LR3 – Zooming In and Out

In the Library and Develop modules, set your preferred zoom views to either  Fit or Fill and 1:1 or Custom Zoom by clicking on the preferred option in the Navigator panel’s header. Then, use the Spacebar to zoom between your preferred zoom views (or click with the Zoom tool while in Loupe view).

You can also use the Command (Mac) / Control (Win) + “+” or “-” (plus or minus) toggles through all zoom levels (from Grid to Fit, Fill, 1:1 and Custom zoom).