Photographic Image Quality:
Focal length (expressed in mm)
A typical lens description might look like this: “Canon EF 100mm f/2.8 L Macro IS USM.”
Canon is obviously the manufacturer’s name; we’ll sort out the rest of the jargon on this page,
beginning with the 100mm part.
▸ Focal length, quite simply, is the distance from the focal point of the lens to the plane of the sensor.
On a given sensor, a lens will produce a specific angular field of view; in this case, a 100mm lens on a FF
camera (which it’s designed for) gives us a FOV of about 20° (horizontal). A lens is always labeled
with its real focal length, regardless of the size that it’s mounted on—thus the FOVCF that we discussed
in the sensor section. On an APS-C camera, the 100mm FOV would be about 13° (like 150mm on FF).
▸ Again as discussed in the sensor section, 50mm is considered the “normal” focal length
on a FF sensor, since it approximates the human field of view (about 40°). Anything shorter (less)
than that is considered “wide angle” and anything longer (more) is considered “telephoto.”
Wide-angle lenses can be moderate (24 to 36mm) or as extreme as 17mm or less. Longer lenses can also
be moderate (90 to 135mm) or go all the way to 500 or 800mm or more, all with reference to the FF sensor.
▸ Different focal lengths also change the perspective of an image: wide angle makes close objects
seem much larger than farther ones while telephoto makes objects appear “flatter” or closer together.
(To be more accurate, this is actually a result of the distance between the photographer and the objects,
which of course is aided by the length of the lens.)
▸ Understanding FOVCF makes it possible to understand why compact cameras can be designed to
literally fit into a shirt pocket—the lens only has to cover a tiny 11mm circle around the sensor
and the “normal” (50mm FF) focal point is just 9mm away from the plane of the compact camera sensor.
Maximum aperture, usually called “speed” (expressed as an f-stop)
This is the f/2.8 part of our example lens description. The f-stop, or aperture, numbers
tell how much light the lens will admit to the sensor. They are sequenced so that each value is
approximately = [the previous one × √2]: 1, 1.4, 2, 2.8, 4, 5.6, and so on. You will
also see intermediate values such as 1.7 or 3.5.
▸ Here’s the hard part for many people to get used to: a smaller aperture number is a
larger opening, lets in more light and is said to be “faster”; a
larger number is a smaller opening, lets in less light and is said to be
“slower.” We’ll look at this more thoroughly when we discuss exposure.
▸ When you are shooting, you (or the camera) can always set the aperture to let in less
light, but you can’t set it to any more than the maximum specified on the lens. In general,
a faster lens lets you shoot in lower light but with some side effects that we’ll also discuss later.
Common maximum apertures generally range from 2 to 5.6. Also in general, a faster lens is larger and
heavier (and more expensive) than a slower one, given the same camera size.
And again for reasons of size, weight and cost, longer lenses tend to have smaller maximum apertures
than shorter ones.
▸ You may also see the aperture marked on the lens itself as a ratio (1:2.8 instead
of f/2.8, for example).
Lens mount and series
Most small cameras have the lens built-in. Not only is this less expensive to build, it means that
there are no extra lenses to buy and carry around, and there is much less chance of dust getting into
the body of the camera. Great for traveling. On the other hand, interchangeable-lens cameras provide
much more versatility—almost all DSLRs and mirrorless system cameras have this feature.
▸ Each manufacturer of interchangeable-lens cameras has one or more unique designs for mounting
the lens to the camera body. The mount type is normally specified in the lens description, such as
“EF” in our example. Occasionally, adapters are available to allow mounting
a lens of one type on a body of a different type, but these don’t always allow for full
functioning of the lens features.
▸ Some manufacturers will offer lenses that are similar in focal length and aperture but
different in build quality, features and price. The more “up-scale” of these will usually
carry a series designator such as the “L” in our example.
▸ There is absolutely no standardization between manufacturers in how the mount types and
special series designators are coded or listed in the lens name.
Decades ago, most lenses were built with a single focal length, as in our 100mm example; these are
called “prime” lenses. Today, by far the majority are “zoom” lenses that cover
a range of focal lengths; this is true for both built-in and interchangeable lenses.
▸ Zoom lenses are described by their shortest and longest focal length, for example 24–105mm.
Some manufacturers specify the range of their zoom lens instead, for example 5× zoom for a 5–25mm lens.
(This would obviously be for a compact 5.6 FOVCF camera.)
▸ Zoom lens maximum apertures tend to be smaller than those of prime lenses. Some of the best
zoom lenses maintain a constant maximum aperture throughout the zoom range, for example f/4 or
even f/2.8; however, most zooms get slower as they get longer, for example the Sony
18–55mm f/3.5–5.6 kit lens for their NEX series cameras.
▸ Tradeoffs: zooms are versatile at the cost of speed, weight, size, price, and in some cases
quality. Few if any zooms will do everything. Primes reverse the pros and cons, and for some
photographers help them concentrate on the subject more than the technology. Many smaller cameras,
however, simply don’t offer this option.
▸ Some consumer-level cameras advertise a “digital zoom” feature in addition to
the optical zoom that is built into their lens. This is marketing nonsense and simply dishonest—what
really happens is that the camera records only a smaller and smaller portion of the image center,
exactly as if it were cropped later. Useless.
Until quite recently, long lenses required very steady hand holding at high shutter speeds—or
mounting on a tripod—to avoid a blurred image due to camera shake. Many modern telephotos have
addressed this problem by incorporating “smart” sensing and control mechanisms that help stabilize
the lens as you shoot. Almost needless to say, this raises the price, size, and weight of the lens.
Each manufacturer uses its own label for these systems; the “IS” in our example lens is one of the
more obvious ones. Although still far from perfect, IS systems do seem to provide a real advantage
in using these lenses.
▸ A few interchangeable-lens systems provide image stabilization in the camera body, eliminating the
need for stabilized lenses. However, if a stabilized lens
is mounted on a stabilized body, one or the other has to be turned off to keep them from conflicting.
Most modern lenses can be focused under control of the camera rather than by the photographer
turning a ring on the lens barrel. The various ways this is done are beyond the scope of our discussion
here, but again there must be some mechanism within the lens in order to accomplish it. The
“USM” of our example lens is simply the name of one manufacturer‘s AF
system. Once again add weight, size, and cost.
Macro lenses (like our example) are designed to focus much more closely to the
subject than usual, providing up to 1:1 or greater magnification. (In other words, a quarter coin
would completely fill the short dimension of a FF image.) These are always prime lenses that can also
be used in normal shooting. Applications include small-product and scientific photography.
Fisheye lenses have a very short focal length relative to the sensor size, usually covering
nearly a 180° field of view. Instead of forming a normal rectilinear image, they form the kind of
circular image that the name implies. Very specialized.
Tilt/shift lenses are designed for architectural photography—they provide lens
movements similar to (but more limited than) those of a view camera. One example of how these are used
is to make buildings appear straight vertically rather than tilted backwards. In many cases today,
similar results can be obtained in software during post processing. Expensive.
Mirror lenses are designed like mirror telescopes, to fold a long optical path
into a shorter physical package. Typical focal lengths are 600mm or more and they always have a fixed
aperture, usually f/8. A tripod is mandatory; image quality can be questionable in the less expensive versions.
Although a consumer can make a fairly educated guess about the image quality of a lens by looking at
all of the characteristics we’ve discussed so far—and by checking prices and the reputation of
various manufacturers—there are still several quality attributes of a lens that can be measured
scientifically. These include sharpness, chromatic aberration (off-colors), vignetting (darker corners),
distortion (straight lines that bow inward or outward), and autofocus speed. All of these can be found
easily in lens reviews on the Internet. Although some reviews can get a bit “geeky,” they
can still help to identify lenses that may perform very well at a reasonable price (and lenses that
should be avoided at any price).
Lens hoods and filters
Many lenses come with a hood that attaches to the front and shades the lens from stray light that
could degrade the image. The hood also provides some additional protection from physical damage to
the front of the lens. The most useful ones can be attached in reverse to the lens for storage.
▸ Most lenses also have threaded fronts (sized in mm) that accept filters. Historically,
filters were used for color correction and special effects; those are mostly no longer needed.
However, all lenses should have a clear UV or “skylight” filter attached
(if at all possible) for physical protection. Buy it with the lens; install it when the lens comes
out of the box; leave it on always. Don’t forget to keep it clean both by dusting regularly with a
lens brush and by periodic wiping with a solution and cloth formulated for this purpose. Dust
and fingerprints will destroy image quality. If it gets scratched or cracked, replace it
quickly—it’s a lot cheaper than replacing the lens! And of course, keep the lens cap on when
the camera is not in use.