Lens Speed: A look at why some lenses are called "fast" lenses and their advantages.
A Lens Aperture Primer, Part III
Fast lenses are the perfect solution to low-light scenes, like this nighttime scene of Times Square.
By using a wide aperture I was able to toss the background behind these bleeding heart flowers totally out of focus. Lens aperture choice is a crucial part of creative photography.
Lens Speed
One term you'll often hear photographers toss
around when they're talking about lenses is lens speed. The speed of a
lens is described by its maximum aperture size. If you have a 100mm
lens with a maximum aperture (largest lens opening) of f/4, for
instance, the speed of the lens is f/4. The larger the maximum aperture
is, the faster the lens is considered to be. This is because the wider
the maximum aperture, the faster light can get to the sensor since more
of it enters though a larger opening (and also the faster the shutter
speed you can use when the lens aperture is wide open). Lenses that
offer a large maximum aperture are commonly referred to as "fast"
lenses (or "fast glass" in photographer-speak) and lenses with smaller
maximum apertures are regarded as "slow." As with sports cars and
racehorses, faster is generally considered better--at least among
equipment junkies.
Lens speed is important for two reasons:
First, the faster a lens is the brighter the viewfinder is when you're
focusing (again, because more light is coming into the camera) and that
can be particularly useful in low-light situations. I've done a lot of
concert photography over the years and when I'm photographing
performers like blues singer Miss Marie (see the photo above), the
venues are typically very dark even with stage lighting and so having a
bright viewfinder really helps me compose such tight shots.
Second,
in low-light situations having a wider maximum aperture means that you
can shoot at a faster shutter speed (or work at a lower ISO or both).
Personally I am only concerned with the first advantage because I
almost always use a tripod and so, unless I am trying to restrict depth
of field (see below), I would rarely use the widest aperture. In fact,
if I were to quickly scan the metadata for 1,000 recent photos, I doubt
that you would find 25 pictures shot at the maximum aperture. As a
rule the faster a lens is the more expensive it is and the heavier and
larger it is. In order to get more light into a lens, manufacturers
have to use larger elements and more refined types of glass, both of
which add to the cost of manufacturing. Also,
single-focal-length-lenses are faster than zoom lenses--usually by a
stop or two. Some manufacturers make the same focal length lens in
several different speeds. For instance, a 135mm prime focal length lens
might be available with a lens speed of f/2 and f/3.5. You can guess
which one is more expensive. Several new cameras, like the Nikon D90
and D700 have very fast ISO
speeds that help with exposure in dim lighting, but it still takes a
wide maximum aperture to see your subject clearly.
Zoom lenses
(with the exception of very expensive pro lenses) are usually a stop or
two slower than prime lenses within their focal-length range. A 50mm
lens might have a speed of f/2, for instance, while a 50-135mm zoom
might have a maximum aperture of only f/4.5 or even f/5.6. The loss of
two stops of light will make the viewfinder image significantly dimmer.
Lens
speed is really only an important issue with DSLR lenses and some
advanced zoom cameras because compact cameras typically have smaller
zoom ranges and typically have larger maximum apertures. Still, if
you're in the buying mode, it's worth reading the camera's
specifications to see what the maximum aperture is because it will tell
you something about the quality of the lens and the brightness of your
viewfinder (including your LCD panel).
(Entire tutorial
copyright 2009 Jeff Wignall; teachers may use this as a teaching aid
and anyone may blog this with a link back to this site.)
Having a fast maximum aperture makes it much easier to compose and photograph nighttime views. This is the famous volcano at the Mirage Hotel and Casino in Las Vegas and while it's explosively bright when it's errupting, it's actually very dim otherwise. Having a fast maximum aperture makes it easy to pre-compose the shot.