OSLO provides many ways of identifying surfaces beyond direct specification. These methods fall into three groups: pickups, solves, and variables.

Pickups pick up parameters from another surface and apply them to the current surface. This is useful, for example, in designing a system with a mirror that reflects the light back through some part of the system. The surfaces in the path back from the mirror are the same physical surfaces as in the path toward the mirror. By specifying the radii of the surfaces in the path back as pickups from those in the forward path, you guarantee they will always be identical.

Solves tell the program to calculate the value of a parameter that satisfies a given condition. For example, specifying the thickness of the next-to-last surface of a lens by an axial ray height solve with height zero tells the program to calculate the thickness of this surface so that the axial ray will pass through the vertex of the image surface. This is the same as saying the image surface remains at the paraxial focus. As other surfaces change, OSLO changes the thickness of the next-to-last surface to satisfy the condition.

The Variable specification (which is only applicable for directly specified items) tells OSLO it may vary the item during optimization. In addition to allowing the free variation of surface data during optimization, variables can be constrained (in OSLO PRO and OSLO SIX) to targeted values using Lagrange multipliers. This allows you to construct elaborate constraints that go beyond the solves and pickups enabled as basic data specifications.

The various ways to specify lens data rely on the fact that whenever lens data is changed, OSLO retraces the axial and chief ray through the system, resolving all pickup and solve requests. The routine that does this is called lens setup. It is an essential part of an interactive program.