- Add lens_type as a custom dropdown attribute with all lens types as attribute options/labels. Set 'Use in layered nvigation'to 'Yes'
- Add this new attribute to the default attribute group.
- Reindex database
- Insert the products and select the lens type for each product.
- Add the products to the desired category.
- Set this category's option 'Is anchor' to 'Yes'
Now you have a layered category with a drop down option where you can choose teh lens type.