Understanding camera lenses can help add more creative control to
digital photography. Choosing the right lens for the task can become a
complex trade-off between cost, size, weight, lens speed and image
quality. This tutorial aims to improve understanding by providing an
introductory overview of concepts relating to image quality, focal
length, perspective, prime vs. zoom lenses and aperture or f-number.
LENS ELEMENTS & IMAGE QUALITY
All but the simplest cameras contain lenses which are actually
comprised of several "lens elements." Each of these elements directs the
path of light rays to recreate the image as accurately as possible on
the digital sensor. The goal is to minimize aberrations, while still
utilizing the fewest and least expensive elements.
Optical aberrations occur when points in the image do not translate
back onto single points after passing through the lens — causing image
blurring, reduced contrast or misalignment of colors (chromatic
aberration). Lenses may also suffer from uneven, radially decreasing
image brightness (vignetting) or distortion.
INFLUENCE OF LENS FOCAL LENGTH
The focal length of a lens determines its angle of view, and thus
also how much the subject will be magnified for a given photographic
position. Wide angle lenses have short focal lengths, while telephoto lenses have longer corresponding focal lengths.
Note: The location where light rays cross is not necessarily equal to the focal length,
as shown above, but is instead roughly proportional to this distance.
as shown above, but is instead roughly proportional to this distance.
Many will say that focal length also determines the perspective of an
image, but strictly speaking, perspective only changes with one's
location relative to their subject. If one tries to fill the frame with
the same subjects using both a wide angle and telephoto lens, then
perspective does indeed change, because one is forced to move closer or
further from their subject. For these scenarios only, the wide angle
lens exaggerates or stretches perspective, whereas the telephoto lens
compresses or flattens perspective.
The following table provides an overview of what focal lengths are
required to be considered a wide angle or telephoto lens, in addition to
their typical uses. Please note that focal lengths listed are just rough ranges, and actual uses may vary considerably; many use telephoto lenses in distant landscapes to compress perspective, for example.
Lens Focal Length* | Terminology | Typical Photography |
---|---|---|
Less than 21 mm | Extreme Wide Angle | Architecture |
21-35 mm | Wide Angle | Landscape |
35-70 mm | Normal | Street & Documentary |
70-135 mm | Medium Telephoto | Portraiture |
135-300+ mm | Telephoto | Sports, Bird & Wildlife |
*Note: Lens focal lengths are for 35 mm equivalent cameras.
If you have a compact or digital SLR camera, then you likely have a
different sensor size. To adjust the above numbers for your camera,
please use the focal length converter in the tutorial on digital camera sensor sizes.
Other factors may also be influenced by lens focal length. Telephoto
lenses are more susceptible to camera shake since small hand movements
become magnified, similar to the shakiness experience while trying to
look through binoculars. Wide angle lenses are generally more resistant
to flare, in part because the designers assume that the sun is more
likely to be within the frame. A final consideration is that medium and
telephoto lenses generally yield better optical quality for similar
price ranges.
FOCAL LENGTH & HANDHELD PHOTOS
The focal length of a lens may also have a significant impact on how easy it is to achieve a sharp handheld photograph. Longer focal lengths require shorter exposure times to minimize blurring caused by shaky hands.
Think of this as if one were trying to hold a laser pointer steady;
when shining this pointer at a nearby object its bright spot ordinarily
jumps around less than for objects further away.
This is primarily because slight rotational vibrations are magnified
greatly with distance, whereas if only up and down or side to side
vibrations were present, the laser's bright spot would not change with
distance.
A common rule of thumb for estimating how fast the exposure needs to be for a given focal length is the one over focal length rule.
This states that for a 35 mm camera, the exposure time needs to be at
least as fast as one over the focal length in seconds. In other words,
when using a 200 mm focal length on a 35 mm camera, the exposure time
needs to be at least 1/200 seconds — otherwise blurring may be hard to
avoid. See the tutorial on reducing camera shake with hand-held photos for more on this topic.
Keep in mind that this rule is just for rough guidance; some may be
able to hand hold a shot for much longer or shorter times. For users of digital cameras with cropped sensors, one needs to convert into a 35 mm equivalent focal length.
ZOOM LENSES vs. PRIME LENSES
A zoom lens is one where the photographer can vary the focal length
within a pre-defined range, whereas this cannot be changed with a
"prime" or fixed focal length lens. The primary advantage of a zoom lens
is that it is easier to achieve a variety of compositions or
perspectives (since lens changes are not necessary). This advantage is
often critical for dynamic subject matter, such as in photojournalism
and children's photography.
Keep in mind that using a zoom lens does not necessarily mean that one no longer has to change their position;
zooms just increase flexibility. In the example below, the original
position is shown along with two alternatives using a zoom lens. If a
prime lens were used, then a change of composition would not have been
possible without cropping the image (if a tighter composition were
desirable). Similar to the example in the previous section, the change
of perspective was achieved by zooming out and getting closer to the
subject. Alternatively, to achieve the opposite perspective effect, one
could have zoomed in and moved further from the subject.
Why would one intentionally restrict their options by using a prime
lens?Prime lenses existed long before zoom lenses were available, and
still offer many advantages over their more modern counterparts. When
zoom lenses first arrived on the market, one often had to be willing to
sacrifice a significant amount of optical quality. However, more recent
high-end zoom lenses generally do not produce noticeably lower image
quality, unless scrutinized by the trained eye (or in a very large
print).
The primary advantages of prime lenses are in cost, weight and speed. An inexpensive prime lens can generally provide as good (or better) image quality as a high-end zoom lens.
Additionally, if only a small fraction of the focal length range is
necessary for a zoom lens, then a prime lens with a similar focal length
will be significantly smaller and lighter. Finally, the best prime
lenses almost always offer better light-gathering ability (larger
maximum aperture) than the fastest zoom lenses — often critical for
low-light sports/theater photography, and when a shallow depth of field is necessary.
For compact digital cameras, lenses listed with a 3X, 4X, etc. zoom
designation refer to the ratio between the longest and shortest focal
lengths. Therefore, a larger zoom designation does not necessarily mean
that the image can be magnified any more (since that zoom may just have a
wider angle of view when fully zoomed out). Additionally, digital zoom is not the same as optical zoom, as the former only enlarges the image through interpolation. Read the fine-print to ensure you are not misled.
INFLUENCE OF LENS APERTURE OR F-NUMBER
The aperture range of a lens refers to the amount that the lens can
open up or close down to let in more or less light, respectively.
Apertures are listed in terms of f-numbers, which quantitatively
describe relative light-gathering area (depicted below).
Note: Aperture opening (iris) is rarely a perfect circle,
due to the presence of 5-8 blade-like lens diaphragms.
due to the presence of 5-8 blade-like lens diaphragms.
Note that larger aperture openings are defined to have lower
f-numbers (often very confusing). These two terms are often mistakenly
interchanged; the rest of this tutorial refers to lenses in terms of
their aperture size. Lenses with larger apertures are also described as being "faster,"
because for a given ISO speed, the shutter speed can be made faster for
the same exposure. Additionally, a smaller aperture means that objects
can be in focus over a wider range of distance, a concept also termed
the depth of field.
f-# | Corresponding Impact on Other Properties: | |||
---|---|---|---|---|
Light-Gathering Area (Aperture Size) |
Required Shutter Speed | Depth of Field | ||
Higher | Smaller | Slower | Wider | |
Lower | Larger | Faster | Narrower |
When one is considering purchasing a lens, specifications ordinarily
list the maximum (and maybe minimum) available apertures. Lenses with a
greater range of aperture settings provide greater artistic flexibility,
in terms of both exposure options and depth of field. The maximum
aperture is perhaps the most important lens aperture specification,
which is often listed on the box along with focal length(s).
An f-number of X may also be displayed as 1:X (instead of f/X), as
shown below for the Canon 70-200 f/2.8 lens (whose box is also shown
above and lists f/2.8).
Portrait and indoor sports/theater photography often requires lenses
with very large maximum apertures, in order to be capable of a narrower
depth of field or a faster shutter speed, respectively. The narrow depth
of field in a portrait helps isolate the subject from their background.
For digital SLR cameras, lenses with larger maximum apertures provide significantly brighter viewfinder images — possibly critical for night and low-light photography. These also often give faster and more accurate auto-focusing in low-light. Manual focusing is also easier
because the image in the viewfinder has a narrower depth of field (thus
making it more visible when objects come into or out of focus).
Typical Maximum Apertures | Relative Light-Gathering Ability | Typical Lens Types |
---|---|---|
f/1.0 | 32X | Fastest Available Prime Lenses (for Consumer Use) |
f/1.4 | 16X | Fast Prime Lenses |
f/2.0 | 8X | |
f/2.8 | 4X | Fastest Zoom Lenses (for Constant Aperture) |
f/4.0 | 2X | Light Weight Zoom Lenses or Extreme Telephoto Primes |
f/5.6 | 1X |
Minimum apertures for lenses are generally nowhere near as important
as maximum apertures. This is primarily because the minimum apertures
are rarely used due to photo blurring from lens diffraction,
and because these may require prohibitively long exposure times. For
cases where extreme depth of field is desired, then smaller minimum
aperture (larger maximum f-number) lenses allow for a wider depth of
field.
Finally, some zoom lenses on digital SLR and compact digital cameras
often list a range of maximum aperture, because this may depend on how
far one has zoomed in or out. These aperture ranges therefore refer only
to the range of maximum aperture, not overall range. A range of
f/2.0-3.0 would mean that the maximum available aperture gradually
changes from f/2.0 (fully zoomed out) to f/3.0 (at full zoom). The
primary benefit of having a zoom lens with a constant maximum aperture
is that exposure settings are more predictable, regardless of focal
length.
Also note that just because the maximum aperture of a lens may not be
used, this does not necessarily mean that this lens is not necessary. Lenses
typically have fewer aberrations when they perform the exposure stopped
down one or two f-stops from their maximum aperture (such as
using a setting of f/4.0 on a lens with a maximum aperture of f/2.0).
This *may* therefore mean that if one wanted the best quality f/2.8
photograph, a f/2.0 or f/1.4 lens may yield higher quality than a lens
with a maximum aperture of f/2.8.
Other considerations include cost, size and weight. Lenses with
larger maximum apertures are typically much heavier, larger and more
expensive. Size/weight may be critical for wildlife, hiking and travel
photography because all of these often utilize heavier lenses, or
require carrying equipment for extended periods of time.
source: cambridgeincolour