Of all the many parts
of your kit, the lens is your most powerful creative tool. Lenses, more
than any other part of the camera, create the look and feel of your
photographs. And it's not just about
focal-length and aperture, either: the character of the glass and
construction of your lens creates a certain visual quality. Some lenses
are tack sharp, others silky smooth. Some lenses are warm and bright,
while others are cool and
moody. Photographers go to great lengths to seek out coveted lenses
for the special way they draw an image.
In this tutorial
we'll look at different kinds of lenses - zooms versus primes - for
night photography. We'll consider the features and drawbacks of each,
and how to choose your lenses for photographing at night.
Focusing Light
Before
we launch into choosing zooms versus fixed focal length lenses for
night photography, we had better set some baselines. Any discussion
about lenses, focal length, and aperture begins with terms of reference.
Full Frame
The
135 format is also referred to as "full frame," the modern term for
digital sensors with the same image area as 35mm film. When
the focal length for a lens and camera combination is given it is
usually described in terms of a 35mm-format "focal length equivalent."
The
focal length of a lens is normally given in millimetres, with shorter
focal lengths providing wider angles of view. This angle of view can be
split into three distinct categories: wide angle, normal and telephoto.
The
"normal" lens for a camera and lens combination is equivalent to the
image diagonal of the camera format. The image area dimensions for the
135 format are 24mm x 36mm: a diagonal of the image area of about 43mm.
Therefore
a 50mm lens on a full frame camera approximately matches the diagonal
of 43mm and is considered "normal." A normal lens gives approximately
the same perspective as the human eye. Focal lengths for lenses of less
than 40mm are referred to as wide angle and focal lengths above 60mm are
called telephoto.
Crop Factor
Cameras
with a different size sensor to full frame use a "crop factor" to
express the 135mm counterparts to a given focal length.
This gives you the full frame "focal length equivalent" for any lens and
camera combination. APS-C cameras have a crop factor about 1.5 times
that of full frame cameras. For Micro Four Thirds you need to
multiply focal length by a factor of two. This means that on a
Micro Four Thirds camera a 50mm normal lens now becomes a 100mm
telephoto lens once you multiply by the two times crop factor.
Full-frame (far left), APS-C (middle) and Micro Four Thirds (far right)
camera formats. A crop factor for camera systems other than 135 format
needs to be applied to give the full frame equivalent camera and lens
combination.Crop factor also helps to
determine the depth-of-field or zone of focus at any given aperture. As
film format or sensor size decreases depth of field at the same aperture
increases. You gain about an extra f-stop-worth of apparent
depth-of-field for APS-C cameras and two stops worth for Micro Four
Thirds systems. This
17mm f2.8 Olympus Micro Four Thirds prime lens mounted on a Panasonic
GH2 body gives approximately the same viewing angle as a 35mm focal
length lens on a full frame camera due to the two times crop factor
(17mm X 2 crop = 34mm). However, depth of field at its maximum aperture
of f2.8 is equivalent to f5.6 on a full frame system.For
night photography, crop-sensor cameras can produce just as good results
as full frame cameras. They don't have the same image quality at very
high ISOs, but used in the 200 ISO to 1600 ISO range many APS-C and
Micro Four Thirds cameras are very capable of holding their own when
making night exposures. These cameras also have the advantage of being
smaller, using smaller lenses, and requiring less power. The also
typically cost less than their larger counterparts.
Aperture
Aperture
refers to the diameter of the opening through which light enters the
lens. It has a numerical scale measured in f-stops. Fast lenses have a
large aperture and allow more light into the camera and onto the film
plane or image sensor.
By adjusting this aperture the amount of
light entering the camera can be controlled in the same way our pupils
control the light entering our eyes. When a bright light source is
present the pupil contracts allowing less light into the eye. This has
the effect of increasing the depth of field meaning more of the scene
will be in focus.
The same rule applies when you decrease (stop
down) or increase (open up) the aperture of a lens. As aperture size
increases more light enters the camera and depth of field decreases and
vice versa: as aperture size decreases less light enters the camera and
depth of field increases.
Understanding the role of focal length
and aperture is critical for controlling the "zone of focus" in your
images. You can use this focus control to isolate a specific subject
within the frame or to ensure the whole scene is in sharp focus.
A Matter Of Perspective
Lens choice also influences another important composition tool: perspective.
Normal
Subjects
shot with a 50mm lens exhibit no discernible geometric distortion and
appear... normal. Funny about that! However, If you substitute the 50mm
lens for a 20mm extreme wide angle lens a completely different picture
emerges.
Wide
Objects appear to be
smaller and further away from the camera as the viewing angle increases.
The distance between separate objects also seems to increase. Straight
lines may seem to curve especially towards the edges of the frame and
objects can appear distorted. The
extreme distortion of an 8mm fisheye lens has warped the straight lines
of these buildings making them appear curved. The perspective in this
shot implies a feeling of confinement as the buildings tower over the
viewer.
Telephoto
If
you again substitute the 20mm wide angle lens for a 100mm telephoto
lens it has the effect of zooming in on your subject by making the
viewing angle narrower. Telephoto lenses also compress the distance
between subjects making them appear closer together. Normal
or telephoto focal lengths are good for lightning photography as you
need to keep your distance from the subject due to the inherent danger.
This shot was taken with a Nikkor 80-200mm f2.8 zoom lens set to 90mm
and was taken on a Nikon D200 APS-C format camera making the focal
length equivalent to a 135mm lens on a full frame camera. You
can also increase or decrease subject size within the frame by moving
the camera closer to or further away from your subject. Moving the
camera changes perspective and you can control the scale of objects in
the scene with the placement of the camera.
Controlling
perspective is an important tool in every photographer's arsenal. It can
be used in a variety of creative ways to compose your scenes.
The Sweet Spot
Very few lenses perform optimally at the
extremes of aperture. In other words, you should avoid shooting at
maximum and minimum apertures if possible. At their wide-open and
closed-down settings lenses exhibit a number of optical flaws. This can
degrade image quality considerably.
At maximum aperture with the
lens fully open you will get a softening of the image due to coma, also
known as astigmatism. You may also observe more vignetting and colour
fringing along with a loss of contrast when the lens is fully open.
When
a lens is stopped down to its minimum aperture you will also observe a
decrease in image quality from the effects of diffraction. Diffraction
causes a loss of resolution, making images appear soft. This is more
obvious on larger-sensor cameras.
So, image quality is best at middle apertures. When you
combine these apertures with low ISO values and a stable camera
platform you can increase the quality of your night photography images
considerably.
As a rule of thumb you should use apertures at least two stops up from
the maximum and two stops down from the minimum. I generally shoot using
apertures between f/5.6 and f/11. Rules are made to be broken and with
experience you will know when you can bend or break these rules. This
chart shows aperture in full increment f-stops starting at a maximum
aperture of f1.4. As you move away from maximum and minimum apertures a
lenses image quality will generally improve. This "sweet spot" is seen
here marked in red .Having a large maximum aperture - say f/1.4 or f/2 -
makes the image in the viewfinder brighter. This assists in
ensuring critical focus in low light conditions. It also means these
lenses are performing optimally at apertures around f2.8 to f/4, which
happens to be the max aperture of most professional-grade zoom lenses.
Those fancy zooms still
require stopping down to f4 or f5.6 before maximum image quality is
assured.
Zoom Lenses or Prime Lenses?
Lenses come in two distinct categories: zooms and primes.
The Prime Directive: Quality and Character
Prime
lenses have a fixed focal length. This means that the design of the
lens is optimized. Prime lenses also generally have a faster maximum
aperture. This allows more light into the viewfinder making it brighter
and easier to focus, which is handy in low light scenarios.
My
personal preference is to use full-frame, fast-aperture, manual-focus
primes: a set of Nikon AI-S lenses, older now, but with wonderful
optical qualities. For my style of night photography these lenses offer
several advantages, including calibrated infinity stops, wider
apertures, and versatility.
Prime lenses, especially older, manual-focus ones, have an infinity
stop. An infinity stop is a calibrated physical limit on focus. This
ensures focus on distant objects in low light. Subjects I like to shoot,
like lightning and astrophotography, benefit greatly from having this
feature and I use it all the time.
Older primes also have marked hyperfocal ranges. Knowing you can
set focus and gain maximum depth of field without even looking through
the viewfinder can come in very handy at times.
My prime lenses work on the three camera
formats I shoot with. They are equally at home on an APS-C or Micro four
thirds camera as they are on a full frame body. This versatility makes
them a good long term investment and I can use them for video, time
lapse and photography work.
On the other hand, depending on the subjects you primarily photograph, prime lenses can be less versatile
than zooms because they may need to be changed frequently. This leaves
you susceptible to getting dust in the camera or onto the digital
sensor.
The Zoom: Flexibility for a Price
Zoom
lenses have a variable focal length, making them more versatile then
prime lenses. A general purpose zoom lens can fulfill a variety of
roles. Zooms
can be used in a range of photographic situations. This full frame
28-105mm Nikkor zoom lens ranges from wide angle through to telephoto
focal lengths and has limited macro capabilities too. Most
entry level cameras come with a zoom lens that allows you to vary the
viewing angle from wide angle through to telephoto. Some modern
"superzooms" cover a very large range of focal lengths. They can replace
a whole suite of prime lenses, meaning you could leave one lens
attached to your camera indefinitely.
This versatility does have a
cost. Zoom lenses generally have compromises in their optical design
and image quality is usually - but not always - inferior. Due to design
limitations they are also generally slower, allowing less light in
through the viewfinder making it more challenging to gain critical focus
in low light.
Zoom lenses have further disadvantages for night
photography. Most zooms,
and especially recent ones, do not have infinity stops or marked
hyperfocal
distance ranges. They're often also more optically bland and clinical,
lacking the attractive character that older lenses can have. Beware that
many zoom lenses
today have an image circle that does not cover full frame sensors. When
considering a zoom, double check to make sure it will fit your camera.
Recent advancements in design and manufacturing technology, however, have shrunk the quality gap
between primes and zooms. I have added several zooms to my lens
collection for added versatility and for when I don't want or need to
carry a suite of prime lenses.
The new Sigma 18-35mm f/1.8 zoom, for example, could replace several of my prime lenses and its fast too! Add a fast
50mm prime and a 70-200mm f/2.8 zoom and with these three lenses you
would be covered for most night photography situations. F8 and be there....It works for me and it should work for you too.
Your Lens is a Powerful Creative Choice
Now
that you have some background information on lenses you can begin to
make informed decisions about the type of lenses you might like to have
for the type of work you wish to do.
Is the focal range versatility of a zoom more important to you than the ultimate image quality of a prime lens? Do you really need
full frame lenses if you only intend to shoot on APS-C camera bodies?
Only you can answer these questions. From my perspective prime lenses
are generally a better investment for night photography: contrast and
resolution is sharper, and they have
fewer defects from spherical and chromatic aberrations. I encourage you
to come to conclusions of your own.
Knowing what equipment
will serve you best and understanding how to get the most from your gear
takes time. Research your options. If you are curious about a certain
lens, try renting it before you buy. Experiment with different lenses
and observe how changing aperture and focal length effect image quality,
field of view, perspective and depth of field.
See you next time
where I will look at some of the basic equipment you will need for
night photography and make suggestions regarding what gear to buy on
specific budgets.
