# Choose or make a lens

> A lens decides what is pulled out of each passage. Use a built-in one, or make your own from it.

A lens says what is pulled out of each passage and what gets compared.

## The built-in lenses

| Lens | Its cards |
|---|---|
| **nonfiction** | Concepts, claims, cases and practices |
| **fiction** | Motifs, characters, places and objects |

A collection names its lens in its note, in the `reader` property.

## Make your own

1. Open **Settings → Clusters → Lenses** and select **Customize…** beside a built-in lens. Or run **Clusters: Customize this lens…**.
2. Clusters shows what it will write, then makes a folder in Model Hub's skills folder.
3. Add your changes to its `reader.yml`.
4. In Model Hub, reload the skills. Your lens then shows under **Yours** on the Lenses tab.
5. Name it in a collection note's `reader` property.

Your lens **extends** the built-in one: it holds only what you change, so you keep the built-in's later improvements.

A lens is a Model Hub skill: a folder with `SKILL.md`, `reader.yml` and a `prompts` folder. Model Hub needs a skills folder set in its settings.

## Check your lenses

Run **Clusters: Check my lenses**, or use the check button on the Lenses tab. It lists any problem in plain words, with the line at fault. Clusters also checks on its own after an update.

A lens in an older format still works, and the check offers to rewrite it in the current one. The rewrite happens only when you select **Rewrite**, and it has no undo.

## What a lens changes later

Changing a lens changes what is sent for each passage, so the next index run reads the affected passages again. The dry run shows how many.

## Related

- [Make a collection](collections.md)
- [Survey a collection](survey.md)