

Take that seriously, but not fearfully. Two things soften it. Adapters let most modern mirrorless bodies take the same maker's older SLR lenses with autofocus intact, so an existing collection is rarely stranded. And good glass holds its value, so switching mounts costs you the spread between selling and buying rather than the whole sum.

What is genuinely hard to undo is a mount with a thin catalogue in the role you care about. Before you buy anything, list the two or three lenses you expect to own in three years and check they exist in that mount at a price you would pay. If they do not, no body in that system is the right purchase.

The rule has one clean exception and two smaller ones. The clean exception: when autofocus is the thing stopping you. If your failures are missed focus on a moving subject, a faster lens makes the problem worse, because it narrows depth of field and gives the focus system less to work with. That is a body purchase, and autofocus and burst explains which specifications are doing the work.

• Video. If you need a specific codec, frame rate or recording time, no lens supplies it. Buy the body that has it.
• Reliability and sealing. A camera that fails in rain, or whose shutter is at the end of its life, is not a candidate for optimisation. Replace it.
• A genuinely obsolete body. No viewfinder, no stabilisation, contrast-only autofocus: at that point the body is the bottleneck for everything, including the new lens.

Outside those, the order holds. If you are still deciding what kind of camera you need in the first place, start with how to choose a camera and come back to the spending question afterwards.