Preview is one of those under-recognized apps designed by Apple. It is the default PDF reader for OS X, which many people end up replacing with Adobe Reader.
Preview has a lot of features many people don’t know about. I’ve written before about Instant Alpha and signing PDFs with your real signature. Some new features in Yosemite include the ability to annotate with Loupes and Speech Bubbles.
For this example, I will be editing a photo of a brainless-cat.
Open a photo in Preview and navigate to Annotate > Loupe.
This will insert a Loupe, which will zoom in on an area of the photo. This isn’t just for examining the photo. You can actually save the image with the loupe in the picture.
You can zoom in/out or make the loupe larger using the colored dots. Just slide them around the circle.
As you can see, this cat is very strange and has a large tooth that protrude while he is sleeping. In the screenshot below, I adjust the loupe to zoom into his pink nose.
Another [slightly cheesy] trick is to insert speech bubbles. This is also done through the Annotate menu. Considering this cat is sleeping in Firefox-mode, the annotation seems appropriate.