Especially if you're new to photography, you might not know what to expect from different lenses. This online tool lets you compare lenses according to actual pictures photographers have taken with that lens.
To use Lens Vs. Lens, you select two specific camera lenses, and the site uses photos pulled from Flickr to show you what photos have been taken with that lens. Beyond that, though, you can adjust the aperture and focal length, and the tool will refine its search to look for photos accordingly. This way, you can compare specifics between two lenses to see how they perform at different settings. To test the tool out for yourself, head to the link below.