If you use Apple’s Preview program regularly to view images and PDF documents on your Mac, then you might find yourself opening some documents where you might need to zoom in on some details. There are several ways to do this in Preview using its standard Zoom function, but in addition you can use the Loupe to keep your document mainly at one zoom level, and only increase the zoom of the area under the loupe.
To activate the Loupe, press Control-Command-L, or choose Loupe from the Tools > Annotate menu. When done, you will see the circular loupe on your document, which will remain with the window as you move it around and scroll up and down. You can also click and drag the loupe to various parts of the Preview window to focus it on a particular aspect of your document.
While a single loupe may be enough for most purposes, you can create and place multiple loupes, if needed. The loupe is basically an annotation object, just like any arrow, box, or speech bubble. As such, you can simply hold the Option key while dragging the loupe, to pull a duplicate off of the original loupe to a destination of your choice. You can also press Control-Command-L again to add another loupe of the default size and zoom level.
As with other annotation objects, you can hold the shift key and click your loupes to add them to a selection, and delete them individually or in selection.
Loupes are good for having a specific area zoomed, but there are two other options for viewing an area of a document in Preview that might be too small. These include the standard zoom level, which you can toggle by clicking the plus and minus toolbar options, or by pinch-zooming on your document. In addition, you can tap the grave accent button (the same as the tilde key on the standard English keyboard layout), which will activate the magnifier that will follow your mouse (turn this feature off by tapping the Escape key).
This is under Tools/Magnifier in 10.9. You should have commented on that…
I think that is the “magnifier” tool that Topher mentions in the very last sentence. Like the loupe, it is round for image files (but not for PDFs!), but you can only have one active at the time and in general behaves differently than what Topher describes for Yosemite’s loupe.
Yes, there are some things we are missing for refusing to upgrade…