I listen to a lot of photography pod casts and read a lot blog posts and a frequent question that comes up centers around what we call "fast glass". The glass in this case is just a generic reference to our lenses but what exactly does "fast" mean and why should we care about? In most cases, photographers who primarily shoot portraiture and weddings will tell you that these fast lenses (lenses with a maximum aperture of 2.8 or more) are most valuable in isolating the subject from a distracting background by creating that nice blurred background effect, something we call "bokeh".
But I get a bit ahead of myself – first, you might be wondering what I mean by having an aperture of "2.8 or more" (I know it confused me in the early days). The aperture is always stated in "f-stops" and the number itself, simply stated, represents a ratio of the lens's focal length vs. the diameter of the opening where the light comes in (called the "entrance pupil"). Now, you don't need to get deep into the technical side of how aperture works in order to understand it enough to control your images so we won't get into a deep technical discussion here. Instead we'll leave off saying that most of what you need to know is that the smaller the f-stop number, the larger the opening is and the more light you’re letting into your camera; f/2.8 is a larger opening than f/4 and f/1.8 is larger than f/2.8.
Also, I want to make sure you understand that a lens rated at f/2.8 (or f/4 or??) isn't "stuck" at that f-stop; the f/# rating of a lens is just the maximum aperture, the maximum amount of light it can let it. In almost all cases you can adjust the aperture to any value between this maximum and the lens’s minimum aperture (usually around f/22 but does vary). In fact, it's not uncommon for zoom lenses (lenses with variable focal lengths) to have a variable maximum aperture. With these lenses you'll find their f-stop rating expressed something like f/3.5-5.6, which means that at the lens's widest field of view (shortest focal length – e.g. 18mm) the maximum aperture is f/3.5 but at the lens's longest focal length (e.g. 200mm) the maximum aperture changes to just f/5.6.
Ok, so what about this "fast glass" stuff – what exactly is "fast"? Well, like I said in the beginning, a fast lens is generally thought of as any lens with an aperture of f/2.8 or larger – f/2.8, f/2.0, f/1.8, f/1.4, etc. Since I've also said that an f/2.8 lens doesn't need to always be left at f/2.8 (you can happily shoot at f/8 or f/16 if you like), why not always get the fastest lens you can? Well, there are a couple reasons; fast lenses tend to be both more expensive and heavier than their slower counterparts. Every time that f/# goes up or down, it's a halving or doubling of the amount of light coming in and so an f/2.8 lens can let it twice the light of an f/4 lens, which means the lens needs to have a larger diameter to hold the larger/heavier glass lens elements. This often also results in the lens body going from lightweight materials like plastic to metal in order to help with the structural integrity of the lens. Of course all this adds to the price, making that f/2.8 lens 30-40% more expensive than its f/4 cousin.
So, now you start thinking to yourself, "hmm… mostly I'm going to be shooting with reasonably bright light and don't really need anything faster than f/4 anyway. After all, if most of what I shoot is going to be at f/4 – f/8, why in the world pay extra for that expensive f/2.8 beast-of-a-lens?". Great question, glad you asked, and really the major theme of this post.
In most posts I've read and podcasts I've listened to, the guidance would be "you don't" [need the fast lens]. If you’re mostly going to shoot with an aperture at f/4 or smaller (typical of landscape shooters) then you really don’t need a fast lens, right? Yeah, well, you've probably guessed by now that I have a different take on this, and you'd be right. You see, the f-stop that you set your camera to when you take the shot is only activated at the point when the shutter is opened. The rest of the time the aperture is wide open, as open as it can go, in order to make it easier for you to view and frame your scene. Even more important, this "always open" aperture behavior allows your focus system to get the most light possible in order to quickly lock focus. In fact in some camera systems (Canon comes to mind), the focus system includes special high-precision focus sensors that ONLY come into play if you’re using f/2.8 or faster lenses!
Think about that – if you only bought lenses "slower" than f/2.8 you'd never be able to take advantage of those features and they're exactly the sort of features that would most benefit someone shooting a fast-paced soccer game on a bright day…the same shooter who probably doesn't think they need a fast lens because they're almost always between f/8 and f/11, and never want to go below f/4! In fact, they really should be investing in that fast lens in order to help their cameras perform to their full potential, and maybe lock into that critical moment when a slower lens might have been struggling to track the action.
There are a lot of other great things to talk about with lenses and f/stops, with variable apertures vs. fixed apertures but I'll save that for another time. The main point here is that you don’t want fast lenses just to get a nice blurry background behind your subject. Fast lenses can also unlock potential in your camera that can make or break a shot and if the types of shooting you do relies on quick focusing or accurately tracking moving subjects, you'll find fast glass to be exactly what you should be putting on your wish list and in your bag.