If you’re talking about Window > Screen Mode > Full Screen (F) in Lightroom 5, you can control how an image fits that full screen mode. It’s based on the same magnifications you see at the top of the Navigator panel in Loupe or Develop view, particularly Fit and Fill.
When the proportions of an image are different than the proportions of the display:
- If the magnification is set to Fit, the image is shown completely, but with black borders to fill in the non-matching areas.
- If the magnification is set to Fill, the image is enlarged to fill the display completely, but the sides that don’t match the display proportions are hidden outside the edges of the display.
That magnification setting in the Navigator panel is maintained as you enter Lightroom Full Screen mode. Whatever it is — Fit, Fill, 1:1, zoomed in, or zoomed out — is how it enters Full Screen mode. So if you want to see an image in Full Screen mode with no black borders and non-matching areas hidden outside the display edges, set the magnification to Fill on top of the Navigator panel.