A standard house key works a pin tumbler cylinder. Inside the cylinder, a row of spring-loaded pin stacks holds the plug so it cannot turn. Each cut along the blade lifts one stack to a set height, and when the split in each stack aligns exactly with the shear line (the seam between the plug and the housing), the plug turns and drives the bolt. Common residential keys carry five or six cuts at fixed spacing, and each cut is ground to one of a short list of set depths.

That last detail is where the risk comes from. Because every depth is drawn from a fixed list, the pattern of cuts, called the bitting, reads as a plain sequence of numbers. Anything that shows the cut edge of the blade clearly enough also shows that sequence. The code is on the outside of the key, in view every time the key leaves your pocket. Put that sequence together with the brand and profile of the key, and a person holding a matching blank has what they need to cut a working copy without ever touching your original.

The first factor is the kind of key you carry, because it decides how easy the next step is for anyone holding a photo.

A restricted system comes with paperwork from the locksmith who set it up, and that registration is what proves it, whatever is stamped on the bow. If you are unsure which you have, a locksmith can tell from the key and the cylinder.

The second factor is the image itself. The more of the cut edge that is visible and sharp, the fewer cuts a copier is left to guess. A picture where fingers, another key or the ring itself covers part of the blade leaves gaps in the sequence. A shot where the head of the key is clear adds the brand and profile, which narrows down the blank.

Do not lean on poor image quality as protection. An ordinary phone camera can capture enough detail to read the depth of each cut, and a photo that looks casual to you can still hold every cut at a usable resolution when enlarged. The useful question is narrower: was the blade in clear view, side on, and in focus? If the answer is yes, treat the key as readable.

A copy is only worth something to a person who knows which door it fits. A stranger who photographs a set of keys on a café table has a code and, on its own, no door to try it on. The risk climbs when the picture and the address travel together.

If the key cannot be tied to your address, the photo is a weak lead. If it can, the photo deserves the same weight as a missing key.

Put the factors together, and the call becomes clearer. The table below sets out how they combine.

Rekeying changes the pins inside the existing cylinders, so the cut pattern in the photo no longer matches. Every other key to those doors stops working too, so plan who needs a new copy before the work is done. If the photo is one of several near misses (keys lent out, a housemate who moved on, a tradesperson who held a key for months), it may be the point to move the front and back doors onto a restricted profile, so that future copies have to come through a locksmith who checks who is asking.

A few habits that cost nothing keep your key cuts out of pictures.

Keep the blade out of pictures, know whether your profile is standard or restricted, and rekey when a clear photo and your address fall into the same hands. That puts control of copies back with whoever holds the original.