Okay, so it doesn’t have the same ring as “fast cars and freedom” but the two do go together. Fast lenses give you freedom to shoot in lower light while still maintaining a shutter speed fast enough for your purposes.
How is that? The wider the maximum aperture, the faster the lens. These days, a fast lens is generally one that opens to at least F/2.8 – so that you can use a faster shutter speed and still achieve correct exposure. This allows you to shoot in lower light situations even when hand-holding the camera. A fast lens is a must for taking pictures of people indoors without a flash.
The danger of opening up the aperture so wide is the lack of depth of field. It is important to be aware of your point of focus and how far you are from your subject when shooting wide open.
For example, I like to photograph babies by window light. I don’t want to go under 1/100 for my shutter speed (because babies move unexpectedly). Unless I’m getting A LOT of light from the window while using my lens that opens only to F/4, I cannot get enough light for proper exposure. So, in this situation, I almost always use my 50mm lens that opens at f/1.8. This way, I can use a wider aperture AND a fast shutter speed (a.k.a. fast lens”). It’s also great for getting shots where you want to isolate a subject and blur out the background with the help of a shallow depth-of-field associated with wider apertures.
Another situation fast lenses are great for is shooting sports – especially indoor ones. When photographing a basketball game, the photographer generally is not close enough for his flash to help. Also, the athletes are moving (right?) so he or she needs a fast shutter speed. A lens with a wide enough aperture to achieve proper exposure remedies this situation.
So why isn’t every lens fast? Faster lenses are generally pricier because they usually have higher quality optics (or if you want to sound cool, quality “glass”). The fastest zoom lens (that I’ve seen anyway) opens to F/2.8. It is more costly to manufacture quality zoom lenses, so these can appear pricey, but the value in improved image quality is definitely there.
Fast fixed lenses (a.k.a. prime lens) are less expensive than zoom, but the faster ones can still be expensive. For example, 50mm f/1.8* fixed lenses can usually be found for around $100, but a 50mm f/1.4 fixed lens runs around $400. And for a 50mm f/.2 lens, you could be looking at over $1,700! Is it worth it? That all depends on what you’re using it for and under what conditions. For my portrait work, I’ve never felt that I’ve needed to open up wider than f/1.8, but I know other portrait photographers absolutely love shooting with a 50mm f/1.4 fixed lens. You can always rent one from a local camera shop and see if it meets your needs before sinking your cash into a fast lens.
*A lens is generally described with its focal length followed by its maximum aperture.