When I first started to seriously pursue photography, I’d hear people talking about “fast lenses” and how awesome they are and I had no idea what they were talking about. I had yet to dive into the details of f-stops, focal lengths, shutter speeds and ISOs and how they all worked together, so I’d just nod along, thoughtfully scratching my chin in silent agreement. Yes, yes, fast lenses are superb!
Now that I’ve learned a bit more, I’d like to give you a high-level explanation so you have a general understanding of what makes a lens fast.
What is a fast lens?
When you take a look at an SLR lens, there are two main components that describe it: the focal length (for example 50mm or 70-200mm) and the f-stop (also called f-number and described with “f/” in front of the value. For example f/1.8, f/4.0 or even a range like f/3.5-5.6).
This specific f-stop value refers to the maximum aperture (or actual opening) of that lens. While it’s hard to say precisely what makes a lens fast – mainly because there are so many factors in play – in general f/2.8 and wider (f/2, f/1.8, etc) are considered fast. So the higher the f-stop, the slower the lens.
What are the pros of fast lenses?
Fast lenses are excellent tools for low-light situations, sports, fast-moving objects (like kids and pets) and for achieving extremely low depth of field. When traveling, you’re not always in control of your environment or your subjects, so having a fast lens helps you shoot successfully more often. For example you’re less likely to need a flash in those dark European churches or a tripod for that sunset shot in Hawaii.
What are the cons of fast lenses?
Fast lenses are heavier and more costly.
Why do you need a fast lens?
Fast lenses are typically the best lenses you can buy and are usually geared towards professionals so they’re very well built to withstand the elements (wind, dust, etc). And because they have the added ability to shoot in low light, they’re very versatile and you can use them pretty much all the time to produce beautiful results.
You don’t always need a fast lens, and out of all the photography gear I have right now I’d only consider one of my lenses to be super fast. Because during the day, especially, fast lenses aren’t quite as necessary and you can easily get by with ones with maximum apertures of f/4.0 or so.