Here’s a good question pondered by many new photographers: how exactly can a lens have a "speed"? Those of us in the know—or at least with a little more experience–know the answer to this question likely because, at some point early on in our photographic adventures, we heard someone refer to a "fast" lens and we said something profound along the lines of, "Wait. What?" And then they told us how lenses can have a particular speed. So if you’ve heard of lenses referred to as fast or slow, or if the idea of speed in general doesn’t make sense to you when it comes to lenses… Well, read this post on the Pixiq photo blog. It explains how a lens has speed based on its ability to use fast shutter speeds. A wide aperture, which allows for a fast shutter speed, is a "fast" lens. And one thing the Pixiq piece doesn’t mention is that speed is relative. An f/4 lens might be fast if it’s a telephoto, but relatively slow for a wide angle. The same goes for different camera formats, too.